kow ler and erdos here tal 1995

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Vis ion Res . Vo l. 35, No. 13, pp. 189%1916, 1995Copyright © 1995 Elsevier Science Ltd

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T h e R o l e ,of A t t e n t i o n in t h e P r o g r a m m i n g

o f S a c c a d e sEILEEN KOWLER,*~" ERIC ANDERSON,* BARBARA DOSHER,J; ERIK BLASER:~

Received 17 Ma rch 199 4; ir~ revised for m 4 O ctober I9 94

Accurate sacc adic programm ing in natural visual scene s requires a s ignal designat ing which of the many

potent ial targets is to be the goal of the saccade. Is this s ignal controlled by the al locat ion o f perceptual

attention, or do sacc ades have their ow n independent selective f i l ter? W e found evidence for the

involvement o f perceptual at tent ion, namely: (1) summoning perceptual attent ion to a target also

facil i tated saccades; (2) perceptual ident if icat ion was better at the saccadic goal than elsewhere; and

(3) at tempts to dis sociate the locus of at tention fro m the saccad ic goal were unsuccessful , i .e . i t was not

possible to prepmre to look quickly and accurately at one target wh ile at the sam e t ime making highlyaccurate perceptual judgements about targets elsewhere. W e also studied the trade-off between sacca dic

and perceptual performance by means o f a novel applicat ion o f the "attent ional operat ing characterist ic"

(A OC ) to oculo mo tor performance. T his analysis revealed that some attention could be diverted from

the saccadic goal with virtually no cost to either saccadic latency or accuracy, showing that there is a

ceil ing o n the attent ional demands o f saccades. Th e f inks we discovered between sacc ades and attention

can be explained by a m odel in which perceptual at tent ion determines the endpoint of the saccad e, wh ile

a separate trigger s ignal init iates the saccad e in response to transient changes in the at tentional locus.

Th e m odel w i l l be d iscussed in the contex t o f current neurophysio log ica i work on saccad ic control .

Saccades Attention Attention operating characteristic Eye movement

INTRODUCTION

Selective at tention is the gateway to conscious experience,

affecting our ability to perceive, distinguish and

remember the various stimuli that come our way (James,

1890). In contemporary usage, selective attention denotes

the allocation of limited processing resources to some

stimuli or tasks at the expense of others (Norman &

Bobrow, 1975; Reeves & Sperling, 1986; Shaw, 1982,

1984; Sperling & Dosher, 1986). Most of what is known

about selective attention concerns its effects on perception

or memory, but selective attention may also be a

significant contributor to, motor control, determining

which of the various objects in the visual field is to be

the target used to plan and guide movement. This

paper examines the role of selective attention in the

programming of saccadic eye movements.

Understanding the role of selective attention in

saccadic programming is important for understanding

how it is possible to direct a saccade accurately to a chosen

visual object in a highly structured visual display. The

difficultyencountered when scanning such displays is that

* D e p a r t m e n t o f P s y c h o l o g y , R u t g e r s U n i v e r s i t y , N e w B r u n s w i c k ,

NJ 08903 , U. S . A.

t T o w h o m a l l c o r r e sp o n d e n c e s h o u l d b e addressed.

:~Depar tment o f Cogni t ive Sc ience , Univer s i ty o f Cal i fo rn ia , I rv ine ,

CA 92717 , U. S . A.

the saccadic system must "know" which of the many

available objects is to be the target. I n t h i s p a p e r , w e a s k e d

w h e t h e r t h e s a cc a d i c s y s t e m " k n o w s " w h i c h is th e e f fe c t i ve

t a r g e t b y m e a n s o f t h e s a m e a t t e n t i o n a l m e c h a n i s m t h a t

s e r ve s p e r c e p t i o n . In effect, we asked whether the saccadic

target is selected by shifting perceptual attention to the

saccadic goal, or, alternatively, whether it is possible

to shift perceptual attention to one location while

simultaneously invoking a separate selective mechanism

that will direct the saccade elsewhere. Determiningwhether separate selective mechanisms serve perception

and eye movements, or, alternatively, whether a single

mechanism serves both, will shed light on the nature of the

central mechanisms tha t cont rol high-level aspects of

saccadic planning and execution and will contribute to the

understanding of the processing steps leading up to the

execution of an accurate saccade.

A role for perceptual attention in saccadic program-

ming is often assumed because of the intuitively appealing

observation that people prefer to shift attention to where

they are about to look (e.g. Hendersen, Pollatsek &

Rayner , 1989). This may be a sensible s trategy to use whilereading or while scanning complex displays, but by itself

the observation reveals nothing about the role of

attention in saccadic control. Shifts of attent ion preceding

saccades might serve a variety of purposes unrelated to

saccadic control, such as to evaluate whether a particular

1897



1898 EILEEN KOW LER e t a l .

eccentr ic ta rge t i s a su i table goa l for the saccade , or (as

H e n d e r s e n e t a l . , 1989 , p r op ose d ) t o ge t a he a d s t a r t on

p r oc e s s ing the ne x t i t e m in a s e q ue nc e .

A n a l o g o u s a r g u m e n t s a p p l y t o t h e p h y s i o l o g i c a l

l i te r a tu re . L inks be twe e n a t t e n t ion a n d sa c c a de s a r e o f t e n

a s s u m e d b a s e d o n f in d i ng s i n m o n k e y o f p r e -s a c c ad i c

a c t iv i ty in ne u r on s im p l i c a t e d in a t t e n t iona l c on t r o l . S uc h

ne ur ons ha ve be e n f ound in a r e a s suc h a s i n f e r io r

t e m por a l c o r t e x ( C he la z z i , Mi l l e r , Dunc a n & De s im one ,

1993) , pu lv ina r ( P e t e r se n , R ob inson & Mor r i s , 1987 ;

R ob inson & Mc C lu r k in , 1989) , a nd pa r i e t a l c o r t e x

( Ande r se n , E ss i c k & S iege l, 1987 ; G na d t & A nde r se n ,

1988 ; And e r se n & G na d t , 1989) . Th i s p r e - sa c c a d ic

a c t iv i ty m igh t p l a y a n e s se n t ia l r o l e i n s e t t i ng the spa t i a l

pa r a m e te r s o f t he s a c c a de . A l t e r na t ive ly , t h i s a c t i v it y

m igh t ha ve no f unc t iona l r o l e i n s a c c a d ic p r og r a m m ing

a t a l l , bu t m igh t i n s t e a d se r ve t o e nha nc e pu r e ly

pe r c e p tua l o r c ogn i t i ve a spe c t s o f t he a n im a l ' s t a sk , suc h

a s t a r ge t de t e c t ion , l oc a l i z a t i on o r r e c ogn i t i on , t ha t

ha ppe n to c o inc ide w i th s a c c a d ic p l a nn ing .

The be ha v io r a l a nd phys io log ic a l e x pe r im e n t s de -

sc r ibe d a bove p r ov ide e v ide nc e tha t sh i f t s o f a t t e n t ion

p r e c e de sa c c a de s , bu t do no t r e ve a l t he f unc t ion o f the se

p r e - sa c c a d ic a t t e n t iona l sh i f t s . De te r m in ing the r o l e o f

a t t e n t ion in s a c c a d ic p r og r a m m ing r e q u i r e s p syc ho phys i -

c a l a nd oc u lom oto r e x pe r im e n t s e x p r e s s ly de s igne d to

d i sc ove r whe the r a t t e n t ion sh i f t s a r e ne c e s sa r y t o

p r o g r a m a c c u r a te s a c c ad e s . U n f o r t u n a t e l y , t h e m e t h o d o -

l o gi c al o b s t a cl e s to o b t a i n in g a n u n a m b i g u o u s a n s w e r t o

th i s q ue s t ion ha ve p r ove n to be f o r m ida b le . P r io r

a t t e m p t s t o s tudy the r o l e o f a t t e n t ion in s a c c a d ic

p r og r a m m in g ha ve l e d t o c on fl i c ti ng r esu l t s, a nd , i n som e

c a se s , t o a r t i f a c tua l ou t c om e s ( i s sue s t o be r e v i e we d in

m or e de t a i l be low) .

T h e i n d e t e r m i n a te o u t c o m e o f t h e p r io r w o r k l e a v es

o p e n b r o a d a n d b a s i c q u e s t i o n s a b o u t s a c c a d e s a n d

a t t e n t ion , suc h a s :

- - D o e s p e r c e p t u a l a tt e n t i o n p l a y a n y r o le i n sa c c ad i c

c on t r o l ? I f so , how g r e a t a de m a nd doe s s a c c a d ic

p r og r a m m ing p l a c e on l im i t e d a t t e n t iona l r e sou r c e s

a n d a t w h a t s t a g e o f s a c ca d i c p r o g r a m m i n g d o e s

a t t e n t ion c om e in to p l a y?

- - I f pe r c e p tua l a t t e n t ion i s no t r e spons ib l e f o r t he

se l e c tion o f s a c c a d ic t a r ge ts , t he n wh a t so r t o f

se le c tive m e c h a n i sm i s do ing the j ob?

O ur s tudy a dd r e s se d a l l o f t he se is sue s.

O u r a p p r o a c h w a s i n f lu e n c ed b y t h r ee a s p e c t s o f p ri o r

w o r k :

( 1) p r io r r e se a r c h de m ons t r a t i ng a r e l a t i onsh ip b e -

t w e e n s m o o t h e y e m o v e m e n t s a n d a t te n t i o n ;

( 2) p r io r s tud i e s show ing sa c c a d ic e r r o r s du r ing the

sc a nn ing o f s t r uc tu r e d v i sua l d i sp l a ys ( whe r e

a t t e n t iona l a l l oc a t ion m igh t b e ne e de d to s e l e c t t heta rge t ) ; and

( 3) t he c on f l i c ti ng ou tc om e s o f p r io r e x pe r im e n t s on

sa c c a de s a nd a t t e n t ion .

The se t h r e e i n f lue nc e s a r e r e v i e we d be low.

A t t e n ti o n a n d sm o o t h e y e m o v e m e n t s

I nve s t iga to r s s inc e Dodge a nd Fox ( 1928) a nd Te r

B r a a k ( 19 57 ) h a v e f o u n d t h a t s m o o t h e y e m o v e m e n t s c a n

be use d to m a in t a in a n a c c u r a t e l ine o f sigh t on e i t he r

s t a t i ona r y o r m ov ing t a r ge t s i n t he p r e se nc e o f

ba c kgr ound s t im u l i m ov ing a t a d i f f e r e n t ve loc i ty ( e . g .

D u b o i s & C o l le w i jn , 1 9 7 9 ; M u r p h y , K o w l e r & S t e in m a n ,1975; Te r B raak & Buis , 1970) . The inf luence of the

b a c k g r o u n d s o n e y e v e lo c i ty c a n b e a s s m a l l a s 2 - 4 %

( K owle r , va n de r S t e e n , Ta m m inga & C o l l e wi j n , 1984) .

This h igh degree of se lec t iv i ty i s due to a t tent ion:

K h u r a n a a n d K o w l e r ( 1 9 8 7 ) s h o w e d t h a t p e r c e p t u a l

j udgm e n t s a r e be t t e r f o r t r a c ke d t a r ge t s t ha n f o r

u n t r a c k e d b a c k g r o u n d s . T h e i r s t u d y , w h i c h c o n t r o l l e d

f o r d i f f e re n t i a l e ff e c ts o f r e ti na l im a ge spe e d a n d pos i t i on ,

s h o w e d t h a t a s in g le a t t e n t iona l f il t er de t e rm ine s t he i npu t

bo th t o s m o o th e ye m o ve m e n t s a nd p e r c e p t ion , i .e . i t i s

no t po ss ib l e t o f u l ly a t t e nd one t a r ge t a nd a t t he s a m e t im e

a c c u r a t e ly pu r sue a no the r . The p r e se n t s tudy r a i s e sa n a l o g o u s q u e s t io n s a b o u t s a cc a d es .

S a c c a d e s i n s t r u c t u r e d v i s u a l d i s p l a y s

Give n the e f f e c t ive s e l e c tion o f t he t a r ge t f o r sm oo th e ye

m o ve m e n t s ju s t de sc r ibe d , i t i s su r p r i sing to d i sc ove r t ha t

the e f f e c tive ne ss o f t a r ge t s e l e c tion f o r s a c c a de s ha s be e n

q ue s t ione d . S e ve r a l i nve s t iga to r s ha ve r e po r t e d tha t

sho r t - l a t e nc y sa c c a de s m a de in s t r uc tu r e d v i sua l fi elds c a n

be ina c c u r a t e , l a nd ing ne a r t he c e n te r o f t he e n t i r e

s t im u lus c on f igu r a t ion , r a the r t ha n a t t he de s igna t e d

t a r ge t w i th in t he c on f igu r a t ion ( F ind la y , 1982; O t t e s , Va nGisbe r g e n & Egge r m o n t , 1985; C oe f f e & O 'R e g a n , 1987).

The se sa c c a d ic e r r o r s ( wh ic h wo u ld be d i sa s t r ous i f t he y

oc c u r r e d du r ing na tu r a l s c a nn ing ) i n sp i r e d the p r oposa l

tha t t he r e i s a l ow- l e vel , a u tom a t i c , a v e r a g ing m e c h a n i sm

tha t de t e r m ine s s a c c a d ic e ndpo in t s , a t l e a s t wh e n sa c c a d ic

l a t e nc y is sho r t ( F ind la y , 1982 ; O t t e s e t a l . , 1985 ; W ise &

De s im one , 1988) .

Low - le ve l a ve r a g ing , how e ve r , i s un l ike ly be c a use t he

e ndpo in t s o f t he so - ca l l e d sho r t -l a t e nc y " c e n te r ing "

sa c c a de s c a n be b i a se d by h igh - le ve l f a c to r s , suc h a s t he

p r o b a b i l i t y o f t h e t a rg e t a p p e a r i n g i n o n e o r a n o t h e r

loc a t ion ( H e & K ow le r , 1989), o r vo lun ta r y e f f o r t ( H e &K ow le r , 1991). A m or e p l a us ib l e e x p la na t ion o f t he

sa c c a d ic e r r o r s o bse r ve d in h igh ly - s t r uc tu r e d v i sua l fi elds

i s t ha t t he y we r e no t " e r r o r s " i n t he u sua l s ense. I n s t e a d ,

sa c c a de s we r e p r og r a m m e d wh i l e a t t e n t ion wa s d i s -

t r i bu t e d a c r os s w ide r e g ions o f t he v i sua l f i e ld i n a n

a t t e m pt t o l oc a t e t he de s igna t e d t a r ge t a c c u r a t e ly .

According to th is v iew, spa t ia l ly- se lec t ive a t tent ion

de te r m ine s t he e f f e c tive i npu t t o s a c c a de s ( H e & K ow le r ,

1989, 1991; a lso Co ef fe , 1987). This sug gest ion w as only

t e n t a t i ve , howe ve r , be c a use de sp i t e m a ny p r io r a t t e m pt s

to do so ( see be low) , a c l e ar l i nk be twe e n a t t e n t ion a nd

s a c c ad i c e y e m o v e m e n t s h a s n o t b e e n d e m o n s t r a te d .

