electrophysiological evidence for a postperceptual locus of suppression during the attentional blink

8/14/2019 Electrophysiological Evidence for a Postperceptual Locus of Suppression During the Attentional Blink

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Journal of Experimental Psychology: Copyright 1998 by the American Psychological Association, Inc.Human Perception an d Performance 0096-1523/98/$3.001998, Vol. 24. No. 6 ,1 656-16 74

E le c tr op h ys io log i c a l E v id e n c e for a P os tp er c e p tu a l L oc u s o fSupp ress ion D ur ing the Attent ional B l inkEd w a r d K . Vo g e l a n d S t e v e n J. Lu c kU n i v e r si ty o f I o w a Ki m r o n L . Sh a p i roU n i v e r s i t y o f W a l e s , B a n g o r

When an observ er detects a target in a rapid stream of visual st imuli , there is a brief period oftim e during which the detection o f subsequent targets is impaired. I n this study, event-relatedpotentials (ERPs) w ere recorded from normal adult observers to determine w hether this"attentional blink" reflects a suppression of perceptual processes or an impairment inpostperceptual processes. No suppression was observed during the attentional blink intervalfor ERP components corresponding to sensory processing (the P1 and N1 components) orsemantic analysis (the N400 com ponent). How ever, complete suppression wa s observed fo r anER P comp onent that has been hypothesized to reflect the updating of working m em ory (the P3comp onent). Resu lts indicate that the attentional blink reflects an impairme nt in a postpercep-tual stage o f processing.

O v e r t h e p a s t s e v e r a l d e c a d e s , t h e v a s t m a j o r i ty o f s t u d i e so f v i s u a l a t t e n ti o n h a v e e x a m i n e d t h e o p e r a t i o n o f a t t e n ti o nacro ss sp ace . I n t h e v i su a l s ea rch t a sk , f o r ex amp le , a t a rg e ti t e m m u s t b e d e t e c t e d w i t h i n a n a r r a y o f d i s tr a c t o r i te m s t h a ta r e p resen t ed a t d i f f e r en t l o ca t i o n s f ro m th e t a rg e t . I n r ecen ty e a r s , h o w e v e r , t h e r e h a s b e e n g r o w i n g i n t e r e s t i n t h eo p e r a t i o n o f a t t e n ti o n o v e r t im e r a t h e r t h a n o v e r s p a c e .T i m e - b a s e d a t t e n t i o n h a s t y p i c a l l y b e e n s t u d i e d w i t h t h erap id se r i a l v i su a l p r esen t a t i o n (RS VP ) t ask , wh ich can b et h o u g h t o f a s a t i m e - b a s e d a n a l o g o f t h e v i s u a l s e a r c h t a sk .S p ec i f i ca l l y , wh ereas v i su a l s ea rch t a sk s t y p i ca l l y r eq u i r et h e o b s e r v e r t o d e t e c t a t a r g e t i n a n a r r a y o f i t e m s p r e s e n t e ds imu l t an eo u s ly a t d i f f e r en t l o ca t i o n s , RS VP t ask s t y p i ca l l yr eq u i r e t h e o b se rv er to d e t ec t a t a rg e t i n a seq u en ce o f i temsp resen t ed seq u en t i a l l y a t a s i n g l e l o ca t i o n . Becau se v i su a ls e a r c h a n d R S V P a r e s o a n a l o g o u s , i t s e e m s r e a s o n a b l e t os u p p o s e t h a t t h e s a m e m e c h a n i s m s o f a t t e n t i o n w o u l do p e r a t e i n b o t h t a s k s . H o w e v e r , s p a c e p l a y s a u n i q u e l yi m p o r t a n t r o l e i n t h e o r g a n i z a t i o n o f t h e p r i m a t e v i s u a ls y s t e m , a n d s e v e r a l i n v e s t i g a t or s h a v e p r o p o s e d t h a t s p a c ea l so p l ay s a u n iq u e ro l e i n v i su a l a t t en t i o n (Hi l l y a rd &

Edward K. Vogel and Steven J. Luck, Department of Psychol-ogy, University of Iowa; Kimron L. Shapiro, Department ofPsychology, University of Wales, Bangor, Wales.This study was supported by Gra nt 95-38 fro m the McDonnell-Pew Program in Cognitive Neuroscience. Portions of the data inExperiment 2 were previously published in "W ord Meanings CanBe Accessed but N ot Reported During the Attentional B link," byS. J. Luck, E. K. Vogel, and K. L. Shapiro, 1996, Nature, 382, pp .616-618.We thank the following people for contributing directly orindirectly to the ideas and data discussed in this article: LyndonBerglan, Marvin Chun, Vince Di Lollo, Howard Egeth, JimHoffman, Bill Maki, and Jane Raymond.Correspondence concerning this article should be addressed toEdward K. Vo gel or Steven J. Luck, Department of Psychology,University of Iowa, El i Seashore Hall , Iowa City, Iowa 52242-1407. Electronic m ail may be sent to [email protected] orsteven-luck @uiowa.edu.

M t in t e , 1 9 8 4 ; Ni s sen , 1 9 8 5 ; T re i sm an & Gela d e , 1 9 8 0 ; Tsa l& Lav i e , 1 9 9 3 ). Co n seq u en t l y , sp ec i a l a tt en t i o n a l me ch a-n i s m s m a y b e u s e d f o r s p a c e - b a s e d a t t e n t i o n t h a t a r e n o tav a i l ab l e fo r t ime-b ased a t t en t i o n . In t h i s s t u d y we ad -d res sed t h i s i s su e b y co mp ar in g t h e n a tu re o f a t t en t i o n a lse l ec t i o n i n a t ime-b ased a t t en t i o n t a sk wi th p rev io u sf i n d in g s f ro m sp ace-b ased a t t en t i o n t a sk s .

T i m e -B a s e d A t te n t io n a n d th e A t t e n t io n a l B l i n kM u c h o f t h e r e c e n t i n te r e s t i n t i m e - b a s e d a t t e n ti o n d e r i v e sf r o m a p h e n o m e n o n t h a t w a s i n d e p e n d e n t l y d i s c o v e r e d b y