P r i o r a t t e m p t s t o l i n k a t t e n t i o n w i t h s a c c a d e s

The ba s i c i de a be h ind the p r io r a t t e m pt s t o s tudy the

r o l e o f a t t e n t ion sh i ft s in s a c c a d ic p r og r a m m in g wa s to



THE ROLE OF ATTENTION IN THE PROGRAMMING OF SACCADES 1899

evaluate performance on a perceptual task carried out

while saccadic preparation was in progress. The critical

variable in such experiraents was the location of the

perceptual target relative to the intended endpoint of the

saccades. If shifts of attention precede saccades, then: (1)

perceptual performance should be better for targets

located at the saccadic goal; and (2) drawing atten tion to

one region of space should reduce the latency of saccades

made there at the expense of saccades made elsewhere.

Experiments using this logic have produced diametri-

cally opposite results. Posner (1980), for example,

summarized two studies, one in which reaction time to

detect the appearance of a stimulus was shorter at the

saccadic goal, and the other in which reaction time was

shorter at a location opposite to the saccadic goal. Posner

(1980) rejected strong links between attention and

saccades, concluding instead that movements of attention

depend on the importance of the target, not on the

occurrence of saccades.

Remington (1980) also rejected strong links between

saccades and attention on the basis of his finding

(Experiment 3) that detection of a brief luminance

increment was equally accurate regardless of the location

of the increment relative to the goal of the saccade. But

inspection of his data shows that saccadic latencies were

prolonged when the luminance increment and saccadic

goal were in different places, calling into question the

independence he had proposed. Klein (1980) and later

Klein, Kingstone and Pontefract (1992), kept alive the

notion o f independence by finding that cues signaling the

likely location o f a target :for a manual response did notinfluence subsequent saccades. They speculated, however,

that while their results rejected a role for saccades in the

control of attention, their experiments were insufficient o

rule out the involvement of attentional shifts in what they

referred to as "saccadic execution".

Shepherd, Findlay and Hockey (1986) believed that

they had evidence showing that shifts of attention

preceded saccades. This was based on their finding of

shorter manual reaction times to the appearance of a

target at the saccadic goal. But this result was not likely

to have had anything to do with attention at all because

the manual response occurred well after the saccadebrought the line of sight to the target. Thus, the results

could be a ttributed to effects of retinal eccentricity, rather

than to attention.

Subramaniam and Hoffman (1992), who did remove

the target before the saccade occurred, thus eliminating

confounding effects of retinal eccentricity, found that

identification of a target letter was more accurate at the

intended goal of the saccade than elsewhere. Their results

provide the most convincing demonstration to date of a

saccadic and attentional link. However, the issue still

remains open because they did no t explicitly ask subjects

to try to make a saccade to one place while shiftingattention to another. In the absence of such instructions,

subjects may have chosen to attend to where they were

told to look rather than to make the explicit effort to

dissociate the locus of attent ion from the designated goal

of the saccade. If instructed to do so, subjects might have

been able to make the dissociation--or, at the very least,

improve identification of targets at non-goal locations.

The conflicting results of the prior work illustrate how

difficult it is to study, inside the laboratory, a task that

people are continually doing outside the laboratory,

namely, making saccades and perceiving objects at the

same time. Part of the difficulty s finding a perceptual task

that is sufficientlysensitive to the allocation ofatt ent ional

resources. Indeed, the role of attention in the detection of

targets (the task used in most of the prior studies of

saccades and attention described above) has been

controversial. Many investigators have argued that

detection occurs at a "pre-attentive level" and attention,

in the sense of allocation of processing resources, does not

influence target detection at all (Shaw, 1984; Sperling &

Dosher, 1986; Norman & Bobrow, 1975; Kinchla, 1992;

Palmer, 1994). These investigators have developed formal

models showing how effects of location probabili ty on the

time to detect an abruptly-appearing target can be

attributed to adjustments in decision criteria, rather than

to changes in allocation of processing resources. To the

extent that the prior work on saccades and attent ion has

been dominated by studies of detection tasks, whose

attentional demands are uncertain at best, the diverse

pattern of results obtained in the prior work may not be

surprising.

O V E R V I E W

We did three sets of experiments to determine the role

of attent ion in the programming of saccades. Theseexperiments, like the prior attempts to address this issue,

made concurrent measurements of saccadic and

perceptual performance. But unlike nearly all of the prior

attempts, the perceptual task we used (letter identifi-

cation) is known to be sensitive to the allocation of

attention, when attentional allocation is governed by

means o f either visual or verbal cues (Sperling &

Melchner, 1978; Krose & Julesz, 1989). (Our own results

will verify the effectiveness of such cues.) In addition,

each set of experiments added conditions that allow

increasingly more stringent control of the strategies

employed by the subject, something we found to be ofcritical importance for interpreting the patterns of results

we obtained as we proceeded with the work.

The first set of experiments (1A ,B ." D rawing a t t en t ion to

an eccentric target) measured the latencies of saccades

made in or opposite to the location o f an eccentric target

that summoned attention. The second set of experiments

(2 and 3 : Cen t ra l cue) used a central cue to direct saccades

while perceptual performance was assessed at the saccadic

goal and elsewhere. The last experiment (4: A t t en t i o n a l

operat ing character ist ic) measured the trade-off between

attent ional allocation at the saccadic goal and elsewhere

when subjects were required to assign different weights to

the saccadic and perceptual task.

The bottom line is that saccades require shifts of

attention, but there is a clear ceiling on the attentional

demands of saccades, leaving considerable attentional

resources available for processing perceptual material.




GENERAL METHODS

Subjects

Two subjects were tested, EK, one of the authors, and

MC, who was naive abou t the purpose of the experiment.

MC had some prior experience as an eye movement

subject in a prior study dealing with different aspects ofsaccades (He & Kowler, 1992).

Apparatus

Stimuli were generated on a display monitor (Tektronix

608, P4 phosphor) located directly in front of the subject's

right eye. Displays were refreshed every 20 msec, a rate

high enough to prevent visible flicker. The intensity

was set so that an array of 40 × 40 points subtending

2.2 x 2.2 cm on the display had a measured intensity

of 100 cd/m2 at the 20 msec refresh rate. This works

out to a luminous directional-energy of 12 cd-#sec per

point (Sperling, 1971). Displays were controlled by aminicomputer (LSI 11/24).

The stimuli were seen against a dim (3.7 cd/m2),

homogeneous background produced by a raster on a

second display monitor located perpendicular to the first.

The views of the two displays were combined by a pellicle

beam splitter. The combined displays were viewed in a

dark room through a collimating lens which placed them

at optical infinity. Subject EK, who is myopic, viewed the

displays through a negative lens, placed between the eye

and collimating lens, which kept the stimuli in sharp

focus.

Eye movement recording

Two-dimensional movements of the right eye were

recorded by a Generation IV SRI Double Purkinje Image

Tracker. The left eye was covered and the head was

stabilized on a dental biteboard.

The voltage output of the tracker was fed on-line

through a low pass 50Hz filter to a 12 bit

analog-to-digital converter (ADC). The ADC, under

control of the computer, sampled eye position every 10

msec. The digitized voltages were stored for subsequent

analysis.Tracker noise-level was measured with an artificial eye

after the Tracker had been adjusted so as to have the same

first and fourth image reflections as the average subject's

eye. Filtering and sampling rate were the same as those

used in the experiment. Noise level, expressed as a

standard deviation of position samples, was 0.4 min arc

for horizontal and 0.7 min arc for vertical position.

Recordings were made with the Tracker's automati-

cally movable optical stage (auto-stage) and focus servo

disabled. These procedures are necessary with Generation

IV trackers because motion of either the auto-stage or the

focus-servo introduces large artifactual deviations of

Tracker output. The focus-servo was used, as needed,

only during intertrial intervals to maintain subject

alignment. This can be done without introducing artifacts

into the recordings or changing eye position/voltage

analog calibration. The auto-stage was permanently

disabled because its operation, even during inter-trial

intervals, changed the eye position/voltage analog

calibration.

Analysis of eye movement data

The onset and offset of saccades were detected by

means of a computer algorithm employing an accelera-tion criterion. The cr iterion was determined by examining

a large sample of analog records of eye position. Saccades,

as small as the microsaccades that may be observed during

maintained fixation (Steinman, Haddad, Skavenski &

Wyman, 1973), could be reliably detected by this

algorithm. Saccade size was defined as the difference

between eye position at saccade onset and saccade offset.

Experiment la,b: Drawing Attention to An Eccentric

Target

In this experiment the locus of attention was controlled

by presenting an eccentric stimulus that summoned

attention. The s timulus was a single numera l presented in

a display o f letters. The logic was tha t if saccades require

shifts of attention to the saccadic goal, then latency

should be longer for saccades made opposite to the

numeral than for saccades made to the numeral because

additional time would be needed to shift attention from

the numeral to the saccadic goal. If, on the other hand,

saccadic endpoints are determined by a separate selective

system, unrelated to perceptual attention, then the

numeral would simply act to provide information about

where the saccade should be directed and no special

advantage should accrue to saccades made in or opposite

to its location.

On the face of things, the "opposite" condition seems

similar to Hallet (1978) "anti-saccade" task. But there

was an important difference. A visible stimulus (a letter)

was presented at the location opposite the numeral, so

that, unlike the typical anti-saccade task, saccades never

had to be directed into a blank region of the visual display.

The addition of a visible stimulus is important because

difficulties in making anti-saccades, reported in the past,

could have stemmed from the need to choose a saccadic

endpoint within the blank region of the display, rather

than from any involvement of attention.

We used a single numeral located among letters to

summon attention, rather than the eccentric boxes or lines

favored by other investigators, because we wanted the

stimulus summoning attention to be equivalent on a

sensory level to the other characters in the array. This

would allow us to be sure that any effect of the numeral

on saccades would be attributable to attention, rather

than to sensory differences among the target locations in

the display. Such differences would be troublesome

because they might affect saccades independently of

any at tentional involvement (see Palmer, 1994, for a

discussion of an analogous issue in research on perceptual

attention). A brief psychophysical experiment, described

in the Methods section, will confirm the assumption

that the numeral was indeed effective in summoning

attention.



T H E R O L E O F A T T E N T I O N I N T H E P R O G R A M M I N G O F S A C C A D E S 1901

i ii " " i

o " | " ~ . . I

z ! ' : ' i " !

i ii !. . . . - i i! !°o°..

i ' " " " : " : 3 "L.-' ( ::

] . . . . .~|I

F I G U R E 1. A n e x a m p l e o f t h e c h ar a c t e r a rr a y u s e d in E x p e r i m e n t 1

c o n t a i n i n g 7 l e t te r s a n d a s i n g l e n u m e r a l .

Eliminated tr ials

T r i a ls c o n t a i n i n g e p i s o d e s i n w h i c h t h e e y e t r a c k e r l o s t

" l o c k " ( 4 % f o r M C ) w e r e el i m i n a t e d . W e a l so e li m i n a t e d

t h o s e t r i a l s i n w h i c h t h e s u b j e c t d i d n o t a p p e a r t o h a v e

m a d e a g e n u i n e a t t e m p t t o l o o k a t t h e t a r g e t . T h e s e

i n c l u d e d t r ia l s w i th n o d e t e c t e d s a cc a d e s ( 6 % f o r M C ) ,

t r i a ls w i t h la t e n c i e s < 1 00 m s e c ( 0 . 6 % f o r M C ) a n d t r i a l si n w h i c h t h e e r r o r o f th e f i r st s a c c a d e w a s > 5 0 ' ( i. e .

> 4 0 % o f t h e d i s t a n c e t o th e t a r g e t) ( 6 % f o r E K a n d 5 %

f o r M C ) . F i n a l l y , w e e l i m i n a t e d t r i a l s i n w h i c h t h e

n u m e r a l w a s n o t i d e n ti f ie d c o r r e c t l y ( 4 % f o r E K a n d 1 7 %

f o r M C ) b e c a u s e i n s u c h tr i al s t h e s u b j ec t c o u l d n o t h a v e

b e e n a b l e t o f o l lo w t h e i n s t r u c t io n s t o l o o k a t o r o p p o s i t e

t o t h e n u m e r a l .

T h e r e s u l t s r e p o r t e d w e r e b a s e d o n t h e r e m a i n i n g 2 1 7

t r i a l s f o r E K a n d 3 2 3 t r i a l s f o r M C .

E x p e r i m e n t 1 A w i l l r e p o r t t h e e f f ec t o f t h e n u m e r a l o ns a c c a d e s . T h e n , f o l l o w i n g t h e p r e s e n t a t i o n a n d b r i e f

d i s c u s s i o n o f t h e s e r e s u lt s , E x p e r i m e n t 1 B w i l l b e

d e s c r i b e d . E x p e r i m e n t 1 B i n c lu d e s a p e r c e p t u a l t a s k t o b e

p e r f o r m e d c o n c u r r e n t l y w i t h t h e s a c c a d i c t a s k .

E x p er i m en t l a. " M e t h o d s

Tr&ls wi th more than one saccade

T h e r e s u l ts d e s c r i b e d a r e b a s e d o n t h e f i rs t s a c ca d e o f

t h e tr i al . T h e m a j o r i t y o f tr i al s ( 7 8 % f o r E K a n d 8 8 % f o r

M C ) c o n t a i n e d o n l y a s in g le s a c c ad e . F o r t h o s e t r i a ls w i t h

2 o r m o r e s a c ca d e s, a l a rg e r p r o p o r t i o n ( 6 3 % f o r E K a n d

7 1 % f o r M C ) o c c u r r e d u n d e r i n s tr u c t io n s t o l o o k

o p p o s i t e t h e t a r g e t.

S t imulus

S t i m u l i w e r e a r r a y s o f 8 c h a r a c t e r s ( 2 1 ' w i d e x 3 0 '

h i g h ) , lo c a t e d a t e q u a l i n t e r v a ls a l o n g t h e p e r i m e t e r o f a n

i m a g i n a r y c i r c le w i t h r a d i u s o f 2 d e g . In t h e c e n t e r w a s a

sma l l (5 ' ) f i xa t i on c rossha i i r .T h r e e f r a m e s w e r e p r e s e n t e d o n e a c h t r i al . F r a m e 1 w a s

th e p 're-mask ( d u r a t i o n 5 0 0 m s e c ) a n d c o n s i s t e d o f 8

l e t t e r s . F r a m e 2 w a s t h e cr i t ica l f rame ( 2 0 0 m s e c )

c o n t a i n i n g 7 le t te r s a n d a s i n g le n u m e r a l . A n e x a m p l e o f

t h i s a r r a y i s s h o w n i n F i g . 1 . F r a m e 3 ( 5 0 0 m s e c ) w a s t h e

p o s t - m a s k a n d c o n t a i n e d a l l le t te r s .