B r o a d b e n t a n d B r o a d b e n t ( 1 9 8 7 ) a n d b y S p e r l i n g a n dco l l eag u es (Reev es & S p er l i n g , 1 9 8 6 ; Weich se lg ar t n e r &S p er l i n g , 1 9 8 7 ) an d t h a t was su b seq u en t l y n amed t h ea t t en t i o n a l b l i n k b y R a y m o n d , S h a p i r o , a n d A m e l l ( 1 9 9 2) .T h e a t t en t i on a l b l i n k i s t y p i c a l ly o b s e r v e d i n R S V P t a s k s i nw h i c h t h e s t i m u l i a re p r e s e n t e d a t a r a t e o f a p p r o x i m a t e l y 1 0i t e m s p e r s e c o n d a n d t h e o b s e r v e r s a r e r e q u ir e d t o d e t e c t o rd i sc r imin a t e two t a rg e t s o n each t r i a l . Th e two t a rg e t s a r et y p i c a l ly c a l le d T 1 a n d T 2 , a n d t h e l a g b e t w e e n T 1 a n d T 2 i st y p i ca l l y v a r i ed r an d o mly ac ro ss t r i a l s ( e .g ., o n a Lag 3 t ri a l ,T2 i s t h e t h i rd i t em a f t e r T1 ) . I n ex p er im en t s o f t h i s n a tu re ,t h e c o r r e c t d e t e c ti o n o f T 1 i s f o l l o w e d b y a p e r i o d o f t i m ed u r in g wh ich t h e o b se rv er s f r eq u en t l y f a i l t o d e t ec t T2 , an dt h is p e r i o d o f i m p a i r e d T 2 d e t e c t i o n i s c a l le d t h e " a t t e n -t i o n a l b l i n k " b e c a u s e i t is s i m i l a r t o t h e s o r t o f i m p a i r m e n tt h a t w o u l d b e p r o d u c e d i f a n e y e b l i n k w e r e t r i g g e r e d b y T 1 .S p ec i f i ca l l y , T2 d e t ec t i o n accu racy t y p i ca l l y d ro p s t o i t sl o w e s t p o i n t a t a T 1 - T 2 l a g o f th r e e i t e m s a n d t h e n r e c o v e r sb y a l ag o f s i x t o e i g h t i t ems (Ch u n & P o t t e r , 1 9 9 5 ; M aid ,F r ig en , & P au l so n , 1 9 9 7 ; Ra y m o n d e t a l. , 1 9 9 2) . I n t e r es t -i n g ly , t h e re i s u su a l l y l i t t l e o r n o imp a i rmen t a t Lag 1 ,a l t h o u g h t h i s d ep en d s o n t h e d e t a i l s o f t h e ex p er ime n ta ld es ig n ( see , e .g . , Du n c an , Ward , & S h ap i ro , 1 9 9 4 ).T h e a t t e n t i o n a l b l i n k i s a n a l o g o u s t o i m p a i r m e n t s o fd i s t r a c t o r p r o c e s s i n g t h a t h a v e b e e n o b s e r v e d i n v i s u a ls e a r c h t a s k s . F o r e x a m p l e , T s a l a n d L a v i e ( 1 9 8 8 ) a s k e do b serv er s t o i d en t i fy a v i su a l s ea rch t a rg e t o f a p a r t i cu l a r

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POSTPERCEPTUAL LOCU S OF SUPPRESSION 16 57c o l o r a n d t h e n r e p o r t a s m a n y a d d i t i o n a l i t e m s f r o m t h ea r r a y a s p o s s i b l e . T h e y f o u n d t h a t t h e o b s e r v e r s w e r ei m p a i r e d a t r e p o r t i n g i t e m s t h a t w e r e f a r f r o m t h e t a r g e tl o c a t i o n c o m p a r e d w i t h i t e m s t h a t w e r e a d j a c e n t t o t h e t a rg e t( fo r r e l a t ed f i n d in g s , s ee Ch e l az z i , M i l l e r , Du n can , &D e s i m o n e , 1 9 93 ; K i m & C a v e , 1 9 9 5 ; L u c k , F a n , & H i l l y a r d,1 9 9 3 ) . Th i s p a ra l l e l s t h e t y p i c a l a t t en t i o n a l b l i n k p a t t e rn , i nw h i c h T 2 i s re p o r t e d a c c u r a t e l y w h e n i t i m m e d i a t e l y f o l lo w sT 1 b u t i s m i s s e d w h e n i t i s t e m p o r a l l y m o r e d i s t a n t .P e r f o r m a n c e e v e n t u a l l y r e c o v e r s a f t e r th e a t t e n t io n a l b l i n k ,w h e r e a s n o r e c o v e r y i s o b s e r v e d a t l a r ge r d i s ta n c e s i n v i s u a ls e a r c h. H o w e v e r , t h i s d i f f e r e n c e p r o b a b l y r e f l e ct s t h e f a c tt h a t a t te n t io n i s e v e n t u a l l y w i t h d r a w n f r o m T 1 i n t he R S V Pp a r a d i g m ; v i s u a l s e a r c h p e r f o r m a n c e w o u l d s i m i l a r l y b ee x p e c t e d t o r e c o v e r o n c e a t t e n t io n w a s w i t h d r a w n f r o m t h etarget .

A l t h o u g h t h e R S V P t a s k i s a n a l o g o u s to t h e v i s u a l s e a rc ht a s k a n d t h e a t t e nt i o na l b l i n k p h e n o m e n o n i n t h e R S V P t a s ki s an a lo g o u s t o t h e su p p ress io n o f d i s t r ac to r i t ems i n t h ev i su a l s ea rch t a sk , t h e re i s g o o d r easo n t o b e l i ev e t h a t t h eu n d er ly in g a t t en t i o n a l m ech an i s ms a r e n o t en t i r e l y i d en t i ca l.S p ec i f i ca l l y , wh ereas a t t en t i o n a l se l ec t i o n i n sp a t i a l a t t en -t i o n p a r a d i g m s a p p e a r s t o b e g i n a t a n e a r l y s t a g e , b e f o r es t imu lu s i d en t i f i ca t i o n i s co mp le t e , i t ap p ear s t h a t t h ea t t en t i o n a l b l i n k r e f l ec t s a p o s tp e rcep tu a l a t t en t i o n a l mech a-n i s m t h a t o p e r a t e s a t t h e s t a g e o f w o r k i n g m e m o r y . I n t h efo l l o win g sec t i o n s we d i scu ss t h e r esea rch t h a t su p p o r t st h ese p ro p o sa l s .

A n E a r l y L o c u s o f S p a c e - B a s e d S e l e c t io nT h e o r i e s o f a t t e n t i o n h a v e g e n e r a l l y b e e n d i v i d e d b e -t w e e n early-selection theories, w h i c h p r o p o s e t h a t a t t e n t io n

i n f lu e n c e s p e r c e p t u a l p r o c e s s e s , a n d late-selection theories,w h i c h p r o p o s e t h a t a t t e n t i o n o p e r a t e s o n l y a f t e r p e r c e p -t i o n i s c o m p l e t e . T h e t e r m s early and late h a v e b e e nc o n s t r u e d i n a v a r i e t y o f w a y s , b u t t h e m o s t f u n d a m e n t a ld i s t i n c t i o n i s wh e th er a t t en t i o n a l se l ec t i o n o ccu r s b e fo re o ra f t e r a s t imu lu s h as b een fu l l y i d en t i f i ed ( fo r p a r t i cu l a r l yc l ea r s t a t em en t s o f t h is d i s ti n c t i o n , s ee Du n c an , 1 9 8 0 ;D u n c a n & H u m p h r e y s , 1 9 89 ) . I n th i s a r ti c le , w e a l s o u s e t h et e r m s perceptual and postperceptual t o r e f e r t o t h i s s a m edis t inct ion . 1

T h e r e i s l i t t l e d e b a t e a b o u t w h e t h e r a t t e n t i o n c a n s o m e -t i m e s o p e r a t e a t a l a t e , p o s t p e r c e p t u a l s t a g e . F o r e x a m p l e ,t h e r e a r e m a n y s i tu a t io n s i n w h i c h a n o b s e r v e r w i l l i d e n t i fy as t i m u l u s b u t c h o o s e n o t t o m a k e a r e s p o n s e t o i t , w h i c h i sc l e a r l y a c a s e o f p o s t p e r c e p t u a l s e l e c t i v e p ro c e s s i n g . T h ep r i m a r y f o c u s o f t h e l o c u s - o f - s e le c t i o n d e b a t e h a s t h u s b e e nt h e q u e s t io n o f w h e t h e r a t t e n t io n s o m e t i m e s o p e r a t e s a t a ne a r l y s t a g e , b e f o r e p e r c e p t io n i s c o m p l e t e . D e s p i t e d e c a d e so f r e s e a r c h , h o w e v e r , t h e t r a d i t io n a l b e h a v i o r a l t e c h n i q ue so f c o g n i t i v e p s y c h o l o g y h a v e b e e n u n a b l e t o f i r m l y d e t e r -m i n e w h e t h e r e a r l y s e l e c t i o n i s p o s s i b l e . T h e p r i m a r yd i f fi c u lt y i s t h a t t h e se t e c h n i q u e s r e l y o n m e a s u r e m e n t s o fb e h a v i o r a l o u t p u t , w h i c h r e f l e c t s t h e c o m b i n e d e f f e c t s o fea r l y an d l a t e p ro cesses . Th i s mak es i t d i f f i cu l t t o d e t e rmin ewi th ce r t a i n ty t h e s t ag e o f p ro cess in g t h a t i s i n f lu en ced b y ag i v e n e x p e r i m e n t a l m a n i p u l a t i o n . C o n s i d e r , f o r e x a m p l e ,

t h e E r ik sen f l an k er p a rad ig m, i n wh ich t o -b e - ig n o red d i s t r ac -t o r i t e m s a r e f o u n d t o i n t e r f e r e w i t h t h e p r o c e s s i n g o f at o -b e-a t t en d ed t a rg e t i t em ( see C . W. E r ik sen , 1 9 9 5 ) . Wh ensu ch i n t e r f e r en ce i s fo u n d , i t i s o f t en u sed as ev id en ce t h a tt h e d i s t r ac to r s a r e fu l l y i d en t i f ied an d t h a t s e l ec t i o n t h e re fo reo ccu r s a t a p o s tp e rcep tu a l s t ag e ( e .g . , B . A . E r ik sen &Er ik sen , 1 9 7 4 ; Ha g en a ar & v an d er He i j d en , 1 9 8 6 ). Ho w -ev er , t h e o b se rv ed i n t e r f e r en ce co u ld j u s t a s eas i l y r e f l ec t ap e r c e p t u a l - le v e l s e l e c t i o n m e c h a n i s m t h a t o n l y p a r t i a ll y o rp ro b ab i l i s t i ca l l y a t t en u a t es t h e d i s t r ac to r s ( see T re i sman ,1 9 6 4 ) . S imi l a r l y , wh en n o i n t e r f e r en ce i s o b se rv ed , t h i sr esu l t i s o f t en u sed t o a rg u e fo r ea r l y se l ec t i o n ( e .g . ,F ran co l i n i & Eg e th , 1 9 8 0 ; Lav i e , 1 9 9 5 ; Yan t i s & Jo h n s to n ,1 9 9 0 ) . Ho wev er , i f t h e i n t e r f e r en ce n o rmal ly a r i ses a t t h es t a g e o f r e s p o n s e s e l e ct i on , i t c o u l d e a s i l y b e e l i m i n a t e d b yan a t t en t i o n a l mech an i sm th a t o p era t es a t a s t ag e t h a t i si n t e rp o sed b e tween s t imu lu s i d en t i f i ca t i o n an d r esp o n sese l ec t i o n ( see Al lp o r t , T ip p er , & C h m ie l , 1 9 8 5 ; Dr iv e r &Tip p er , 1 9 8 9 ) . Th u s , n e i t h e r t h e p resen ce n o r t h e ab sen ce o fi n t e rf e r e n c e f r o m d i s tr a c t o rs c a n u n a m b i g u o u s l y b e u s e d t od e t e rm in e t h e l o cu s o f se l ec t i o n .

To av o id am b ig u i t i e s o f t h i s n a tu re , s ev e ra l a t t en t io nr e s e a r c h e r s h a v e t u r n e d t o e v e n t - r e l a t e d p o t e n t ia l ( E R P )r e c o r d in g s , w h i c h p r o v i d e a c o n t in u o u s m e a s u r e o f p r o c e s s -i n g b e t w e e n a s t i m u l u s a n d a r e s p o n s e a n d c a n t h e r e f o r e b eu sed t o p in p o in t t h e t ime a t wh ich a t t en t i o n b eg in s t oi n f lu e n c e p r o c e s s i n g . E R P s a r e s c a l p - r e c o r d e d v o l t a g e f l u c -t u a t i o n s t h a t r e f l ec t sy n ch ro n o u s n eu ro n a l ac t i v i t y as so c i -a t ed wi th sen so ry , mo to r , o r co g n i t i v e ev en t s ( fo r r ev i ews ,see Do n c h in , Ka r i s , Bash o re , Co l es , & Gra t t o n , 1 9 8 6 ;Hi l l y a rd & P i c to n , 1 9 8 7 ) . As sh o wn in F ig u re 1 , t h e ERPw a v e f o r m i s c o m p o s e d o f s e v e r a l n e g a t i v e a n d p o s i t i v ev o l t ag e f l u c tu a t i o n s t h a t a r e t r i g g ered b y an ev en t su ch as av i s u a l s t im u l u s . T h e p e a k s a n d v a l l e y s o f th e w a v e f o r m a r eca l l ed wa ves o r components a n d r e f l e c t th e t e m p o r a l p r o g r e s -s i o n o f p r o c e s s i n g , b e g i n n i n g w i t h s e n s o r y a c t i v i t y a n dp r o g r e s s i n g t h r o u g h h i g h e r l e v e l c o g n i t i v e p r o c e s s e s a n dr e s p o n s e - r e l a t e d p r o c e s s e s . E a c h E R P c o m p o n e n t i s t y p i -c a U y n a m e d a c c o r d i n g t o i t s p o l a ri t y a n d t e m p o r a l p o s i t io nw i t h i n t h e o v e r a l l E R P w a v e f o r m ( e . g . , P 2 f o r t h e s e c o n dp o s i t i v e c o m p o n e n t , o r P 2 1 5 t o i n d i ca t e a p r ec i se p eak

1This division of cognitive processing into perceptual a ndpostperceptual stages is crude because i t cannot easily accom mo -date factors such as top-down influences on perception and the useof partial information by decision and response processes. Sim i-larly, give n the possibility that a stimulus m ay be reinterpreted afterfurther information has been acquired, there may never be aspecific t ime at which a st imulus has been "fully identified."However, these difficult ies can be minimized by defining the termsperceptual and postperceptual in a task-specific manner. Specifi-cally, the completion of perception can be defined as the point atwhich the ob serv er has extracted the stim ulus attributes that are tobe reported fo r the specific task being performed. Acco rding to thisdefinition, a picture of a red spoon would be considered fullyidentified in a color discrimination task as soon as its color wasexplicitly represented, but this same picture would not be consid-ered fully identified until i ts name b ecam e available in a namingtask or until i ts mem bership in the si lverware category wa sdetermined in a categorization task.