T h e i d e n t i t y o f t h e l e tt e rs , t h e i d e n t i t y o f t h e n u m e r a l

( 0 - 9 ) , a n d t h e l o c a t i o n o f t h e n u m e r a l w e r e s e le c t ed

r a n d o m l y a n d i n d e p e n d e n tl y , w i t h o u t t h e k n o w l e d g e o f

t h e s u b j e c t .

T h e l e t t e r s B , I , G , O , Q ,. S a n d Z w e r e n o t i n c l u d e d i n

t h e c r i ti c a l f r a m e b e c a u s e o f t h e i r s t r o n g r e s e m b l a n c e t othe numera l s 3 , 1 , 6 , 0 , 0 , 8 and 2 .

Procedure

T h e s u b j e c t l o o k e d a t t h e c r o s s h a i r a n d s t a r t e d a t r i a l

b y p r e s s i n g a b u t t o n w h e n r e a d y . 1 0 0 m s e c l a t e r t h e

s e q u e n c e o f 3 f r a m e s w a s s h o w n . T h e s u b j e c t w a s

i n s t r u c t e d t o m a k e a s i n g le s a c c a d e e i t h e r in t h e d i r e c t i o n

o f th e n u m e r a l o r t o t h e c h a r a c t e r o p p o s i t e t o t h e

n u m e r a l . T h e s u b j e c t s h a d t o id e n t i f y t h e n u m e r a l , b y

m e a n s o f a b u t t o n p r e s s, a t t h e e n d o f e a c h t r ia l .

E x p er i m en t a l s e s s io n s

E K w a s r u n i n 4 a n d M C i n 8 s e s s i o n s c o n t a i n i n g 6 0

t r i a ls e a c h . I n h a l f t h e s e s s io n s , t h e i n s t r u c t i o n s w e r e t o

l o o k i n a n d i n t h e o t h e r h a l f t o l o o k o p p o s i t e t h e n u m e r a l .

I n a n d o p p o s i t e s e s s i o n s w e r e a l t e r n a t e d .

Ver i f y ing the e ff ec tiveness o f the numera l in summ onin g

a t t en t ion

W e d i d a p s y c h o p h y s i c a l e x p e r i m e n t , b e f o r e t h e

s a c c a d i c e x p e r i m e n t s w e r e p e r f o r m e d , t o t e s t o u r

a s s u m p t i o n t h a t t h e n u m e r a l c a p t u r e d a t t e n t i o n . W ea d d e d a t e s t r a m e ( 1 0 0 m s e c d u r a t i o n ) a f t e r F r a m e 2 . T h i s

t e s t f r a m e - - F r a m e 3 i n t h e s e q u e n c e - - - c o n t a i n e d 7 l e tt e rs

a n d a s i ng l e n u m e r a l e i t h e r in t h e s a m e o r i n th e o p p o s i t e

l o c a t i o n a s t h e n u m e r a l i n F r a m e 2 , w h i c h w a s a s s u m e d

t o b e s u m m o n i n g a t te n t io n . T h e i d e n t it y a n d l o c a t i o n o f

t h e t es t n u m e r a l w e r e c h o s e n r a n d o m l y . I f t h e n u m e r a l i n

F r a m e 2 r e a ll y d i d c a p t u r e a t t e n t i o n , t h e n t h e t e s t

n u m e r a l i n F r a m e 3 s h o u l d b e i d e n ti f ie d m o r e a c c u r a t e l y

w h e n i t w a s i n th e s a m e l o c a t i o n a s t h e n u m e r a l i n F r a m e

2 t h a n w h e n i t w a s i n t h e o p p o s i t e l o c a t i o n . W e t e s te d E K

i n 2 0 0 a n d M C i n 5 0 0 t r i a l s . F o r t r i a l s i n w h i c h t h e

n u m e r a l i n F r a m e 2 w a s i d e n ti f ie d a c c u r a t e l y ( 8 4 % f o rE K a n d 7 4 % f o r M C ) , t h e a c c u r a c y o f id e n t if y i n g t h e te s t

n u m e r a l i n F r a m e 3 d id d e p e n d o n l o c a ti o n . A n u m e r a l

i n t h e s a m e l o c a t i o n a s t h e n u m e r a l i n F r a m e 2 w a s

i d en t if ie d m o r e a c c u r a t e l y ( 9 2 % f o r E K a n d 9 0 % f o r M C )

t h a n a n u m e r a l i n t h e o p p o s i t e l o c a t i o n ( 6 6 % f o r E K ;

5 9 % f o r M C ) . T h i s o u t c o m e v e ri fi es o u r a s s u m p t i o n t h a t

t h e n u m e r a l i n F r a m e 2 c a p t u r e d p e r c e p t u a l a t t e n ti o n a n d

p a v e s t h e w a y f o r d o i n g t h e s a c c a d i c e x p e r i m e n t , w h o s e

r e s u lt s a r e d e s c r i b e d b e l o w .

Exp erim en t l a." Re sul ts

I t w a s e a s i e r t o l o o k i n t h e d i r e c t i o n o f t h e n u m e r a l

s u m m o n i n g a t t e n t i o n t h a n t o l o o k o p p o s i t e t o t h e

n u m e r a l . V e r y l a r g e s a c c a d ic e r r o r s , w h i c h w e d e f i n e d a s

a d i r e c t i o n a l e r r o r o f > 6 7 d e g ( i. e. 1 .5 t im e s t h e

d i r e c t i o n a l s e p a r a t i o n o f a d j a c e n t t a r g e t s ) w e r e m o r e



1 90 2 E I L E E N K O W L E R e t a l .

f r e q u e n t i n opp os i t e t r i a l s [ F ig . 2 (c )] . O f t he r e m a i n i n g

t r i a ls , in w h i c h d i r e c t i ona l e r r o r s w e r e < 67 de g , s a c c a de s

m a d e t o t h e n u m e r a l h a d s h o r t e r l a t e n c ie s [F i g . 2 (a )] a n d

s ma l l e r d i r e c t i ona l e r r o r s [ F i g . 2 ( b ) ] t ha n s a c c a de s ma de

o p p o s i t e t o t h e n u m e r a l . D i r e c t i o n a l e r r o r w a s u s e d a s th e

i n d e x o f s a c c a d i c a c c u r a c y b e c a u s e s a c c a d e v e c t o r

m a g n i t u d e , u n l i k e d i r e c t i o n , w a s a b o u t t h e s a m e f o r

s a c c a d e s m a d e i n a n d o p p o s i t e t o t h e n u m e r a l ( m e a n

v e c t o r m a g n i t u d e = 10 4 ' f o r E K a n d 1 18 ' f o r M C ) .

F i g u r e 2 a l so s h o w s i n d i v i d u a l d i f fe r e n ce s : M C ' s s a c c a d i c

l a te n c i e s w e r e l o n g e r t h a n E K ' s a n d h i s la t e n c y d i f fe r e n c e s

b e t w e e n i n a n d o p p o s i t e s a c c a d e s w e r e s m a l l e r . M C ' s

l a t e n c y d i f f e r e n c e , a l t h o u g h s m a l l e r t h a n E K ' s , w e r e

s ta t i s t i ca l ly re l i able (Z = 2 .76; P < 0 .003) .

Experiment l a: Discussion

S a c c a d e s m a d e t o a s t i m u l u s t h a t c a p t u r e s a t t e n t i o n

h a v e s h o r t e r la t e n c i es a n d b e t t e r a c c u r a c y t h a n s a c c a d e s

m a d e t o a t a r g e t 1 8 0 d e g a w a y . T h e g r e a t e r d i f f i c u l t ye n c o u n t e r e d w h e n l o o k i n g o p p o s i t e to t h e n u m e r a l m i g h t

r e fl e c t t h e c o s t o f h a v i n g t o s h i f t a t t e n t i o n a w a y f r o m t h e

n u m e r a l t o t h e s a c c a d i c g o a l . I f t h e s e d i ff e r e n ce s w e r e

r e a l ly d u e t o t h e a t t e n t i o n a l s h i f t, w e w o u l d e x p e c t t o f i n d

s o m e s i g n o f t h i s a t t e n t i o n a l s h i f t i n p e r c e p t u a l

p e r f o r m a n c e .

T o t e s t t h i s id e a , w e r e p e a t e d t h e e x p e r i m e n t w i t h t h e

a d d i t i o n o f a c o n c u r r e n t p e r c e p t u a l t a s k . S u b j e c ts h a d t o

i d e n t i f y t h e l e tt e r lo c a t e d o p p o s i t e t o t h e n u m e r a l . W e

c hos e l e t t e r i de n t i f i c a t i on be c a us e i t i s s e ns i t i ve t o t he

a l l o c a t i o n o f a t t e n t i o n a l r e s o u r c e s, i .e . w h e n a t t e n t i o n i s

c o n t r o l l e d b y m e a n s o f v i s u a l o r v e r b a l c u e s , p e r f o r m a n c e

i s b e t t e r a t a t t e n d e d t h a n a t u n a t t e n d e d l o c a t i o n s

( S pe r l i ng & M e l c hn e r , 1978; S ha w , 1984; K r os e & J u l e s z ,

1989; R e e ve s & S pe r l i ng , 1986). I f s h i f t s o f pe r c e p t u a l

a t t e n t i o n p r e c e d e s a c c a d e s , t h e n i d e n t i f i c a t i o n o f t h e

l e t t e r s h o u l d b e b e t t e r w h e n s a c c a d e s w e r e d i r e c t e d

o p p o s i t e t o t h e n u m e r a l , a n d i n th e d i r e c t i o n o f t h e l et t er ,

t h a n w h e n s a c c a d e s w e r e d i r e c t e d t o t h e n u m e r a l . I f

p e r c e p t u a l a t t e n t i o n a n d s a c c a d e s c a n b e d i s s o c i a t e d ,

i d e n t i f i c a t i o n s h o u l d b e e q u a l l y a c c u r a t e r e g a r d l e s s o f

s a c c a d i c d i r e c t i on .

5O 0

45 0

> " 2 5 0( 3

IO 0

0

Ag ,1 5

5~ 5z

a

Po

LIJ

E K M C

(a )

5 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 5i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i ii i i. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .~ ' / " . . . . . . . . .. . . . . . . . . . . . . . . . . . .. . . . ~

. . . . . . . .. . . . . . . 5 S ~ . ~ . . ; ; ; . . . . . . . . . .. . . . . . . . .. . . . . . . . .. . . . r / / . . . . .

............... i l i A . . . . I [ / . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., - i , , ¢ // / / ~

. . . . . . . . . . . . . . . / i l . . . . , ' i / . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ., - / i

. . . . . . . . . . . . . . . / iF - - - / / i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .r/ /// /

. . . . . . . . . . . . . . . / /~ - . . , / , , ' t ................................ I ¢ /*a . . . .

. . . . . . . .. . . . . . . / / i- . . . t i / . . . . . . . . . .. . . . . . . . . .. . . . . . . . . .. . # ~ . .. .e - / t

. . . . . . . . . . . . . . . " . . ' .~ . . . . . . . . . . . . . . . . . Y ~ v z

(b)

. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . } " . . ' ( ' , / ~* " " Z " "~ v . A

0

0 . 2 f ( c )

o . 1 5 1 . . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . .. . . . . . .. . . . . ..

i n o p p i n o p p

F I G U R E 2 . S ac c a d i c l a t e n c y ( a) , a n g u l a r e r ro r ( b ), a n d p r o p o r t i o n o f

t r i a l s w i t h a n a n g u l a r e r r o r o f > 6 7 d e g (c ) a s a f u n c t i o n o f t h e d i r e c t i o n

o f t h e s a c c a d e r e l a ti v e t o t h e l o c a t i o n o f t h e n u m e r a l . S a c c a d e s w e r e

m a d e e i th e r I N t h e d i r e c t io n o f t h e n u m e r a l o r 1 80 d e g O P P O S I T E t h e

n u m e r a l . E a c h b a r i s b a s e d o n a p p r o x . 2 0 0 t ri a l s f o r E K a n d 1 50 tr i a ls

f o r M C . S t a n d a r d e r r o rs ( S E ) a r e s h o w n b y t h e v e rt i c al l i ne s . T h e

c i r c u la r s y m b o l s w i t h o u t e r r o r b a r s s i g n if y t h a t t h e S E s w e r e s m a l l e r

t h a n t h e s e s y m b o l s .

Experiment lb: Methods

Stimuli and procedure

S t i m u l i w e r e t h e s a m e a s i n E x p e r i m e n t 1 A e x ce p t t h a t

t h e d u r a t i o n o f F r a m e 2 , c o n t a i n i n g t h e n u m e r a l , w a s

s e le c t ed a t r a n d o m t o b e e i t h e r 2 00 , 3 0 0 o r 4 0 0 m s e c . T h e

a d d i t i o n a l , l o n g e r d u r a t i o n s w e r e i n c l u d e d b e c a u s e w e

w a n t e d t o b e s u r e o f a l l o w i n g t h e s u b j e c t e n o u g h t i m e t o

c o m p l e t e a l l a s p e c t s o f th e t a s k b e f o r e t h e f r a m e w a s

r e m o v e d , i . e . l o c a t e t h e n u m e r a l , p l a n t h e s a c c a d e a n d

i d e n t i f y t h e l e t te r . S u b j e c t s w e re a s k e d t o m a k e t h e

s a c c a d e a s s o o n a s th e y h a d d e t e r m i n e d t h e l o c a t i o n o f t h e

n u m e r a l , a n d n o t t o w a i t u n t i l t h e e n d o f t h e f r am e .

S ub j e c t s w e r e r e q u i r e d t o r e po r t t he l e t t e r l oc a t e d

o p p o s i t e t o t h e n u m e r a l o n e a c h t r ia l . T h e l e t te r o p p o s i t e

t o t h e n u m e r a l w a s r a n d o m l y s e le c te d f r o m a s e t o f 1 0

( J, K , L , M , N , T , U , V , W o r X ) a n d , o n c e a g a i n , le t te r s

c l o se l y r e s e m b l i n g n u m e r a l s ( B , I, G , O , Q , S a n d Z ) w e r e

n o t i n c l u d e d i n t h e c r i t i c a l s e c o n d f r a m e . F e e d b a c k w a s

g i v e n a f te r e a c h t r i a l b y d i s p l a y i n g t h e f r a m e c o n t a i n i n g

t h e n u m e r a l a n d t h e c r i ti c a l le t te r .