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16 58 VOGEL, LUCK, AND SHAPIRO

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A I ' / . > - ' 7t I I, / **S+ 1 1 . z V P 1 , ' ~ ~ . , . s "

At tended Loc fa2 tion / ' 5 " 3 "St imulus"He go t i n t he c a r andt u r n e d o n t h e M U S T A R D "

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F e m a l e N a m e s ~ . -. , '(Ra re ) - , . . . . __ . / "p ' j "I I I I I I I-100 0 100 200 300 400 500T i m e ( in m s ) r e l a t i v e t o s t i m u l u s o n s e t

Figure 1 . Example event-related potential (ERP) waveforms.No te that negative is plotted upward and that Tim e 0 represents theonset of the ERP-elicit ing stimulus. A: Exam ple of the effects ofspatial attention on the P1 and N1 components. B : Example o f theeffects of semantic mismatch on the N400 com ponent. C: Exampleof the effects of stimulus probability on the P3 com ponent.

l a t e n c y o f 2 1 5 m s p o s t s t i m u l u s ) . B y d e t e r m i n i n g w h i c hco mp o n en t s a r e i n f l u en ced b y a t t en t i o n , i t i s p o ss ib l e t od e t e rm in e wh e th er a t t en t i o n b eg in s t o o p era t e a t an ea r l y o r al a t e s t ag e .S e v e r a l E R P s t u di e s o f v i s u o s p a t i a l a tt e n ti o n h a v e d e m o n -s t r a te d t h a t - - u n d e r a p p r o p r i a t e c o n d i t i o n s - - a t t e n t i o n i n fl u -e n c e s p r o c e s s i n g w i t h i n 1 0 0 m s o f s t i m u l u s o n s e t ( f o rr ev i ews , s ee Hi l l y a rd e t a l . , 1 9 9 6 ; Hi l l y a rd , M an g u n ,Wo ld o r f f , & Lu ck , 1 9 9 5 ; Lu ck , 1 9 9 8 ) . As i l l u s t r a t ed i nF ig u re 1 A, t h e P 1 co mp o n en t i s t y p i ca l l y fo u n d t o b e l a rg e rfo r a t t en d ed - lo ca t i o n s t imu l i t h an fo r i g n o red - lo ca t i o n s t imu l i ,a n d t h i s e f f e c t t y p i c a l ly b e g i n s 7 0 - 1 0 0 m s a f t e r s ti m u l u so n se t ( see , e .g . , Lu ck e t a l . , 1 9 9 4 ; M an g u n , Hi l l y a rd , &Lu ck , 1 9 9 3 ; Nev i l l e & Law so n , 1 9 87 ). N o tab ly , t h e same P 1a t t en t i o n e f f ec t i s o b se rv ed fo r b o th t a sk - r e l ev an t t a rg e ts t imu l i an d fo r t a sk - i r r e l ev an t n o n t a rg e t s t imu l i t h a t b ea r n or e s e m b l a n c e t o t h e t a r g e t s ( H e i n z e , L u c k , M a n g u n , &Hi l l y a rd , 1 9 9 0 ; Lu ck e t a l ., 1 9 9 3 ) . Th e ea r l y o n se t l a t en cy o ft h i s e f f ec t an d i t s i n sen s i t i v i t y t o t h e i d en t i t y o f t h eERP -e l i c i t i n g s t imu lu s p ro v id e s t ro n g ev id en ce t h a t v i su o -sp a t i a l a t t en t i o n b eg in s t o o p era t e a t an ea r l y s t ag e , b e fo res t imu lu s i d en t i f i ca t i o n i s co mp le t e . I n ad d i t i o n , a p o s i t ro n

e m i s s i o n t o m o g r a p h y s t u d y h a s s h o w n t h a t t h e P 1 a t t e nt i o ne f f e c t i s a c c o m p a n i e d b y a c h a n g e i n b l o o d f l o w i n t h ev en t r a l o cc ip i t a l l o b e (He in ze e t a l . , 1 9 9 4 ) , an d a s i n g l e -u n i tr e c o r d i n g s t u d y h a s d e m o n s t r a t e d s i m i l a r a t t e nt i o na l m o d u -l a t io n s i n a r e a s V 2 a n d V 4 o f m o n k e y v i s u a l c o r t e x ( L u c k ,Ch e l azz i , H i l l y a rd , & Des imo n e , 1 9 9 7 ) . To g e th er , t h eses tu d i es p ro v id e s t ro n g ev id en ce t h a t v i su o sp a t i a l a t t en t i o nb eg in s t o o p era t e a t an ea r l y s t ag e , a t l eas t u n d er so meco n d i t i o n s .

A L a t e L o c u s o f T i m e - B a s e d S e l e c t io n ?Al th o u g h t h e re i s su b s t an t i a l ev id en ce i n d i ca t i n g t h a tsp ace-b ased se l ec t i o n b eg in s a t an ea r l y s t ag e , t h e cu r r en te v i d e n c e s u g g e s t s t h a t t i m e - b a s e d s e l e c t i o n o p e r a t e s o n l yaf t e r p e rcep t i o n i s co mp le t e , a l t h o u g h t h i s is su e h as r ece iv e d