F o u r 6 0 - t r ia l s e s si o n s f o r E K a n d t w o f o r M C w e r e r u n

u n d e r e a c h o f t h e i n s tr u c t i o n s , n a m e l y , t o l o o k i n a n d t o

l o o k o p p o s i t e t h e d i r e c t i o n o f th e n u m e r a l . I n a t h i r d

c o n d i t i o n ( n o s a c c a d e ) (4 se s s io n s f o r E K a n d 2 f o r M C ) ,

t h e s u b j e c t s w e r e a s k e d t o i d e n t i f y t h e l e t t e r w h i le k e e p i n g

t h e l i n e o f s i g h t s t a t i o n a r y a n d n o t m a k i n g s a c c a d e s

d u r i n g t h e t r i a l. T h e 3 t y p e s o f s e ss i o n s (i n , o p p o s i t e a n d

n o s a c c a d e ) w e r e t e s t e d o n c e e a c h d a y , w i t h d i f f e r e n t

o r d e r s u s e d o n d i f f e r e n t d a y s .

Eliminated trials

T r i a l s w e r e e l i m i n a t e d a s f o l l o w s : lo s s o f e y e t r a c k e r

" l o c k " (9 % f o r E K a n d < 1 % f o r M C ) , e r r o r o f t h e f ir st

s a c ca d e g re a t e r t h a n 5 0 ' (6 % f o r E K a n d 8 % f o r M C ) o r

a n g u l a r e r ro r g r e a te r t h a n 6 7 d e g ( 5 % f o r E K a n d 2 % f o r

M C ) a n d n u m e r a l n o t i d e n ti fi e d c o r r e ct ly ( 2 % f o r E K a n d

6 % f o r M C ) .



T H E R O L E O F A T T E N T I O N I N T H E P R O G R A M M I N G O F S A C C A D E S 19 03

The results reported were based on the remaining 217

trials for EK and 323 trials for MC.

Experiment lb: Results

Saccadic performance, shown in Fig. 3, can be

compared to that obtained in the prior experiment (1 A)

with the same durat ion (200 msec) of Frame 2. Only MC's

performance changed. Both the mean latency and mean

angular error of his saccades in the direction of the

numeral increased relative to the values observed in

Experiment 1A, and differences between latencies and

angular errors of saccade,; in and opposite to the numeral

were abolished. Figure 3 also shows that EK's saccades

in the direction of the numeral had shorter latencies and

smaller angular errors than did the saccades made

opposite to the numeral, 'with performance not changing

with increasing frame durat ion. MC's latencies increased

with duration.

Analysis of variance confirmed these trends, with

MC showing significant effects of duration on both

latency [F(2,193)= 38, P > 0.001] and angular error

[F(2,193) = 3.02, P< 0.05] but no significant effect

of instruction (i .e. in vs opposite). EK, on the other

hand, showed a significant effect of the instruction on

both latency [F(1,361)= 92, P < 0.001] and angular

error [F(1,361) = 14, P < 0.001] but no significant effect

of duration. None of the interactions (duration x

instruction) reached significance.

Figure 4 shows the perceptual performance. It is

based only on those trials in which the saccade did not

occur until after the critical Frame 2 was replaced by

the mask (81% of the trials for EK and 99% for

MC). EK identified the letter accurately only under the

opposite instruction, when she was preparing to look at

the letter (opposite to the attention-catching numeral).

Similar results were obtained when she shifted attention

to the letter while the eye was stationary. MC's

performance was completely different. He identified the

letters equally well regardless of where he looked or

whether he made any saccades at all. This pattern of

extreme individual differences has implications both

for interpreting the prior literature and for planning

new experiments to discover the role of attention in

saccades.

60 3

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F I G U R E 3 . S a c c a d i c l a t e n c y ( a a n d b ) a n d m e a n a n g u l a r e r r o r (c a n d d ), a n d p r o p o r t i o n o f t r ia l s w i t h a n a n g u l a r e r r o r o f > 6 7

d e g r e e s fo r s a c c a d e s m a d e IN t h e d i r e c t i o n o f t h e n u m e r a l ( • ) a n d OPPOSITE o t h e n u m e r a l ( O ) a s a f u n c t i o n o f t h e d u r a t i o n

o f F r a m e 2 . E a c h d a t u m p o i n t i s b a s e d o n a p p r o x . 6 0 t r ia l s fo r E K a n d 3 0 t r ia l s fo r M C . E r r o r b a r s r e p r e s e n t 1 S E .



1 9 0 4 E I L E E N K O W L E R et al.

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C U E D U R A T I O N (mse¢)

FIGU RE 4. Pro portion of letters identified correctlywhen making a saccade IN the direction of the numeral (0) , OPPOSITEto the nu meral (O), or wh en not m aking saccades at all ([--1), as a function of cue duration. Ea ch datum p oint was bas ed on

approx. 60 trials for EK and 30 trials for MC.

E x p e r i m e n t l b : D i s c u s s i o n

T h e l o g ic b e h i n d t h i s e x p e r i m e n t w a s t h e s a m e a s t h a t

e m p l o y e d i n p r i o r w o r k o n s a c ca d e s a n d a t t e n t i o n

s u m m a r i z e d i n th e I n t r o d u c t i o n , n a m e l y : F i n d i n g s u p e r i o r

p e r c e p t u a l p e r f o r m a n c e f o r t a r g e t s a t th e g o a l o f th e

s a c c a d e s u p p o r t s t h e i n v o l v e m e n t o f a t t e n t i o n a l s h i ft s in

s a c c a d i c p r o g r a m m i n g . E q u i v a l e n t p e r f o r m a n c e f o r

t a r g e t s a t g o a l a n d n o n - g o a l l o c a t i o n s w o u l d s u g g e s t

i n d e p e n d e n c e o f sa c c a d e s a n d a t t e n t i o n . T h e p r i o r

e x p e r i m e n t s ( e . g . P o s n e r , 1 9 8 0 ; K l e i n , 1 9 8 0 ; K l e i n e t a l . ,

1 9 9 2 ; R e m i n g t o n , 1 9 8 0 ; S h e p h e r d e t a l . , 1986;

S u b r a m a n i a m & H o f f m a n , 1 9 9 2 ) o b t a i n e d c o n f l i c t i n g

r e s u l t s , s o m e s u p p o r t i n g a l i n k b e t w e e n s a c c a d e s a n d

a t t e n t i o n a n d o t h e r s s u p p o r t i n g i n d e p e n d e n c e . W e

o b t a i n e d t h e s a m e p a t t e r n o f c o n f li c t in g re s u l ts w i t h i n a

s in g le e x p e r i m e n t . M C ' s p a t t e r n o f p e r f o r m a n c e w a s l ik e

t h a t i n p r i o r s t u d i e s in w h i c h i n d e p e n d e n c e o f s a c c a d e s

a n d a t t e n t i o n w a s o b s e r v e d , w h i l e E K ' s p a t t e r n o f

p e r f o r m a n c e w a s l i k e t h a t i n p r i o r s t u d i e s i n w h i c h

e v i d e n c e f o r a t t e n t i o n s h i f t s p r e c e d i n g s a c c a d e s w a s

f o u n d .

I n d i v i d u a l d i f f e r e n ce s as l a r g e a s t h o s e w e o b s e r v e d a r e

u s u a l l y r e g a r d e d a s f a l li n g s o m e w h e r e b e t w e e n a n u i s a n c e

a n d a d i s a st e r, b u t i n th i s c as e t h e o u t c o m e p r o v e d t o b e

f o r t u n a t e b e c a u s e i t f o r c e d u s t o d e v i s e n e w a n d b e t t e r

m e t h o d s . T h e b e s t g ue s s a b o u t t h e s o u r c e o f t h e i n d i v id u a l

d i f fe r e n c es is t h a t s h i ft s o f a t t e n t i o n d o p r e c e d e s a c c a d e s ,

b u t t h e s u b j e c ts u s e d d i f f e re n t s tr a te g i es : E K k e p t l a t e n c y

s h o r t a t t h e e x p e n s e o f l e t t e r i d e n t i f i c a t i o n w h i l e M C

i n c r e a s e d l a t e n c y i n o r d e r t o i d e n t i f y t h e l e t t e r c o r r e c t l y .

I f t h is i n t e r p r e t a t i o n i s c o r r e c t , t h e n t h e k e y t o d e v e l o p i n g

a m e t h o d t o s t u d y t h e r e l a t i o n s h i p b e t w e e n s a c c a d es a n d

a t t e n t i o n w i l l b e t o e n c o u r a g e e a c h s u b j e c t t o a d o p t e a c h

o f t h e t w o s t ra t e g ie s . T h i s w i ll a ll o w d e t e r m i n a t i o n o f t h e

i n c r e a s e i n s a c c a d i c l a t e n c y t h a t i s r e q u i r e d i n o r d e r t o

i d e n t i f y t a r g e t l e t t e r s c o r r e c t l y a t n o n - g o a l l o c a t i o n s .

F i n d i n g a n y i n c r e a s e a t a l l i n d ic a t e s a r o l e f o r p e r c e p t u a l

a t t e n ti o n i n s a c ca d i c p r o g r a m m i n g . T h e m a g n i t u d e o f th e

i n c r e as e i s a n i n d e x o f t h e e x t e n t t o w h i c h s a c c a d e s r e q u i r e

a t t e n t i o n a l r e s o u r c es .

T h e r e m a i n i n g e x p e r i m e n t s r e q u i r e d s u b j e c t s t o a d o p t

d i f f e r e n t s tr a t e g i e s . W e b e g a n b y s i m p l i f y i n g t h e s t i m u l u sa n d e x p a n d i n g t h e i n s t r u c t i o n s i n w a y s t h a t s e e m e d l i k e ly

t o b e a b l e t o p l a c e s t ra t e g ie s u n d e r e x p e r i m e n t a l c o n t r o l .

I n d o i n g s o , w e h o p e d t o d e t e r m i n e c o n c l u s iv e l y w h e t h e r

s h i ft s o f a t t e n t i o n d o o r d o n o t p r e c e d e s a c c a d e s a n d ,

i n a d d i t io n , a s s e s s th e a t t e n t i o n a l d e m a n d s o f t h e

s a c ca d e s b y c o m p a r i n g p e r f o r m a n c e u n d e r i n s t r u ct i o n s t o

e m p h a s i z e e i t h e r t h e s a c c a d i c o r t h e p e r c e p t u a l t a s k s .

E x p e r i m e n t 2 : C e n t r a l C u e s

O n c e a g a i n , c o n c u r r e n t s a c c a d i c a n d p e r c e p t u a l t a s k s

w e r e p e r f o r m e d . T h e m a i n n e w f e a t u r e i s t h a t a s i m p l e

c e n t r a l c u e - - a s in g le p o i n t e r d i r e c t e d t o o n e o f t h e

l e t t e r s - - w a s u s e d t o d e s i g n a t e t h e s a c c a d ic g o a l in s t e a d o f

a n e c c e n t r ic n u m e r a l . I d e n t i f y i n g th e o r i e n t a t i o n o f th i s

p o i n t e r i s e a s i er a n d f a s t e r t h a n t h e n u m e r a l s e a r c h t a s k

u s e d i n E x p e r i m e n t 1 . B y s w i t c h i n g t o t h i s e a s i e r a n d

f a s t e r ta s k , t h e r e w o u l d b e l es s o p p o r t u n i t y f o r s u b j e c ts

t o s n e a k a t t e n t i o n a l g l im p s e s o f t h e e c c e n t r ic t ar g e t s , a

t r ic k t h a t c o u l d i m p r o v e p e r c e p t u a l p e r f o r m a n c e w i t h o u t

c o s t t o s a c c a d e s .

P e r f o r m a n c e w a s c o m p a r e d u n d e r t w o d i f f e r e n t

c o n d i t i o n s . I n t h e R a n d o m r e p o r t c o n d i t i o n , t h e s u b j e c t

h a d t o m a k e t h e s a c c a d e a s s o o n a s p o s s i b le f o l l o w i n g th e

a p p e a r a n c e o f t h e c e n t r a l p o i n t e r a n d , a f t e r t h e t ri a l,

i d e n t i fy a l e tt e r c h o s e n a t r a n d o m f r o m t h e 8 le t te r s in t h e

d i s p l a y . I f s h i f t s o f a t t e n t i o n p r e c e d e s a c c a d e s , t h e n

i d e n t i f i c a t i o n s h o u l d b e b e t t e r f o r l e t t e r s l o c a t e d a t t h e

s a c c a d i c g o a l t h a n f o r l e t te r s l o c a t e d e l s e w h e r e . O f c o u r s e ,



T H E R O L E O F A T T E N T I O N I N T H E P R O G R A M M I N G O F S A C C A D E S 1 90 5

s u b j e c t s m a y s i m p l y d e c i d e t o s h i f t a t t e n t i o n t o t h e

s a c c a d i c g o a l e v e n if t h e y d o n o t h a v e t o . T o t e s t t h is

p o s s i b i l i t y , a Fixed report c o n d i t i o n w a s i n c l u d e d i n a n

a t t e m p t t o f o r c e a d i s s o c ia t i o n b e t w e e n s a c c a d e s a n d

a t t e n t i o n . I n t h e Fixed report c o n d i t i o n , s u b j e c t s w e r e

a s k e d t o t r y t o i d e n t i f y a. l e t te r a t t h e s a m e l o c a t i o n o n

e a c h t r i a l, s a c r i f i c in g s a c c a d i c l a t e n c y i f n e c e s s a r y , but

only as much as necessary, t o i d e n t i f y t h e l e t t e r . A n y

i n c r e a s e i n s a c c a d i c l a t e n c y i n t h e Fixed report c o n d i t i o n

r e l a t i v e t o t h a t i n t h e Random report c o n d i t i o n w o u l d

r e f l e ct t h e c o s t o f ha vi nl ~; t o m o v e p e r c e p t u a l a t t e n t i o n

f r o m t h e p e r c e p t u a l t a r g e t l o c a t i o n t o t h e g o a l o f t h e

s a c c a d e o r , a l t e r n a t iv e l y , t h e c o s t o f h a v i n g t o a t t e n d t o

t w o l o c a t i o n s ( s a c c a d i c a n d p e r c e p t u a l t a r g e t s ) c o n c u r -

r e n t ly . O n t h e o t h e r h a n d , i f a c c u r a t e p e r c e p t u a l r e p o r t s

i n t h e Fixed report c o n d i t i o n w e r e o b t a i n e d w i t h n o

i n c r e a s e in s a c c a d i c l a t e n c y , w e w o u l d h a v e e v i d e n c e t h a t

p e r c e p t u al a t t e n t i o n w a s not r e q u i r ed i n o r d e r t o p r o g r a m

s a c c ad e s , a n d t h a t a s u b j e c t c o u l d p r o g r a m s a c c a d e s to

o n e l o c a t i o n w h i l e p a y i n g a t t e n t i o n t o a s t i m u l u s l o c a t e d

e l s e w h e r e .

Experiment 2: Method

Stimulus

T h r e e f r a m e s w e r e p r e s e n t e d , a l l c o n t a i n i n g 8 l et te r s .