re l a t i v e ly l it t le s t u d y . Th e m o s t co m p el l i n g r easo n t o p red i c ta l a t e l o c us o f s e l e c t io n i n a t i m e - b a s e d t a s k s u c h a s R S V P i st h a t t h e r e a p p e a r s t o b e n o n e e d f o r s e l e c t i v e p e r c e p t u a lp r o c e s s i n g i n R S V P : T h e v i s u a l s y s t e m a p p e a r s to b e a b l e t oid en t i fy s t imu l i a t su ch a r ap id r a t e t h a t t h e re i s t y p i ca l l y n on eed t o l imi t p e rcep tu a l p ro cess in g t o a su b se t o f t h ein co min g s t imu l i . F o r ex amp le , P o t t e r (1 9 7 6 ) fo u n d t h a tu n t r a in e d o b s e r v e r s c o u l d a c c u r a t e l y e x t r a c t t h e m a i n t h e m ef r o m p i c t u r e s o f c o m p l e x r e a l - w o r l d s c e n e s p r e s e n t e d a tr a t e s o f m o r e t h a n 8 p i c tu r e s p e r s e c o n d . T h e s e s c e n e s w e r em u c h m o r e c o m p l e x t h a n t h e s i m p l e a l p h a n u m e r i c c h a r a c -t e r s o r wo rd s t y p i ca l l y u sed i n RS VP t ask s . I t i s t h e re fo rer e a s o n a b l e t o s u p p o s e t h a t t h e v i s u a l s y s t e m i s c a p a b l e o ff u l l y i d e n t i f y i n g e v e r y i t e m i n a t y p i c a l R S V P s e q u e n c e .T h u s , t h e r e w o u l d s e e m t o b e n o r e a s o n f o r t h e u s e o f ap e r c e p t u a l - l e v e l a t t e n t i o n a l m e c h a n i s m i n m o s t t i m e - b a s e da t t en t i o n t a sk s . P o t t e r d id , h o wev e r , i n d i ca t e t h a t a t t en t i o n a lse l ec t i o n sh o u ld b e imp o r t an t a t a l a t e r s t ag e . S p ec i f i ca l l y ,s h e f o u n d t h a t o b s e r v e r s b e c a m e i n a c c u r a t e a t h i g h r a te s o fp r e s e n t a t io n i f t h e t a s k r e q u i r e d t h e m n o t o n l y t o p e r c e i v et h e p i c tu r e s b u t a l s o t o s t o re t h e m i n w o r k i n g m e m o r y . T h i ss u g g e s t s th a t w o r k i n g m e m o r y i s o v e r l o a d e d i n R S V P t a s k san d t h a t a t t en t i o n a l se l ec t i o n may p l ay an imp o r t an t ro l e i nd e t e r m i n i n g w h a t i n f o r m a t i o n i s e n c o d e d o r c o n s o l i d a te d i nw o r k i n g m e m o r y .M o r e d i r e c t e v i d e n c e f o r a p o s t p e r c e p t u a l l o c u s o fse l ec t i o n i n t h e a t t en t i o n a l b l i n k p a rad ig m was p ro v id ed b yS h ap i ro , Dr iv e r , Ward , an d S o ren sen (1 9 9 7 ) , wh o u sed anR S V P t a s k w i t h t h re e t a r g e ts ( T 1 , T 2 , a n d T 3 ). W h e n T 2 w a sp resen t ed d u r in g t h e a t t en t i o n a l b l i n k p e r i o d t r i g g ered b yT l - - a n d w a s t h e r e f o r e n o t c o r r e c t l y d i s c r i m i n a t e d b y t h eo b s e r v e r s - - T 2 s t i l l s i g n i f i c a n t l y p r i m e d T 3 . T h i s p r i m i n gef f ec t i n d i ca t es t h a t T2 was i d en t i f i ed ev en t h o u g h t h eo b serv er s co u ld n o t accu ra t e ly r ep o r t i t . Th e mo s t p l au s ib l eex p l an a t i o n ~ o r t h i s f i n d in g i s t h a t t h e v i su a l sy s t e m fu l l yi d e n ti f ie d e v e r y i t e m i n t he R S V P s t r e a m b u t t h a t T 2 w a s n o ts t o r e d in a r e t r i e v a bl e f o r m i n w o r k i n g m e m o r y , p r e s u m a b l yb e c a u s e t h e w o r k i n g m e m o r y s y s t e m w a s b u s y w i t h T 1 .Becau se T2 was fu l l y i d en t i f i ed , h o wev er , i t was ab l e t oc a u s e p e r c e p t u a l - l e v e l p r im i n g o f T 3 ( s e e a l s o S h a p i r o,Ca ld w el l , & S o ren sen , 1 9 9 7 ) . S imi l a r ly , M ak i e t a l . (1 9 9 7 )fo u n d t h a t d i s t r ac to r s t imu l i p r esen t ed d u r in g t h e a t t en t io n a lb l i n k p e r i o d c a n p r i m e a s u b s e q u e n t l y p r e s e n te d T 2 s t im u -


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P O S T P E R C E P T U A L L O C U S O F S U P P R E S S IO N 1659lus , and th i s a l so ind ica tes tha t s t imul i a r e f u l ly iden t i f i eddur ing the a t ten t iona l b l ink .A l though the f ind ings of Shapi r o , D r iver , e t a l . ( 1997)i n d i c a t e t h a t s o m e i n f o r m a t i o n a b o u t T 2 i d e n t i t y w a sext r ac ted by the v i sua l sys tem dur ing the a t ten t iona l b l inkp e r i o d , t h e s e f i n d i n g s d o n o t c o m p l e t e l y r u l e o u t t h eposs ib i l i ty tha t the a t ten t iona l b l ink i s a t l eas t pa r t i a l lya t t r ibu tab le to an ea r ly se lec t ion mec hanism. Spec i f ica l ly , i ti s poss ib le tha t some per ceptua l - leve l suppr es s ion w aspr esen t dur ing the a t ten t iona l b l ink , bu t no t enough tocom ple te ly e l imina te pr iming on ev er y t ri a l . This poss ib i l i tycould no t be addr es sed b y Shapi r o , D r iver , e t a l. because thee x p e r i m e n t a l d e s i g n d i d n o t p e r m i t a c o m p a r i s o n o f t h em a g n i t u d e o f p r i m i n g d u r i n g t h e a t t e n t i o n a l b l i n k p e r i o dw i t h t h e m a g n i t u d e o f p r i m i n g o u t s i d e o f t h is p e r i o d . T h u s ,a l though the r e a r e sever a l good r easons to suspec t tha t thea t ten t iona l b l ink r e f lec t s a pos tper ceptua l suppr es s ion ofp r o c e s s in g , u n a m b i g u o u s e v i d e n c e f o r t h i s h y p o t h e s i s h a sn o t y e t b e e n p r o v i d e d . T h e m a i n p u r p o s e o f t h e p r e se n t s t u d yw a s t o p r o v i d e t h is e v i d e n c e .