F r a m e s 1 a n d 3 w e r e m a s k s ( d u r a t i o n = 5 0 0 r e s e t ) . T h e

c r it i ca l f r a m e w a s F r a m e 2 ( 2 0 0 m s e c ) , w h i c h c o n t a i n e d

a 3 0 ' c e n t r a l l y - l o c a t e d p o i n t e r d i r e c t e d t o t h e l e t t er t h a t

w a s t o b e t h e t a r g e t o f t h e s a c c a d e ( s e e F i g . 5 ) . O n e o f t h e

l e tt e r s i n F r a m e 2 ( s e le c t e d a t r a n d o m ) w o u l d h a v e t o b e

r e p o r t e d a t t h e e n d o f th e t r ia l . T h i s l et t e r w a s c h o s e n f r o m

t h e s am e s e t ( J - N ; T - X ) u s e d i n E x p e r im e n t l B . T h e o t h e r

d i s p l a y l o c a t i o n s c o u l d c o n t a i n a n y l e tt e r , i n c l u d i n g t h o s e

i n th i s se t. F e e d b a c k w a s g i v e n b y d is p l a y i n g F r a m e 2 f o r

5 0 0 m s e c a f t e r t h e r e s p o n s e w a s g i v e n .

Procedure

I n t h e Random report c o n d i t i o n , s u b j e c t s w e r e

i n s t r u c t e d t o m a k e a s a c c a d e a s q u i c k l y a s p o s s i b le t o t h e

l e t te r d e s i g n a t e d b y t h e c e n t r a l p o i n t e r . A t t h e e n d o f t h e

t r ia l , a l e t te r Q a p p e a r e d a t o n e o f t h e 8 d i s p l a y lo c a t i o n s ,

c h o s e n a t r a n d o m . T h e s u b j e c t h a d t o r e p o r t t h e l e t t e rt h a t h a d a p p e a r e d i n t h a t l o c a t i o n b y m e a n s o f a b u t t o n

p r e s s. I f a t t e n t i o n a l s h i ft s p r e c e d e s a c c a d e s , p e r c e p t u a l

i d e n t i f ic a t i o n s h o u l d b e b e t t e r f o r a l e t t e r lo c a t e d a t t h e

s a c c a d i c g o a l t h a n f o r l e t t er s l o c a t e d e l s e w h e r e .

I n t h e Fixed report c o n d i t i o n , t h e s u b j e c t w a s r e q u i r e d

t o r e p o r t c o r r e c t l y th e l e t t e r i n th e r i g h t m o s t l o c a t i o n o f

t h e d i s p l a y . S a c c a d i c l a t e n c y w a s t o b e s a c r i f i c e d , i f

n e c e s s a r y , b u t only as much as necessary, t o a c h i e v e

p e r f e c t p e r c e p t u a l i d e n t if i c a ti o n .

T w o c o n d i t i o n s w e r e i n c l u d e d i n w h i c h no saccades

w e r e m a d e . T h i s w a s d o n e i n o r d e r t o c o m p a r e t h e

e f f ec t iv e n e s s o f a t t e n t i o n s h i ft s m a d e w h i l e t h e e y e w a ss t a t i o n a r y w i t h t h e e f f ec t iv e n e s s o f a n y a t t e n t i o n s h i ft s

p r e c e d i n g s a c c a d e s . I n t h e Random report~no saccade

c o n d i t i o n , s u b j e c ts w e r e i n s t r u c t e d t o s h i f t a t t e n t i o n t o t h e

l e t te r i n d i c a t e d b y t h e c e n t r a l c u e w h i l e t h e l i ne o f s i g ht

r e m a i n e d f i x a te d a t t h e c e n t e r o f th e d i s p l a y . T h e l e t te r t o

. . - . .

i . . . .

i i :'""," - : r ' " ' i. ' ' | : •

: . . . . . "

i . i . i * 'o . iJ ' i

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i . i....!

F I G U R E 5 . A n e x a m p le o f t h e c h a r a c te r a r r a y u s e d in E x p e r im e n t s 2

a n d 3 . T h e c e n t r a l p o i n t e r ( in t h i s e x a m p l e d i r e c t e d d o w n w a r d ) i n d i c a t e d

t h e t a r g e t o f t h e s a c c a d e .

b e r e p o r t e d w a s s e l ec t e d r a n d o m l y a t t h e e n d o f t h e t ri a l.

I n t h e Fixed report/no saeeade c o n d i t i o n , t h e l e t t e r i n t h e

r i g h t h a n d l o c a t i o n w a s r e p o r t e d w h i l e t h e l in e o f s ig h t

r e m a i n e d a t t h e c e n t e r o f t h e d i s p la y . L e t t e r i d e n t if i c a ti o n

i n t h e Fixed report~no saeeade c o n d i t i o n w a s p e r f e c t ,

c o n f i r m i n g t h a t t h e l e t te r s c o u l d b e r e s o l v e d c le a r l y a n d

t h a t a n y p e r c e p t u a l e r r o r s o b s e r v e d i n t h e r e m a i n i n g 3

c o n d i t i o n s (Random report, Fixed report a n d Random

report/no saceade) w e r e d u e t o i n a t t e n t i o n , n o t t o p o o r

v i s u a l a c u i t y . R e s u l t s f r o m t h e Fixed report/no saeeade

c o n d i t i o n w i l l , t h e r e f o r e , n o t b e d e s c r i b e d f u r t h e r s i n c e

t h i s e x p e r i m e n t a l c o n d i t i o n h a d s e r v e d i t s i n t e n d e dp u r p o s e .

Random a n d Fixed report c o n d i t i o n s w e r e t e s t e d i n

s e p a r a t e e x p e r i m e n t a l s e s s i o n s ( 1 0 0 t r i a l s e a c h ) . E a c h

s u b j e c t w a s t e s t e d i n 3 r e p l i c a t i o n s o f th e 3 t y p e s o f

se ss ions (Random report, Fixed report a n d Random

report~no saecade), w i t h t h e s e ss i o ns o r d e r e d h a p h a z a r d l y

w i t h i n a r e p l i c a t i o n . B o t h t h e n r a n o n e a d d i t i o n a l Fixed

report s e s s io n t o a l l o w f u r t h e r o b s e r v a t i o n o f p r a c t i c e

e f fec t s .

Eliminated trials

T r i a ls w e r e e l i m i n a t e d a s f o l lo w s : l o s s o f e y e t r a c k e r

l o c k ( 5 % f o r E K a n d < 1 % f o r M C ) , e r r o r o f th e fi rs t

s a c ca d e > 5 0 ' ( 4 % f o r E K a n d 1 1 % f o r M C ) a n d

d i r ec t io n a l e r r o r > 6 7 d e g ( 3 % f o r E K a n d 2 % f o r M C ) .

T h e r e s u l t s r e p o r t e d w e r e b a s e d o n t h e r e m a i n i n g 4 9 8

t r i a l s f o r E K a n d 6 8 3 t r i a l s f o r M C .

Trials with more than one saccade

I n a n a t t e m p t t o k e e p s a c c a d i c l a te n c y a s s h o r t a s

p o s s i bl e , t h e i n s t r u c t io n u s e d i n E x p e r i m e n t 1 t o r e a c h t h e

g o a l w i t h a s i n g l e s a c c a d e w a s r e l a x e d . A s e x p e c t e d , t h i s

n e w i n s t r u c t i o n c a u s e d a n i n c r e a s e i n t h e p r o p o r t i o n o f

t ri al s c o n ta i n i n g m o r e t h a n o n e s a c c a de f o r M C , w h o s e

p r o p o r t i o n r o s e t o 0 . 29 f r o m h i s p r e v i o u s v a l u e s o f 0 .1 2

i n E x p e r i m e n t s 1 A a n d 1 B . T h e p r o p o r t i o n o f tr i al s w i t h

m o r e t h a n 1 s a c c a d e w a s 0 . 2 5 f o r E K , a b o u t t h e s a m e a s

h e r p r e v i o u s v a l u e s o f 0 . 22 ( E x p e r i m e n t 1A ) a n d 0 . 2 7 ( 1 B ).




E KI-- 1 t~

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C W C C WQ U E R I E D L O C A T I O N W / R E S P E C T T O T A R G E T

F I G U R E 6 . P r o p o r t i o n o f l e tt e rs i d e n ti f ie d c o rr e c tl y a s a f u n c t i o n o f

l e t t e r l o c a t i o n w i t h r e s p e c t t o t h e s a c c a d i c t a r g e t . © , P e r f o r m a n c e w h e n

t h e l e t t e r t o b e r e p o r t e d w a s i n a f i xe d ( ri g h t h a n d ) l o c a t i o n t h r o u g h o u t

t h e s e ss i o n ; 0 , w h e n t h e lo c a t i o n w a s s e le c t e d r a n d o m l y . A ,

P e r f o r m a n c e w h e n t h e r e p o r t e d l o c a t io n w a s r a n d o m a n d n o s a c ca d e s

w e r e m a d e . E a c h d a t u m p o i n t r e p r e s e n t s a p p r o x . 3 0 - 4 0 t r i a l s.

Experiment 2." Results

Random report

I n t h e Random report c ond i t i on , l e t t e r s a t t he s a c c a d i c

g o a l w e r e i d e n t i fi e d a c c u r a t e l y i n 7 0 - 9 0 % o f t h e t r ia l s,

w h i l e p e r f o r m a n c e w a s n e a r c h a n c e l e v e l s f o r l e t t e r s

l oc a t e d e l s e w he r e ( F i g . 6 , f i l le d c ir c le s ). S t a t e d d i f f e r e n t l y ,

s u b j ec t s u s u a l ly r e m e m b e r e d o n l y o n e o f th e 8 d i s p l a y e d

l e tt e rs , w i t h t h e p a r t i c u l a r l e t t e r r e m e m b e r e d c o i n c i d i n g

w i t h t h e g o a l o f t h e s a c c ad e . T h i s s u g g e s t s t h a t p e r c e p t u a l

a t t e n t i o n d o e s s h i f t t o t h e i n t e n d e d e n d p o i n t o f t h e

s a c c a de ( s e e R e e ve s & S pe r l i ng , 1986 , w ho s how how

a t t e n t i o n c a n b e m o d e l e d a s t h e " g a t e w a y " t o v i s u a l

m e m o r y ) . P e r f o r m a n c e w a s a l m o s t t h e s a m e w h e n t h e l in e

o f s ig h t r e m a i n e d s t a t i o n a r y a n d t h e s u b j e c t w a s

i n s t r u c t e d t o s h i f t a t t e n t i o n t o t h e l e t te r i n d i c a t e d b y t h e

p o i n t e r ( F i g . 6 , t r i a n g l e s ) . T h i s s h o w s t h a t a t t e n t i o n a l

s h i f t s be f o r e s a c c a de s w e r e n o m or e o r l e s s e f f e c ti ve t ha n

a t t e n t i o n a l s h i f ts m a d e w h i l e t h e e y e w a s s t a t i o n a r y .

Fixed report

I n t h e Fixed report c o n d i t i o n , s u b j e c t s w e r e t o l d t o

i d e n t i f y t h e l e t t er i n t h e r i g h t m o s t l o c a t i o n , e v e n i f a

s a c r i f i c e i n s a c c a d i c p e r f o r m a n c e w a s n e e d e d t oa c c o m p l i s h th i s . W e f o u n d t h a t , i n a c c o r d a n c e w i t h t h e s e

i n s t r u c ti o n s , i d e n t i f i c a t io n o f t h e l e t te r i n t h e r i g h t m o s t

l o c a t i o n w a s v e r y a c c u r a t e ( 7 0 - 1 0 0 % c o r r e c t ) (s ee F i g . 6,

ope n c i rc l e s) . T he i m po r t a n t r e s u l t w a s t h a t t h i s e x c e l l e n t

p e r c e p t u a l p e r f o r m a n c e w a s a c h i e v e d a t t h e c o s t o f a n

i nc r e a s e i n s a c c a d i c l a t e nc y f o r bo t h s ub j e c t s o f 50 - - 75

m s e c , a n d , i n a d d i t i o n , a n i n c r e a s e i n a v e r a g e a n g u l a r

e r r o r ( u p t o 5 d e g ) f o r E K , a s i s s h o w n i n F i g . 7 .

Two additional analyses

(1) Latencies as a unction of location. W a s t h e r e b e t t er

p e r f o r m a n c e i n t h e Fixed report c o n d i t i o n w h e n t h er a n d o m l y - s e l e c t e d sa c c a d i c t a r g e t h a p p e n e d t o f a l l a t t h e

r i g h t m o s t l o c a t i o n , w h e r e a t t e n t i o n h a d b e e n d i r e c t e d a t

t h e s t a r t o f t h e t r ia l ? E K ' s s a c c a d i c la t e n c ie s w e r e s h o r t e s t

40 O

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2 3 , 2 ;S E S S IO N N U M B E R S E S S IO N N U M B E R

F I G U R E 7 . S a cc a d ic l a te n c y ( a a n d b ) a n d a v e r a g e u n s ig n e d a n g u l a r e r r o r a s a f u n c t i o n o f s e ss i o n n u m b e r . E a c h d a t u m p o i n t

i s b a s e d o n a p p r o x . 8 5 t ri a ls . E r r o r b a r s r e p r e s e n t 1 S E .



T H E R O L E O F A T T E N T I O N I N T H E P R O G R A M M I N G O F S A C C A D E S 1907

-=- F I X E D R E P O R T

E K ..e - R A N D K ~ M R E P O R T M e

t ( a ) ( b )

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o . . . . . . o ' ' '- 1 8 0 - 1 3 5 - 6 0 - 4 5 0 4 5 9 0 1 1 8 0 - 1 8 0 - 1 3 5 - 9 0 - 4 5 1 5 1 8 0

C W C C W C W C C W

T A R G E T L O C A T I O N T A R G E T L O C A T I O N

' F I G U R E 8 . S a c ca d i c l a te n c y ( a a n d b ) a n d a v e r a g e u n s i g n e d a n g u la r e r r o r ( c a n d d ) w h e n t h e l e tt e r t o b e r e p o r t e d w a s a t a

f i x e d ( O ) o r r a n d o m ( O ) l o c a t i o n a s a f u n c t i o n o f t h e l o c a t i o n o f t h e sa c c a d i c t a r g e t . Z e r o o n t h e a b s c i s s a r e p r e s e n t s t h e r i g h t h a n d

l o c a t i o n . E a c h d a t u m p o i n t i s b a s e d o n a p p r o x . 3 0 - 4 0 t r i al s . E r r o r b a r s r e p r e s e n t 1 S E .

and directional errors smallest when she looked in

the rightmost location (indicated by 0 on the abscissa

of Fig. 8). But MC's latencies for the rightmostlocation were among his longest. MC was not asked to

prepare for the rare trials in which he would be asked to

look where he was already attending, and such trials may

have had longer than usual latencies because they

represented an infrequent (12.5%), hence, unexpected,

event.

(2) Directional errors. ".['he signed directional errors of

saccades were quite small (5 deg) and not biased toward

the rightmost location containing the perceptual target.

But the rare trials (2-3 %),eliminated because of very large

directional errors (>67 deg; see Methods) showed a

different pattern. All of the large errors occurred in theFixed report condition, showing that the attempt to

dissociate saccades and attention disrupted saccadic

accuracy on at least a small proport ion of the trials. EK's

errant saccades had very shor t latencies (< 200 msec) and

were usually directed toward the perceptual target. But

most of MC's saccades with large directional errors had

long latencies (about 400 rasec), and were usually directed

either upward or downward. Thus, his large saccadic

errors on a few rare trials were due to confusions about

where to look rather than to the diversion of attention to

the perceptual target.

Experiment 2: Discussion

There were two main results. The first was that in the

absence of info rmation about which letter would have to

be identified (Random report), perceptual performance

was accurate at, and only at, the goal of the saccade.

Performance at other locations was near chance. This

result, by itself, suggests a saccadic/attentional link.

The second main result was that attempts to dissociatethe locus of perceptual attention from the saccadic

goal (Fixed report) were unsuccessful. Subjects

preparing to make a saccade in the direction of a

randomly-oriented pointer had to prolong latency

(by 50-75 msec) whenever they were also required to

identify a letter at a different location. The crucial

aspect of the task that led to the longer latencies was

the different locations of the perceptual and saccadic

target, not the requirement to identify a character,

because characters at the saccadic goal were identified

equally well in both the Random and Fixed report

conditions. The difference betweenRandom

andFixedreport latencies is consistent with the idea that attention

must be allocated to the goal of the saccade. The results

also suggest that the attentional demands of saccades,

although real, may be relatively modest, given that

increases in latency of < 75 msec produced substantial

benefits to perceptual performance at locations different

from the saccadic goal.

The next experiments (3 and 4) follow-up two aspects

of the demonstration that attention is allocated to the

saccadic goal. Experiment 3 deals with the spatial extent

of the region at tended preceding saccades. Experiment 4

will employ a more elaborate methodology, drawn from

the attentional literature, to examine the relationship

between perceptual attent ion and saccades, and to obtain

a more precise description of the demands that saccadic

programming and execution place on attentional

resources.



1908 EILEEN KO WL ER e t a l .

E x p er i m en t 3 : C u e B e t w een T w o C h a r a c t e r s

T h i s e x p e r i m e n t w a s t h e s a m e a s th e R a n d o m r ep o r t

c o n d i t i o n o f E x p e r i m e n t 2 e x c e p t t h a t t h e c e n t r a l p o i n t e r

w a s d i r e c t e d b e t w e e n t w o l e t t e r l o c a t i o n s a n d s u b j e c t s

w e r e r e q u i r e d t o i d e n t i f y e it h e r o f t h e t w o l e t t e rs ( s e l e c te d

a t r a n d o m ) . W e w a n t e d t o f i n d o u t w h e t h e r a t t e n t i o n

s h i f te d t o t h e p r e c is e l o c a t i o n o f t h e s a c c a d i c e n d p o i n t ( inw h i c h c a s e , n e i t h e r l e t te r w o u l d b e i d e n t i f i e d a c c u r a t e l y ) ,

o r , a l t e r n a t i v e l y , w h e t h e r a t t e n t i o n s h i f t e d t o a l a r g e r

r e g i o n s u r r o u n d i n g t h e s a c c a d i c g o a l ( in w h i c h c a s e b o t h

l e t t er s m i g h t b e i d e n t i f i e d a c c u r a t e l y ) . I f th e r e s u l t s t u r n e d

o u t t o f a v o r t h e a t t e n t i o n a l - r e g i o n s h y p o t h e s i s , i t w o u l d

m e a n t h a t p e r c e p t u a l a t t e n t i o n , b y i t se lf , i s n o t t h e o n l y

f a c t o r d e t e r m i n i n g t h e p r e c i se l o c a t i o n o f t h e s a c c a d ic

e n d p o i n t . I n s t e a d , p r e c i se p o s i t i o n i n g o f t h e s a c c a d e

w o u l d d e p e n d e i t h e r o n a s e p a r a t e , n o n - a t t e n t i o n a l

m e c h a n i s m , o r e l se o n a l o w e r - le v e l p r o c e s s t h a t

d e t e r m i n e s t h e s a c c a d i c e n d p o i n t b y p o o l i n g i n f o r m a t i o n

w i t h i n s p a t i a l l y a t t e n d e d r e g i o n s .

E x p e r im e n t 3 : M e t h o d

S t imulus and procedure

S t i m u l i a n d p r o c e d u r e w e r e th e s a m e a s t h o s e o f t h e

R a n d o m r ep o r t c o n d i t i o n i n E x p e r i m e n t 2 , e x c e p t t h a t t h e

p o i n t e r w a s d i r e c t e d m i d w a y b e t w e e n t w o r a n d o m l y

s e l e c t e d l e t t e r s . S u b j e c t s w e r e i n s t r u c t e d t o m a k e a

s a c c a d e t o t h e l o c a t i o n i n d i c a t e d b y t h e p o i n t e r . A t t h e

e n d o f t h e t ri a l, o n e o f th e t w o l e tt e r s s u r r o u n d i n g t h e

p o i n t e r , s e l e c t e d a t r a n d o m , h a d t o b e i d e n t i f i e d . E a c h

s u b j e c t r a n i n o n e 1 0 0 - t r i a l s e s s i o n .

Eliminated tr ials

T r i a l s w e r e e l i m i n a t e d a s f o l l o w s : lo s s o f e y e t r a c k e r

l o c k (8 % f o r E K ) , e r r o r o f t h e f ir s t s a c c a d e > 5 0 ' ( 2 % f o r

E K a n d 2 0 % f o r M C ) a n d d i r e c ti o n a l e r r o r > 6 7 d e g ( 1 %

f o r M C ) . T h e r e s u l t s r e p o r t e d w e r e b a s e d o n t h e

r e m a i n i n g 9 0 t r i a l s f o r E K a n d 7 9 t r i a l s f o r M C .

Exper iment 3 : Resu l t s

S a c c a d e s d i r e c t e d m i d w a y b e t w e e n a p a i r o f l e tt e r s w e r e

l e s s a c c u r a t e t h a n t h o s e d i r e c t e d t o a l e t t e r . A v e r a g e

a n g u l a r e r r o r w a s 12 d e g f o r E K ( in c o m p a r i s o n t o a n

e r r o r o f 8 d e g f o r s a c c a d e s t o a l e t t er i n t h e R a n d o m r ep o r t

c o n d i t i o n o f E x p e r i m e n t 2 ) a n d 9 d e g f o r M C ( in

c o m p a r i s o n t o a n e r r o r o f 7 d eg i n R a n d o m r ep o r t ) .

N e v e r t h e l e s s , t h e s e a n g u l a r e r r o r s w e r e s u b s t a n t i a l l y

s m a l l e r t h a n h a l f t h e i n t e r - l e t t e r s e p a r a t i o n ( w h i c h w a s

2 2 . 5 d e g ) , s h o w i n g t h a t s a c c a d e s , f o r t h e m o s t p a r t ,

l a n d e d i n b e t w e e n t h e l e t t e r p a i r s , r a t h e r t h a n a t o n e o r

t h e o t h e r l e t t e r .

I f a t t e n t i o n w e r e c o n f i n e d t o t h e p r e c is e l o c a t i o n o f th e

s a c c a d i c e n d p o i n t , t h e n w e w o u l d e x p e c t p e r c e p t u a l

i d e n t if i c a ti o n o f t h e l e t te r s t o b e p o o r . P e r c e p t u a l

i d e n t i fi c a t io n w a s n o t p o o r . S u b j e c t s w e r e c o r r e c t a b o u t

h a l f t h e t i m e ( s e e F i g . 9 ) , r e g a r d l e s s o f w h i c h l e t t e r t h e y

w e r e a s k e d t o r e p o r t . I n o t h e r w o r d s , t h e y w e r e u s u a l l y

a b l e t o i d e n t i fy o n e o f t h e t w o l e tt e rs s u r r o u n d i n g t h e

s a c c a d ic g o a l, w h i c h w a s l o c a t e d m i d w a y b e t w e e n t h e m .

1-

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45

QUERIED LOCATION W/RESPECT TO TARGET

FIGU RE 9. Propo rtion of letters identified correctly as a function ofletter o cation elative o the saccadic arget locationwhen saccades wereaimed between wo letters (O ) an d w hen saccades were not made (&).

Each datum po int is based on app rox. 40 trials.

T h i s w a s a b o u t t h e s a m e a s p e rc e p t u a l p e r f o r m a n c e w h e n

s a c c a d e s w e r e n o t m a d e a n d s u b j e c ts w e r e s i m p l y r e q u i r e d

t o s h i f t a t te n t i o n i n t h e d i r e c t i o n i n d i c a te d b y t h e p o i n t e r

( F i g . 9 ) . B o t h s u b j e c t s w e r e s l i g h t l y b e t t e r w h e n a s k e d t oi d e n t i f y t h e l e t te r t h a t w a s c o u n t e r c l o c k w i s e w i t h r e s p e c t

t o t h e s a c c a d i c g o a l , b u t t h e r e w a s n o c o u n t e r c l o c k w i s e

b i as f o r j u d g m e n t s m a d e w h e n s a c ca d e s w e r e n o t m a d e .

F i g u r e 1 0 s h o w s t h e r e l a t i o n s h i p b e t w e e n l e t t e r

i d e n t i fi c a t io n a n d t h e e n d p o i n t o f t h e s a c c ad e , W h e n

s u b j e c t s w e r e a s k e d t o i d e n t i f y t h e l e t t e r l o c a t e d c l o c k w i s e

w i t h r e s p e c t t o t h e p o i n t e r , t h e y w e r e m o r e l i k e l y t o b e

c o r r e c t w h e n t h e a n g u l a r e r r o r o f th e s a c c a d e s w a s i n a

c l o c k w i s e d i r e c t i o n . S i m i l a r ly , w h e n i d e n t i f y i n g t h e l e t t e r

l o c a t e d c o u n t e r c l o c k w i s e w i t h r e s p e c t to t h e p o i n t e r , t h e y

w e r e m o r e l i k e ly t o b e c o r r e c t w h e n t h e s a c c a d i c e r r o r w a s

c o u n t e r c l o c k w i s e .

Exper iment 3 : Discuss ion

T h e a b i l it y o f s u b j e c ts t o i d e n t i f y o n e o f t h e t w o l e t t e rs

s u r r o u n d i n g t h e s a c c a d i c g o a l s h o w s t h a t a t t e n t i o n n e e d

n o t b e c o n f i n e d t o t h e p r e c i s e lo c u s o f t h e s a c c a d i c

e n d p o i n t . E i t h e r d i s s o c i a t i o n s b e t w e e n t h e l o c u s o f

a t t e n t i o n a n d t h e e n d p o i n t o f th e s a c c a d e a r e p o s s ib l e

w h e n s a c c a d i c a n d p e r c e p t u a l t a r g e t s a r e c lo s e t o g e t h e r ,

o r a l t e r n a t i v e l y , a t t e n t i o n i s a l l o c a t e d t o a n e x t e n d e d

r e g i o n o f s p a c e a n d a l o w e r - le v e l s e n s o r i m o t o r p r o c e s s

d e t e r m i n e s t h e p r e c i s e s a c c a d i c e n d p o i n t b y p o o l i n g

i n f o r m a t i o n i n t h e a t t e n d e d r e g i o n . T h e r e l a t i o n s h i p

b e t w e e n t h e a n g u l a r e r r o r o f s a c c a d e s a n d t h e a c c u r a c y

o f l e t te r i d e n t i f i c a t i o n ( F ig . 1 0 ) l e n d s s u p p o r t t o t h e l a t t e r

( a t t e n t i o n a l r e g i o n s ) h y p o t h e s i s , h o w e v e r , a d d i t i o n a l

w o r k i s n e e d e d t o r e s o lv e t h e i ss u e c o m p l e t e l y .



THE ROLE OF ATTENTION IN THE PRO GRA MMI NG OF SACCADES 1909

~ -~ 10 l

¢ _ ~ 4 1 -

- 1 0 "-45

~ 0 4O o=

w

IO-45

E K

i i i

- 2 2 . 5 0 2 2 . 5

M C

c o r r e c t- - incorrect

4 5

i i

- 2 2 . 5 0 2 2 . 5

C W C C W

4 5

Q U E R IE D L O C A T IO N W / R E S P E C T T O T A R G E T

FI GURE 10. Mean unsigned ~ngnlar error o f saccades as a function of

the location of the reported letter relative to the saccadic target for

correct (0 ) and incorrect ((3) reports. Each datum point is based on

approx. 20 trials. Error bars represent 1 SE.

Exper iment 4 : A t t en t iona l Opera t ing Character i s t ic

In this experiment, we ~take a closer look at the sacrificesof saccadic performance that are needed in order to

identify targets accurately at locations other than the

saccadic goal. We found such sacrifices in Experiment 2,

when subjects increased saccadic latency by 50-75 msec

in an attempt to follow the instruction to avoid any

perceptual identification errors at non-goal locations.

Increases of this magnitude might not have been

necessary. It is possible that a less conservative strategy

would have been successful as well, resulting in excellent

perceptual performance with a smaller increase in

saccadic latency.

To explore the effect,; of different latency-strategieson perceptual performance, we turned to techniques

developed previously for determining whether two

perceptual tasks draw on a common pool of attentional

resources (Norman & Bobrow, 1975; Navon & Gopher,

1979; Sperling & Melchner, 1978; Sperling & Dosher,

1986). To answer this question, we did experiments that

would allow us to determine the "attentional operating

character istic" (AOC), which describes the functional

relationship between the performance of two concurrent

tasks. The AOCs will allow us to determine whether, and

by how much, performance on one task (e.g. making

saccades) must be sacrificed for performance on another(e.g. ident ifying letters). The unique characteristic o f the

AOC is that performance is measured under instructions

to assign different priorities to one or the other task.

Determining an AOC requires a larger number of

experimental conditions and trials, and makes a greater

demand on a subject's ability to maintain consistent

assignments of task priorities, than the techniques we

(and others) have used so far to study attention and

saccades. Despite these challenges, measuring an AOC is

appropriate at this stage of the research, once links

between saccades and attention have been demonstrated

(Experiments 1-3), because the AOC is the best way to

study the effects of small changes of strategy on

performance. The AOC allows us to avoid confounding

the effects of the strategies used to allocate attentional

resources from the effects of the resources themselves on

the performance of the task. For a detailed treatment of

the theoretical bases for interpreting the AOC, and for

many examples of AOCs obtained from different

perceptual tasks, see Sperling and Dosher (1986).

Exper iment 4 ." Method

S t i m u l u s a n d p ro c ed u re : r a n d o m a n d f i x e d s a c ca d es

The stimulus was the same set of 3 frames (pre-mask,

cue, post-mask) used in Experiments 2 and 3. There

were two main conditions, R a n d o m s a c c a d es and F i x e d

saccades.