O v e r v i e w o f th e P r e s e n t S t u d yI n t h e p r e s e n t s tu d y , w e a p p l i e d t h e E R P t e c h n i q u e t o t h ea t ten t iona l b l ink par ad igm to de te r mine the s tage a t w hichpr ocess ing i s impa i r ed dur ing the a t ten t iona l b l ink . Mor es p e c i f i c a l l y , w e e x a m i n e d t h e E R P w a v e f o r m s e l i c i t e d b ys t imul i p r esen ted dur ing the b l ink pe r iod to es tab l i sh w hichE R P c o m p o n e n t s , a n d t h u s w h i c h p r o c e s s i n g s t a g e s , a r ea f f e c te d b y t h e a t t en t i o n a l b li n k . T h e s p e c if i c E R P c o m p o -nents tha t w er e ex am ined a r e i l lus t r a ted in F igur e 1 . The f i rs te x p e r i m e n t e x a m i n e d t h e P 1 a n d N 1 c o m p o n e n t s , w h i c hr e f le c t p e rc e p t u a l p r o c e s si n g : T h e y h a v e a n e a r l y o n s e t t i m e ,a r e sens i t ive to s t imulus pa r amete r s such as br igh tness andspa t ia l f r equency , and a r e evoked obl iga tor i ly r egar d les s oftask . A s sh ow n in F igur e 1A , these comp onen ts a r e typ ica l lysuppr es sed f or s t imul i p r esen ted a t ignor ed loca t ions inspa t ia l a t t en t ion exper iments ( Cla r k & H i l lya r d , 1996; Lucke t a l . , 1994; Mangun & H i l lya r d , 1991) . I n the f i r s te x p e r i m e n t , w e e x a m i n e d t h e s e c o m p o n e n t s d u r i n g a nR S V P t a s k t o d e t e r m i n e w h e t h e r s e n s o r y p r o c e s s in g w o u l dbe s im i la r ly suppr es sed dur ing the a t ten t iona l b l ink .F i g u r e 1 B i l l u s t r a t e s t h e N 4 0 0 c o m p o n e n t , w h i c h w a se x a m i n e d i n E x p e r i m e n t 2 . T h e N 4 0 0 c o m p o n e n t is h i g h l ys e n s i ti v e t o t h e d e g r e e o f m i s m a t c h b e t w e e n a w o r d a n d a

pr ev ious ly es tab l i shed semant ic contex t ( Besson , K utas , &V an Pe t ten , 1992; K utas & H i l lya r d , 1980a ; K utas , V anPe t ten , & Besson , 1988; O s te r hou t & H olco mb , 1995; Rugg ,1 9 8 4 ). F o r e x a m p l e , a l a r g e N 4 0 0 w o u l d b e e l i c i t e d b y t h el a s t w o r d o f t h e s e n te n c e " H e g o t i n t h e c a r a n d t u r n e d o nt h e m u s t a r d " b u t n o t b y t h e la s t w o r d o f th e s e n t e n c e " H eg o t i n t h e c a r a n d t u r n e d o n t h e r a d i o . " B e c a u s e a w o r d m u s tb e i d e n t i f i e d b e f o r e i t s m e a n i n g c a n b e c o m p a r e d w i t h ap r e v i o u s l y e s t a b l is h e d s e m a n t i c c o n t e x t , t h e p r e s e n c e o f a nN 4 0 0 c o m p o n e n t c a n b e u s e d t o i n d i c a t e t h a t a w o r d h a sb e e n i d e n t i f i e d . T h u s , i n E x p e r i m e n t 2 w e s o u g h t t od e t e r m i n e w h e t h e r w o r d s p r e s e n t e d d u r i n g t h e a t t e n t i o n a lb l i n k p e r io d w o u l d e l i c i t a n N 4 0 0 c o m p o n e n t , w h i c h w o u l d

i n d i c at e t h a t t h e y w e r e i d e n t if i e d e v e n t h o u g h t h e y c o u l d n o tb e r e p o r te d .I n t h e f i n a l e x p e r i m e n t , w e e x a m i n e d t h e P 3 w a v e . A si l lus t r a ted in F igur e 1C, the ampl i tude of the P3 w ave i sinver se ly r e la ted to the pr obabi l i ty of the ta sk- def inedc a t e g o r y o f t h e E R P - e l ic i t i n g s t im u l u s ( s e e t h e r e v i e w b yJohnson , 1986) . For example , i f obse r ver s a r e asked toc a t e g o r i ze n a m e s a s m a l e a n d f e m a l e , a n d i f m a l e n a m e s a r ep r e s e n t e d m o r e f r e q u e n t l y t h a n f e m a l e n a m e s , t h e n t h ef emale nam es w i l l e l i c i t l a r ge r P3 w aves than w i l l the malenames . I t i s impor tan t tha t P3 ampl i tude i s s ens i t ive to thepr obab i l i ty of the ca tegor y , as de f ined by the ta sk , r a the r thanto the pr obabi l i ty of the ind iv idua l s t imulus . N ote , how ev er ,t h a t th e P 3 c o m p o n e n t d o e s n o t a p p e a r t o r e f l e c t t h eca tegor iza t ion pr ocess pe r s e bu t ins tead r e f lec t s a p r ocesstha t f o l low s ca tegor iza t ion . Spec i f ica l ly , the lead ing hy poth -es i s o f the P3 w ave pr op oses tha t i t r e f lec t s the upda t ing ofw o r k i n g m e m o r y ( D o n c h i n , 1 9 8 1 ; D o n c h i n & C o l e s , 1 9 8 8 ).Becau se w e hy pothe s ized tha t the a t ten t iona l b l ink re f lec t sa n i m p a i r m e n t a t t h e s t a g e o f w o r k i n g m e m o r y , w e p r e d i c t e dt h a t th i s c o m p o n e n t w o u l d b e s u p p r e s s e d d u r i n g t h e a t t e n -t iona l b l ink in te r va l .

E x p e r i m e n t 1The f i r s t exper iment t e s ted the hypothes i s tha t the a t ten-t iona l b l ink r e f lec t s a suppr es s ion o f s ensor y pr ocess ing . Wet e s t e d t h i s h y p o t h e s i s b y e x a m i n i n g t h e e f f e c t s o f t h ea t ten t iona l b l ink on the P1 and N 1 w aves , w hich a r e the f i r stm a j o r p o s i t i v e a n d n e g a t i v e E R P c o m p o n e n t s t y p i c a l l ye l ic i t ed by a v i sua l s t imulus . These components r e f lec tsensor y pr ocesses and a r e pr imar i ly sens i t ive to the phys ica l

char ac te r i s t i c s o f the e l ic i t ing s t imulus , such as br igh tness( H i l lya r d & P ic ton , 1987) . N umer o us s tud ies hav e f oundt h a t t h e s e t w o c o m p o n e n t s c a n b e m o d u l a t e d b y s p a t i a la t ten t ion ( Cla r k & H i l lya r d , 1996; Luck e t a l . , 1994;Mangun & H i l lya r d , 1991) . Tha t i s , s t imul i p r esen ted a tignor ed loca t ions e l ic i t smal le r P1 and N 1 w aves thans t imul i p r esen ted a t a t t ended loca t ions . I f the same mecha-n isms of a t t en t ion a r e a l so r espons ib le f or the a t ten t iona lb l i nk , s m a l l e r P 1 a n d N 1 c o m p o n e n t s s h o u l d b e o b s e r v e d f o rs t imul i p r esen ted dur ing the a t ten t iona l b l ink per iod than f o rs t imul i p r esen ted outs ide of th i s pe r iod . H ow ever , i f thea t ten t iona l b l ink oper a tes a t a l a te r p r ocess ing s tage , nos u p p r e ss i o n o f t h e P 1 a n d N 1 w a v e s s h o u l d b e o b s e r v e ddur ing the a t ten t iona l b l ink .T h e u s e o f E R P r e c o r d i n g s in R S V P t a s k s l e a d s t o s o m etechnica l d i f f icu l t ie s tha t p lace im por tan t cons t r a in t s on theexper im enta l des ign . I n pa r ti cu la r , each i t em pr esen ted in anR S V P s t r e a m p r o d u c e s a n E R P r e s p o n s e t h a t l a s t s f o rsever a l hundr ed m i l l i s econds a f te r s t imulus on se t , long pas tthe onse t o f the next s t imulus in the s t r eam. Consequent ly ,t h e E R P e l i c it e d b y a g i v e n i t e m w i l l b e o v e r l a p p e d b y t h eE R P s e l i c it e d b y p r e v i o u s a n d s u b s e q u e n t i t e m s , m a k i n g i td i f f icu l t to i so la te the ER P f or each ind iv idua l s t imulus . Th isi s i l lus tr a ted in F igur e 2 . We e l imin a ted th i s over lap pr ob lemi n t h e p r e s e n t e x p e r i m e n t b y u s i n g a n irrelevant-probetechnique t h a t w a s d e v e l o p e d p r e v i o u s l y i n E R P s t u di e s o fspa t ia l a t ten t ion ( H e in ze e t a l. , 1990; Luc k e t a l ., 1993; Lu ck


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1660 V O G E L , L U C K , A N D S H A P I R ODletractors T 1 T 2(B l ue) ~ (B l ue) (Red)

A3NPN RKMB T D A 3 N P Im F R K M

P r o b e F l e s h(White)Sequence A lo n e ~ Responses to Ind lvldual St lmul i

"quence+nee, .

(Sequence + F l e s h l - S e q u e n c e A l o n e = FleshFigure 2. Example of rapid serial visual presentation stream usedin Experiment 1. All items were successively presented at fixation.Half the trials had a probe present behind the second target (T2);the other half had no probe present. The event-related potentia loverlap problem is simulated in the lower portion o f the figure.& Hillyar d, 1995) Specifically , a task-irr elev ant whitesquare was flashed behind T2 on some trials, and the ERPelicited by this irrelevant probe flash was used as a measu reof sensory processing at the time of T2. A subtractiontechniqu e was used to separate the ERP elicited by the probeflash from the ERPs elicited by the other items in the RSVPstream, as illustrated in Figure 2. To accomplish this, a probeflash was presente d simulta neousl y with T2 on hal f of thetrials, and T2 was presented without a probe flash on theother half.

The probe t echniqu e is based on the fact that voltage fieldssummate linearly. The ERP waveform recorded on probe-present trials reflects the sum of the response to the probestimulus and the response to the other items in the RSVPsequence, whereas the ERP waveform recorded on probe-absent trials simply reflects the response to the other itemsConsequently, the response to the probe stimulus can becomputed by creating a di f ference waveform in which theERP wave form on prob e-absen t trials is subtracted from theERP wavefo rm on probe-pre sent trials. 2 This t echnique hasbeen used in several previous studies of spatial attention,which have shown that the probe-elicited P1 and N1components are suppressed when the probe appears at anignored location relative to when the probe appears at anattended location (Luck et al., 1993; Luck & Hillyard,1995)M e t h o d

Participants All participants in this study were right-handed,neurologically normal college-student volunteers between 18 and

30 years old who were paid or received course credit for theirparticipation. All had normal or corrected-to-normal acuity andreported normal color vision Twenty individual s participated inExperiment 1.Stimuli. The stimuli, which are illustrated in Figure 2, werepresented on a computer-controlled video monitor placed 70 cmfrom the participant. Each trial consis ted of an RSVP stream of 19letters and a single digit. Each character i n the RSVP stream waspresented for 33 ms, and successive characters were separated by ablank interstimulus interval of 50 ms, yielding a presentation rateof approximately 12 characters per second. All characters weredisplayed individually at the center of a gray screen (67 cd/m2).The characters were 0.8 in height and width. The RSVP stream foreach trial consisted of 18 nontargets and 2 targets (T1 and T2).The nontargets in each stream were randomly selected uppercaseletters ( A-Z, with the exception of Y) drawn in blue (CIE colorcoordinates: x = .147, y = .067). T1 was a digit (2-9) , also drawnin blue. Like the nontargets, T2 was an uppercase letter, but it wasdrawn in red (CIE color coordinates: x = 636, y = .344). T1 wasequally likely to be an odd or an even digit, and T2 was equallylikely to be a vowel or a consonant. On half the trials, a solid whiteprobe square (1 1; 63.2 cd/m2) was presented behind T2, withthe same onset and offset time as T2.Procedure T1 was either the 7th or the 10th character pre-sented on a given trial. T2 was always the 1st, 3rd, or 7th characterpresented after T1 (denoted as Lag 1, Lag 3, and Lag 7,respectively). These three lags were used because they typicallyprovide the most relevant data points in the attentional blinkfunction, with little or no impairment at Lags 1 and 7 and the mostsevere impairment at Lag 3. Five hundred milliseconds after theoffset of the final character in the RSVP stream, a question markappeared for 1,000 ms, signaling the part icipant to respond. Thenext trial began 1,000 ms after the offset of the question mark.Two task conditions were used: single target and dual target. Inthe dual-target condition, the participants were instructed to makediscriminative responses to both T1 and T2 at the end of each trial.Specifically, they made a two-alternative forced-choice response toindicate whether T1 was an even or odd number and a secondtwo-alternative forced-choice response to indicate whether T2 wasa vowel or consonant. They were encouraged to make the T1response before the T2 response, and although either order wasallowed, the T1 response was almost always first. In the single-target condition, the participants were instructed to ignore T1 andto make only the vowel-consonan t discrimination response at theend of the trial. Each part icipant received six blocks of 96 trials ineach condition; the two conditions were run in counterbalancedorder.Recording and analysis. Electroencephalographic (EEG) activ-ity was recorded from tin electrodes mounted in an elastic cap,located at standard left- and right-hemisphere positions overfrontal, central, parietal, occipital, and temporal areas (Interna-tional 10/20 System sites F3, F4, C3, C4, P3, P4, O1, 02, T3, T4,T5, T6). Two nonstandard sites were used in addition: OL (halfwaybetween O1 and T5) and OR (halfway between 02 and T6). Thesesites and a right-mastoid site were recorded with a left-mastoidreference, and the data were re-referenced offline to the algebraicaverage of the left and right mastoids. The horizontal electrooculo-

2 The presence of the probe stimulus may alter the processing ofthe other items in the RSVP stream, and the difference wavecreated by this subtraction process therefore reflects ERP activ itycorresponding to this stimulus-stimulus nteraction as well as theresponse to the probe. However, this does not affect the conclusionsof this study in any way.