In the R a n d o m s a c c a d e condition, the subject was

always required to report the letter in the rightmost

display location and, in the same trials, to make a saccade

in the direction indicated by the central cue. The letter in

the rightmost location was chosen from the usual critical

set (J-N; T-X). The direction of the saccade was selected

at random and indicated by the central cue (in Frame 2),

which pointed to one of the 8 letters. Subjects ran under3 types of instructions, which controlled the relative

weight subjects were to assign to each of these tasks. The

first instruction was to give priority to the perceptual task

(P), prolonging saccadic latency if necessary, but only

as much as necessary, to achieve perfect perceptual

performance. The second instruction was to give priority

to the saccadic task (S), which meant keeping saccadic

latency as short as possible and sacrificing perceptual

accuracy if necessary. The third instruction was to achieve

a level of performance intermediate (I) between the two

extremes. Each instruc tion was tested in separate

experimental sessions.In the Fixed saccade condition, which was tested after

the R a n d o m s a c c a de testing was completed, both the goal

of the saccade (indicated, as usual, by the pointer) and the

location of the letter to be reported remained the same

throughout the session. In some types of F i x ed s a c ca d e

sessions, subjects were required to look upward and

report the letter in the leftmost location. In other types of

Fixed saccade sessions, subjects looked to the left and

reported the letter in the top loca tion. Some other changes

were made. The critical letter set was changed to A

through E and P through T. Also, the durations of the

frames were changed to take into account the decrease in

saccadic latency that occurs when saccadic direction is

known in advance of the trial. Specifically, Frame 2

(containing the pointer and critical letter) was reduced to

130 msec, to avoid having saccades occurring while the

critical letter was still displayed. Also, the duration of



1910 EILEEN KOWLERet al.

Fram e 1 (the pre-mask) was rando m (400-700 msec) to

introduce uncertainty about when the saccade would be

required, thus reducing the likelihood that the subject

would be able to precisely time an accurate saccade to

coincide with the onset of Frame 2. The same 3

instructions used in the Random saccade condition were

again tested in the Fix ed saccade condition (priority to

perception, priority to saccades, intermediate priority).

S ing le ta sk

To establish a baseline against which any trade-offs

between saccadic and perceptual performance can be

evaluated, we ran two additional conditions. In one

(saccade only) either the R a n d o m or F i x e d saccadic task

was performed by itself and no reports of letters was

taken. In the other (percep t ion on ly) the letter in the

location tested in the R a n d o m an d F i x ed s a c c a d e

conditions was reported, and no saccades were made.

Trying to achieve independence

Two ot her condit ions were tested after the F i x ed s a c c a d e

sessions were completed. Subjects were given a final

opportunity to try to dissociate saccades and attention

after they had experience in doing each of the tasks

(saccade-only and perception-only) by themselves. They

were asked to try to perform the saccadic and perceptual

tasks together as well as they had just perfor med each task

alone. For some of these sessions, the location of the

saccadic goal and letter to be identified were dif ferent

(specifically, either look up and identify the letter on the

left, or look to the left and identify the letter on top). For

the remaining sessions, the locations of the saccadic goal

and the letter to be identified were the s a m e (either up or

left).

Order o f t es t ing

Subjects ran in two replications (60 trials/session) of

the 5 different conditions for both the R a n d o m and the

F i x ed s a c ca d e sessions. The order of testing in the first

replication was: priority to perception, priority to

saccades, intermediate priority, saccades only and

perception only. The order was the same for the secondreplication except that "priority to perception" and

"priority to saccades" were reversed. Random saccade

sessions were tested before F i x ed s a c ca d e sessions. The 4

sessions in which the subject was asked to try to pe rform

the saccadic and perceptual tasks together as well as they

performed each alone were tested last. These 4 sessions

were run over two days, with a session in which the

saccadic and perceptual targets were at different locations

and one in which the targets were at the same location run

on each day.

Eliminated tr ials

Trials were eliminated as follows: loss of eye tracker

lock (2% for EK an d 0.6% f or MC), latencies < 100 msec

(2% for MC), error o f the first saccade > 50' (4% for EK

and 10% for MC) and directional error > 67 deg ( < 1%

for EK and 0.4% for MC). The results reported were

based on the remaining 1103 trials for EK and 1037 trials

for MC.

E x p er i m en t 4 : R es u l ts

The t rade-o f f o f process ing resources

Figure 11 shows the AOC, with the percentage of

correctly identified letters plotted against saccadic

latency. Note that the values along the latency axis are

o(. )

n

1 0 0

7 5

50

E K

25

e m , , ~ m l-o-Fixed Lm R a n d o m : 1 t a s k J

o L~.,,,,~ ~ o mv L o o k , a t t e n d ~ k ~ f f ~ t [

I

20 O

75

5 0

M C

2 5

2 7 5 2 5 0 2 2 5 1 7 0

S a c c a d i c l a t e n c y ( m s e c )

i

, °

2In

Ii

It

I

0 ' = : , 6 ,2 4 5 2 2 0 1 9 5

S a c c a d i c l a t e n c y ( m s e c )

FIGURE 11. Attentional operating characteristic (AOC)curves showing saccadic atency (abscissa) and proportion of correctlyidentified letters (ordinate). The location of the letter to be reported remained the same throughout the session. The saccadictarget was either selected at random (O ,i) or remained fixed throughout he session (©,IS],A,V). The 3 circles n each functionshow performance under instructions to give priority to the saccadic task (lower circles), to the perceptual task (upper circles)or to adopt an intermediate priority (middle circles). Squares, plotted on the axes, show performance when doing only one task,either the saccadicor the letter identification task. The intersection of he dashed lines emanating from the open squares representsthe independence point, i.e. the point at which there would be no interference in the performance of the two concurrent tasks.The triangles represent attempts to achieve he independence point by trying o minimize atency and maximize etter identificationsimultaneously. Saccadicand perceptual targets were either at the same (upright triangle) or different (inverted triangle) locations.

Each datum point was based on approx. 100 observations. Error bars represent ___1 SE.



THE ROLE OF ATTENTION IN THE PRO GRA MMIN G OF SACCADES 1911

inverted, in keeping with the convention of representing

improvements in performance in AOCs by moving

to the right (Sperling & Dosher, 1986). The results

show how performance on the saccadic task was traded

for performance on the perceptual task. To examine

the trade-off, we need to describe Fig. 11 in some

detail.

Performance in the Random saccade condit ion is shown

by the filled symbols and in the Fixed saccade condition

by the open symbols. The squares, plotted along the axes,

show the performance obtained when either the saccadic

or the perceptual task was done alone. The intersection

of these "task alone" values is the "independence point"

(shown in Fig. 11 for the Fixed saccade condition). I f the

saccadic and perceptual tasks can be done concurrently

as well as each can be done alone, then performance will

fall at the independence point.

Performance never reached the independence point. To

see this, we need to look at the functions (circularsymbols) showing how performance varied under the

three different attentional instructions that were used. In

each function, performance shown by the upper-leftmost

circle was obtained under the instruction to give priority

to the perceptual task (P), and the lower right-most circle

under the instruction to give priority to the saccadic task

(S). The performance shown by the circle in the middle

was obtained when subjects were asked to adopt a

strategy intermediate between these two extremes (I). If

saccadic and perceptual tasks do not compete for the

same processing resources, performance would fall at the

independence point. Figure 11 shows that instead ofindependence, improvement on the perceptual task was

achieved at the expense of performance on the saccadic

task. This result was obtained for both the Random and

Fixed saccade conditions.

Random and Fixedsaccadeswere somewhat different in

that the Random saccadic latencies were longer, and the

differences among the latencies under the different

instructions was smaller, than in the Fixed saccade

condition. The differences among the 3 latencies in the

Random saccade condition were smaller for MC than for

EK (see Fig. 11). Analysis of variance confirmed that

MC's latencies in the Random saccade condition weresignificantly different from one ano ther [F(2,343) = 6.45,

P < 0.01].

Were subjects trying as hard as they could to reach the

independence point? The inverted triangles in Fig. 11

show what happened in the Fixedsaccade condition under

the instruction to do botlh saccadic and perceptual tasks

concurrently as well as each could be done alone.

Performance missed the independence point when

saccadic and perceptual targets were in different locations

(inverted triangles). Subjects missed the independence

point in different ways. EK's sacrificed saccadic latency

(mean = 253 msec, SD--= 34, N = 113, when trying toreach the independence point vs 205 msec, SD = 27,

N = 109, when doing the saccadic task by itself). MC

sacrificed perceptual perl%rmance (78% correct, N = 98,

when trying to reach the independence point vs 98%

correct, N = 120, when doing the perceptual task by itself;

the difference was highly reliable; Z2= 23, df= 1,

P < 0.001).

Performance fell closer to the independence point in

one special case, namely, when perceptual and saccadic

targets were the same (upright triangles in Fig. 11). This

shows that the observed trade-off was not between

making saccades and identifying targets, but rather

between the requirement to pay attention to two different

locations.

Shifting small amounts o f attention to the perceptual target

A striking characteristic of the AOCs in Fig. 11 was that

considerable improvement in perceptual identification

was achieved at the cost of little or no increase in saccadic

latency. This unexpected result can be seen by comparing

performance under the "priority to saccades" and the

"intermediate priority" instructions (i.e. the middle and

lower data points of each AOC). Perceptual performance

well above chance was achieved with saccadic latenciesthat were either the same as (MC) or slightly longer than

(EK) the latencies observed when the saccadic task was

done alone. Achieving the best possible perceptual

performance (which in our task approached 100%

correct) required a much larger increase of saccadic

latency.

Saccadic accuracy

Examining saccadic accuracy was important because

subjects might have chosen to sacrifice saccadic accuracy,

as well as latency, in order to improve perceptualperformance at non-goal locations. If this were the

case, then the latency AOCs, shown in Fig. 11,

underestimate the cost o f giving priority to the perceptual

task and a complete picture requires presentation of

accuracy data as well. It is, of course, possible that the

saccadic latencies were sufficiently long so that the

accuracy obtained in the saccade only condition would be

maintained even under instructions to give priority to the

perceptual task.

Figure 12 shows the AOCs in which the measure of

saccadic performance is average unsigned angular error.

There was no sacrifice in accuracy in the Random saccadecondition; errors did not vary with the instruction (S, I

or P) and were about the same as those observed in the

saccade only condition. In the Fixed saccade condition,

the errors were smaller and the AOCs resembled the

latency AOCs (Fig. 11) in that:

(1)accuracy was sacrificed for improvements in

perception;

(2) a large improvement in perceptual identification

was achieved with little or no sacrifice of saccades

(instructions S vs I), but achieving maximal

perceptual performance (instruction P) required asacrifice of saccadic accuracy, just as it had required

a sacrifice of latency;

(3) subjects failed to reach the independence point

when saccadic and perceptual targets were in

different locations.




¢0

OO

O

o

n

1 0 0

7 5

50

2 5

E K

e R a n d o m ~ a c c a d eo R x e d ¢ , a c ~d e R a n d o m : 1 t a s ko F i x e d : 1 t a s k

I ~ Loo k, a t tend

L L o o k , a t t e n d different

a

I

i

1 0 5

Angular e rro r ( degrees)

10 0

7 5

50

M C

2 5

i &,

I

n

In

It

Ia

4,0 0 i p - - : i

1 5 0 1 5 1 0 5 0

Angular e r ro r ( degrees)

F I G U R E 1 2. S a m e a s F i g . 1 1 , e x c e p t t h a t u n s i g n e d a n g u l a r s a c c a d ic e r r o r i s p l o t t e d o n t h e o r d i n a t e .

We assume tha t saccadic accuracy in the F i x ed s a c ca d e

condition under the instruction to give priority to

perception would have been better had latency been

prolonged even further. Thus, the longest latency shown

in Fig. 11 for the F i x ed s a c c a d e condition underestimates

the time required to achieve both accurate perceptual

performance and accurate saccades.*

Exper iment 4 : Discuss ion

It is not possible to prepare to look to one location,

while simultaneously, and without cost, making accurate

perceptual judgments about an eccentric target located

elsewhere. Making saccades requires a shift in perceptual

attention to the saccadic goal.

The attentional demands of saccades were observed

both when the location of the saccadic goal was known

before the trial ( F i x ed s a c c a d e ) and when the location had

to be determined during the trial (Random saccade) .

Finding a trade-off n both of these conditions means that

the role o f attention is not limited to specifying the initial

selection of the saccadic goal. Had it been, then the

trade-off between saccadic and perceptual performance

would have been found only when the location of the

saccadic goal was chosen randomly. In fact, the observed

trade-offwas weaker in the Random saccade condition, as

shown by the smaller difference between saccadic

latencies under the two extreme instructions (i.e. give

priority to the perceptual and give priority to the saccadic

* I t is p o s s i b le t h a t s u b j e c ts s h o w e d b e t t e r s a c c a d i c p e r f o r m a n c e i n t h e

F i x e d t h a n i n t h e R a n d o m s a c c a d e c o n d i t i o n b e c a u s e t h e t w o

s a c c a d i c t a r g e t l o c a t i o n s t e s t e d i n t h e F i x e d c o n d i t i o n ( l e f t a n d u p )

w e r e e a s i e r t h a n t h e o t h e r l o c a t i o n s . T o d e t e r m i n e i f i t w a s e a s i e r t o

l o o k i n t h e s e t w o d i r e c t i o ns , w e e x a m i n e d l a t e n c y a n d a n g u l a r e r r o r

f o r t h e R a n d o m s a c c a d e c o n d i t i o n a t t h e s e t w o p o s i t i o n s , a n d

c o m p a r e d t h e m t o t h e m e a n l a t e n c y a n d a n g u l a r e r r o r a c r o s s a l l

p o s i t i o n s . D i f f e r e n c e s w e r e s m a l l , i n d i c a t i n g t h a t t h e d i f f e r e n c e s

b e t w e e n t h e F i x e d a n d R a n d o m A O C c u r v e s w e r e n o t d u e t o t ar g e t

l o c a t i o n .

tasks) (Fig. 11) and by the absence of an effect of

instructions on accuracy (Fig. 12). The weaker trade-off

between saccades and perceptual performance in the

Random saccade condition implies that at least some of

the operations unique to the R a n d o m s a c c a d e task (such

as identifying he direction of the pointer on each trial and

choosing the saccadic target) may not have interfered with

the sampling of information from the perceptual target.

Some processing of the perceptual target might have

occurred with no cost to saccades while identification of

the pointer direction and other high-level aspects of target

selection were in progress. Thus, the competition for

attent ional resources between the saccadic and perceptual

target appears to be most acute, not during high-level

aspects of target selection, but rather further downstream,

closer to the time of construction of the saccadic program

for immediate execution.

We found strong evidence for the involvement of

attention in saccadic programming, but at the same time

we found that paying too much attention to the saccadic

target was inefficient. We found that when the subject

switched from the instruction to give priority to saccades

(S) to the instruction to adopt an intermediate priority

between saccadic and perceptual tasks (I), substantial

improvement in the perceptual task was achieved with

little or no cost to saccades. Analogous results have been

obtained for smooth pursuit (Khurana & Kowler, 1987).

The same was n o t true for our perceptual task. We found

no region in which the withdrawal of attention was

harmless.

This outcome shows that increasing the amount of

attent ion benefits saccadic performance, but only up to a

point. The diminishing returns of allocating increasing

amounts of attention to the saccadic task is illustrated in

Fig. 13, which shows hypothetical performance-resource

functions (Norman & Bobrow, 1975) that might have

given rise to the AOCs we observed in Figs 11 and 12. The

performance-resource function for the perceptual task is

nearly linear, while that for the saccadic task (where

performance is represented by latency) levels off with



THE ROLE OF ATTENT ION IN THE PROGR AM MING OF SACCADES 1913

, IN S T R U C T I O N S : ]

S ' = P r i o r it y t o S a c c a d i c a s k /

' r = I n te r m e d i a te /

P ' = P r io r it y o P e r c e p t u a l = k

i

a )

~ o ' "

- I

0

A t t e n t i o n t o S a c c a d i c T a r g e t

Pq)

= o

o

D .

(b

? ' $ ' i

A t t e n t i o n t o P e r c e p t u a l T a r g e t

FIGU RE 13. Hypotheticalperformance esource functionsdemonstrating he effecton task performanceof varyingattentionalallocation. (a) Show s saccadic atency as a function of the am ount of attention allocated o the saccadic arget; (b) shows hepercentage of correctly dentified etters as a functio nof the amo unt of attention allocated o the perceptual arget. The 3 labeledpoints represent the 3 instructions: give priority to the saceadic task (S), the perceptual tas k (P) or a do pt an intermediate

strategy (I).

i n c r e a s in g a m o u n t s o f a l l o c a t e d a t t e n t i o n , i .e . a l lo c a t i n g

i n c r e a s in g a m o u n t s o f a t t e n t i o n t o t h e s a c c a d i c t a s k

r e d u c e d t h e l a t e n c y , b u t o n l y u p t o s o m e l im i t . T o

i l l u s t r a t e t h e e f f e c t o f t r a n s f e r r i n g a t t e n t i o n f r o m o n e

t a r g e t t o a n o t h e r , w e l a b e l e d 3 p o i n t s S , I a n d P , t o

d e s i g n a t e t h e h y p o t h e t i c a l r e l a ti o n s h i p b e t w e e n a t t e n t i o n

a n d l a t e n c y u n d e r t h e 3 i n s t r u c t i o n s t e s te d ( p r i o r i t y t o t h es a c c a d i c ta s k , S , t h e p e r c e p t u a l t a s k , P , o r i n t e r m e d i a t e

p r i o r i t y , I ) . W h e n m o v i n g f r o m p o i n t S t o p o i n t I , f o r

e x a m p l e , a t t e n t i o n i s s h i f t e d f r o m t h e s a c c a d i c t o t h e

p e r c e p t u a l t a r g e t . S a c c a d i c l a t e n c y i n c r e a se s o n l y s l i g h t ly

w i t h t h e d r o p i n a l l o c a t e d a t t e n t i o n w h i l e p e r c e p t u a l

p e r f o r m a n c e i m p r o v e s s u b s ta n t ia l ly .

W e d o n o t k n o w w h e t h e r t h e d im i n i s h i n g r e t u r n s o f

a l l o c a t i n g in c r e a s i n g a m o u n t s o f a t t e n t i o n i s a g e n e r a l

c h a r a c t e r i s t i c o f s a c c a d i c t a s k s , o r w h e t h e r i n c r e a s i n g

a m o u n t s o f a t t e n t i o n w o u l d b e c o m e m o r e v a lu a b l e w h e n

t he d i f f i c u l t y o f t h e s a ca :a d ic t a s k i s inc r e a s e d by , f o r

e x a m p l e , i n c r e a s i n g t h e r e q u i r e d p r e c i si o n o f t h em o v e m e n t . N e v e r t h e l e s s , t h i s i s a n i n t r i g u i n g r e s u l t

b e c a u s e i t s u g g e s t s t h a t t ]h e s a c c a d i c s y s t e m ( a n d p u r s u i t

a s w e ll ; K h u r a n a & K o w l e r , 1 9 8 7) p r o t e c t s u s a g a i n s t t h e

f o l ly o f p a y i n g t o o m u c h a t t e n t i o n t o m o t o r c o n t r o l a n d

i g n o r i n g t h e v e r y t a r g e t s t h a t t h e e y e m o v e m e n t s a r e t h e r e

t o he l p u s pe r c e i ve .

G E N E R A L D I S C U S S I O N

A c c u r a t e s a c c a d e s r e q u i r e s h i f ts o f p e r c e p t u a l a t t e n t i o n

t o t h e t a r g e t . W e h a v e s h o w n t h i s b y f i n d i n g e f fe c ts

o f a t t e n t i o n - c a t c h i ng s ti n~ tu li on s a c c a de s ( E x pe r i m e n t 1 ),b y d e m o n s t r a t i n g s u p e r i o r p e rc e p t u a l j u d g m e n t s a t

t h e s a c c a d i c g o a l ( E x p e r i m e n t 2 ) , a n d b y s h o w i n g

t h a t t h e l o c u s o f p e r c e p tu a l a t t e n t i o n c a n n o t b e f u l ly

d i s s o c i a te d f r o m t h e g o a l o f t h e s a c c a d e (E x p e r i m e n t s 2

a n d 4 ) . O u r e x p e r i m e m s d i f fe r f r o m p r i o r w o r k , i n

w h i c h c o n f l i c ti n g re s u l ts w e r e o b t a i n e d ( s ee I n t r o d u c -

t i o n ) , i n t h a t w e u s e d a p e r c e p t u a l t a s k t h a t m a d e

d e m a n d s o n a t t e n t i o n a l r e so u r c es a n d t e st e d p e r f o r m a n c e

w hi l e s ub j e c t s u s e d a va r i e t y o f s pe c i f i c a nd e x p l i c i t l y -

d e f i n e d s t r a t e g i e s a b o u t a p p o r t i o n i n g e f f o r t b e t w e e n

s a c c a d i c a n d p e r c e p t u a l t a s k s . W e d i d t h i s b e c a u s e b o t h

t h e p r i o r w o r k a n d o u r o w n r e s u l t s ( i . e . t h e i n d i v i d u a ld i f fe r e n c e s o b s e r v e d i n E x p e r i m e n t 1 B ) s u g g e s t e d t h a t ,

w i t h o u t s u c h i n s t r u c t i o n s , s u b j e c t s m a y a d o p t i d i o s y n -

c r a t i c s tr a t eg i e s , d i f f er e n t f r o m t h o s e i n t e n d e d b y t h e

e x p e r i m e n t e r . T h e s e i d i o s y n c r a t i c s tr a t e g ie s p r e c l u d e

u n d e r s t a n d i n g t h e u n d e r l y i n g r e l a t i o n s h i p b e t w e e n

a t t e n t i o n a n d s a c c a d e s .

Implications fo r mo dels of the attentional/saccadic link

O u r r e s u lt s a r e c o n s i s t e n t w i t h t h e i d e a t h a t t h e s a m e

s p a t i a l l y - s e l e c t i v e a t t e n t i o n a l m e c h a n i s m t h a t s e r v e s

p e r c e p t i o n a l s o d e t e r m i n e s t h e g o a l o f t h e s a c c a d e .A t t e n t i o n m a y s e l e ct a n o b j e c t o r a s p a t i a l r e g i o n a s th e

s a c c a d i c goa l , w h i l e t he p r e c i s e l oc us o f t he s a c c a d i c

e n d p o i n t m a y d e p e n d o n s u b s e q u e n t o p e r a t i o n s t h a t p o o l

s p a t i a l i n f o r m a t i o n e x c l u s iv e l y w i t h i n a t t e n d e d r e g i o n s o f

s pa c e ( s e e a l s o H e & K ow l e r , 1989 , 1991 ; M or ga n , H o l e

& G l e nne r s t e r , 1990) .

A n y m o d e l o f h o w a t t e n t i o n a c c o m p l is h e s th i s t as k ,

a n d , i n p a r t i c u l a r a m o d e l w h i c h i s t o b e p l a u s i b l e a t t h e

n e u r o p h y s i o l o g i c a l l ev e l, w o u l d h a v e t o a c c o u n t f o r o u r

f i n d i n g s t h a t d r a w i n g a l i t t l e a t t e n t i o n a w a y f r o m t h e

s a c c a d i c g o a l i s h a r m l e s s , a n d d r a w i n g t o o m u c h

a t t e n t i o n a w a y i m p a i r s e i t h e r s a c c a d i c l a t e n c y , a c c u r a c yo r b o t h . C o n s i d e r in g h o w m o d e l s w o u l d a c c o m p l i sh s u c h

a t a s k i s w o r t h w h i l e , n o t o n l y a s a b a s i s t o i n t e r p r e t

n e u r o p h y s i o l o g i c a l re s u l ts , b u t a l s o t o u n d e r s t a n d e v e n t s

d u r i n g n a t u r a l s c a n n in g , w h e n s a c ca d i c a n d p e r c e p t u a l

t a s k s a r e b e in g p e r f o r m e d c o n c u r r e n t l y a ll th e t i m e . W e




consider two ways of explaining the effects of attention on

saccades.

The first we refer to as the spatial model. In this model

attention can be allocated at the same time to two sites

(the saccadic and the perceptual targets) during the entire

saccadic latency period, with the subject having control

over the strength of attentional activation at each site. In

order to explain the reduction in saccadic latency that

occurs when too much attention is diverted from the

saccadic goal, we would assume that diverting attention

slows processing by, for example, reducing firing rates or

reducing the number o f neurons participating in saccadic

generation.

Using the spatial model to account for the effects of

attentional diversion on saccadic accuracy, however, has

some drawbacks. If the saccadic endpoint is determined

by pooling information across all attended regions, then

dividing attention between two widely separated regions

of the visual display would produce large directional

errors on most every trial. But we found that trials with

large directional errors were extremely rare (2-3% in

Experiment 2; 1% in Experiment 4). For the spatial

model to be able to account for such a low error rate, the

saccadic system would have to be able to distinguish

the saccadic target from the perceptual target even when

both regions receive equivalent amounts of attention.

Making such a distinction would require yet another

signal, in addition to perceptual selective attention, to

identify the saccadic goal. Attention, by itself, would no t

be sufficient.

Another way to distinguish saccadic and perceptual

targets working within the assumptions of the spatial

model would be to have separate groups of neurons

handle saccadic and perceptual selection. In order to

account for the long saccadic latencies observed when too

much attention was directed to the perceptual target, or

the poor perceptual performance observed when too

much att ention was directed to the saccadic target, these

separate groups of neurons would have to be able to

inhibit each other's activity when perceptual and saccadic

targets were in different places.

The temporal model is an alternative, and more

straightforward, explanation for the effects of attention

on saccades. According to the temporal model, the

endpoint of the saccade is determined by the locus of

attent ion during a critical segment of the saccadic latency

period when a saccadic "go" signal is issued. Directing

attent ion away from the saccadic goal during non-critical

portions of the latency period (perhaps early in the latency

period) would have no ill effects. Saccadic errors would

result, however, when a saccade was initiated while

attention was still at a non-goal location. Saccades

would be accurate whenever the saccade was initiated

after the shift of attent ion was completed (see Reeves &

Sperling, 1986, for models of the time course of

attent ional shifts).

The temporal model, unlike the spatial model, does

not require separate attentional areas for saccades

and perception, nor does it require special signals

to distinguish the saccadic target from non-targets.

It does require a distinction between systems that

initiate saccades from those that determine the saccadic

endpoint. If saccades are to be accurate while attention

hops about the visual field, it is necessary for the saccadic

go signal to be issued just as attention has settled

at the saccadic goal. Issuing the go signal too early

leads to errors; issuing it too late prolongs latency

unnecessarily.

The assumption of a separate saccadic initiation

area, responsible for relaying a saccadic go signal, but

not for setting spatial parameters, is consistent with

neurophysiological findings of fixational cells whose

activity inhibits saccades and whose silence facilitates

saccades. [See Munoz and Wurtz (1993a,b), for an

example of such an area in the superior colliculus and a

discussion of its possible role in saccadic control, and see

Schlag, Schlag-Rey and Pigarev (1992) for a representa-

tive description of analogous cells in the cortex

(supplementary eye fields).]

The temporal model has one other virtue. If the

saccadic go signal could be pre-set to occur automatically

in response to a transient change in the locus of attention,

then optimal scanning performance would be ensured--

optimal in the sense that saccadic errors would be small,

latencies would not be unnecessarily prolonged, and

on-line, time-consuming decisions would not be required,

save for the control of attention itself. Saccadic scanning

of complex displays would be easy, effortless and

accurate--which is precisely the way things seem to be in

everyday life.

S U M M A R Y

We developed novel methods to evaluate the role of

attention shifts in saccadic performance and found that

perceptual attention plays an important and necessary

role in saccadic programming. I t was not possible to plan

a saccade to one target while paying full attention to

another.

Two aspects of the attentional demands of saccades

are noteworthy. First, modest diversions of attention

away from the saccadic target were possible with little

loss in saccadic performance. This limit on the attent ional

demands of saccades means that resources will be

available for cognitive processing of the visual

display. Second, the demands made by saccades on

attention appear to concern aspects of saccadic

programming itself, ra ther than "higher-level" decisions

about target selection made well in advance of saccadic

execution.

We described two models to account for our results,

one in which attention is devoted simultaneously to

perceptual and saccadic targets (spatial model) and

the other in which attention shifts during the

saccadic latency period (temporal model). The temporal

model seems simpler, is more in line with current evidence

from neurophysiological studies, and is able to account

for the finely-tuned temporal coordination between

attention shifts and saccades that is experienced by all

of us.



T H E R O L E O F A T T E N T I O N I N T H E P R O G R A M M I N G O F S A C C A D E S 1 91 5

Fina l ly , our experiments employed new and ef f ec t ive

methods , drawn front the a t t ent ion l i t era ture , for

s t u d y in g c o n c u rren t p erc ep tu a l a n d mo t o r p er f o rma n c e.

These t echniques were more success fu l than prior

at tempts to s tudy concurrent saccad ic and perceptua l

performance in that they es tabl i shed more s t r ingent

contro l over subject ' s s t ra teg ies and in so do ing a l lowed

t h e a t t en t io n a l d ema n d s o f t h e mo t o r t a s k t o b e

d e t ermin ed u n a mb ig u o u s ly . S u c h t e c h n iq u es ma y p ro v e

t o b e o f f u r t h er v a lu e f o r u n d ers t a n d in g h o w h u ma n

beings a l loca te l imited process ing resources during the

performance o f a var ie ty o f natura l tasks w i th both

perceptua l and motor requirements .

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