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POSTPERCEPTUAL LOCUS OF SUPPRESSION 16 61100 - ~ S ing le , No Probef

0" "11 - - ~ - - - O S ing le ProbeO . N o ,

0 Dual Probe


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16 62 VOGEL, LUCK, AND SHAPIROc o n d i ti o n . S p e c i f ic a l ly , s e p a r a t e A N O V A s w e r e c o n d u c t e df o r t h e P 1 a n d N 1 w a v e s w i t h t h e f a c t o r s o f c o nd i ti o n , T 2l ag , an d e l ec t ro d e l o ca t i o n . No s i g n i f i can t main e f f ec t s o ri n t e r ac t i o n s i n v o lv in g t h e co n d i t i o n o r l ag f ac to r s wereo b t a i n e d f o r e i t h e r c o m p o n e n t . F o r b o t h c o m p o n e n t s , t h eo n ly s i g n i f i can t e f f ec t was a m ain e f f ec t o f e l ec t ro d e s i te :F(7 , 119) = 11 .33 , p < .005 , fo r the P1 com pon ent , andF (1 3 , 2 2 1 ) = 3 6 .0 9 , p < .0 0 1 , fo r t h e N1 co m p o n e n t ( t h eses ig n i f i can t e f f ec t s s imp ly r e f l ec t t h e we l l - es t ab l i sh ed f ac tt h a t t h e P 1 a n d N 1 c o m p o n e n t s v a r y i n a m p l i t u d e a c r o s ssca lp s i te s ) . I t i s imp o r t an t t h a t t h e Co n d i t i o n La gin t e r ac t i o n e f f ec t t h a t d e f i n es t h e a t t en t i o n a l b l i n k d id n o tap p ro a ch s ig n i f i can ce fo r e i t h e r co m p o n en t (F s < 1 , p s > .5) .Di s c u s s i on

In t h i s ex p er imen t , a n o rmal a t t en t i o n a l b l i n k p a t t e rn waso b s e r v e d i n t h e b e h a v i o r a l a c c u r a c y m e a s u r e s . T h a t i s , T 2d i s c r i m i n a t i o n a c c u r a c y i n t h e d u a l - t a s k c o n d i t i o n w a sre l a t i v e ly h ig h a t Lag s 1 an d 7 , b u t i t d ec rea sed s i g n i f i can t l ya t L a g 3 . H o w e v e r , d e s p i t e t h e s u b s t a n t ia l i m p a i r m e n t i n T 2a c c u r a c y a t L a g 3 , t h e r e w a s n o s u p p r e s s i on o f t h e P 1 o r N 1c o m p o n e n t s e l i c i te d b y t h e p r o b e f l a s h t h at a p p e a r e d c o n c u r -r en t l y wi th T2 . Th i s f i n d in g i s co n s i s t en t w i th t h e h y p o th es i st h a t t h e a t t en t i o n a l b l i n k r e f l ec t s a r e l a t i v e ly l a t e su p p ress io no f i n fo rma t io n an d s t an d s i n sh arp co n t r as t t o t h e r esu l ts o fs p a t i al a t te n t i on e x p e r i m e n t s , i n w h i c h t h e P 1 a n d N 1 w a v e sare t y p i ca l l y su p p ressed fo r s t imu l i p r esen t ed a t i g n o redlo ca t i o n s ( e .g . , E lmer , 1 9 9 4 ; Lu ck e t a l . , 1 9 9 4 ; M an g u n &Hil lyard , 1990 , 1991) .I t i s u n l i k e ly t h a t t h e ab se n ce o f a s i g n i f ican t su p p re ss io ne f f e c t i n t h i s e x p e r i m e n t w a s c a u s e d b y a l a c k o f p o w e r .F i r s t, t h e e f f ec t o f t h e a t t en t i o n a l b l i n k o n b eh av io ra laccu racy i n t h i s ex p er imen t was a t l eas t a s l a rg e as t h ea t t e n ti o n e f f e c t s o b s e r v e d i n m o s t p r e v i o u s E R P s t u di e s o fsp a t i a l a t ten t i o n , i n d i ca t i n g t h a t t h e a t t en t i o n a l man ip u l a t i o nw a s h i g h l y e f f e c ti v e . S e c o n d, o u r s a m p l e s i z e o f 2 0 p a r t i c i-p a n t s w a s l a r g e r t h a n t h e s a m p l e s i z e s u s e d i n m o s t p r e v i o u sER P s tu d i es o f sp a t i a l a tt en t io n . H o we v er , i t i s a lway sd i f f i cu l t t o d raw co n c lu s io n s o n t h e b as i s o f a s i n g l e n u l le f f e c t . E x p e r i m e n t 2 w a s t h e r e f o r e c o n d u c t e d t o p r o v i d ec o n v e r g i n g e v i d e n c e .

E x p e r i m e n t 2Al th o u g h t h e f i n d in g o f n o P 1 o r N1 su p p ress io n d u r in g

th e a t t en t i o n a l b l i n k i n Ex p er imen t 1 su g g es t s t h a t t h ea t t en t i o n a l b l i n k r e f l ec t s a p o s tp e rcep tu a l su p p ress io n o fi n fo rmat io n , i t i s p o ss ib l e t h a t t h e a t t en t i o n a l b l i n k i n s t eadre f l ec t s su p p ress io n a t a r e l a t i v e ly l a te su b s t ag e o f p e rcep -t i o n , a f t e r t h e o f f se t o f t h e P 1 an d N1 co m p o n en t s . Ex p er i -m e n t 2 w a s d e s i g n e d t o r u l e o u t t h i s p o s s ib i l i ty b y m e a s u r i n gt h e N 4 0 0 c o m p o n e n t e l i c it e d b y w o r d s p r e s e n t e d d u r i n g t h ea t te n t io n a l b l i n k i n t e r v a l a s a m e a n s o f d e t e r m i n i n g w h e t h e rth ese w o rd s w ere i d en t if i ed .T h e N 4 0 0 i s a l a r g e n e g a t i v e c o m p o n e n t t h a t p e a k sa p p r o x i m a t e l y 40 0 m s a f t e r th e o n s e t o f a s t i m u l u s a n dr e f l e c t s t h e d e g r e e o f m i s m a t c h b e t w e e n a w o r d a n d ap r e v i o u s l y e s t a b l i s h e d s e m a n t i c c o n t e x t ( B e s s o n e t a l . ,

1 9 9 2 ; Ku tas & Hi l l y a rd , 1 9 8 0 a ; Ku tas e t a l ., 1 9 8 8 ; Os t e rh o u t& H o l c o m b , 1 9 9 5 ; R u g g , 1 9 8 4 ). F o r e x a m p l e , a la r g e N 4 0 0w o u l d b e e l i c i t e d b y t h e l a s t w o r d i n t h e s e n t e n c e " T h ew o m a n d r o v e t o w o r k i n h e r s h i n y n e w n o s e " b u t n o t b y t h el a s t w o r d i n t h e s e n te n c e " T h e w o m a n d r o v e t o w o r k i n h e rs h i n y n e w c a r . " S i m i l a r r e s u lt s a r e o b t a i n e d f o r s e q u e n t i a ll yp r e s e n t e d w o r d p a i r s , i n w h i c h a n N 4 0 0 i s e l i c i t e d b y t h es e c o n d w o r d i f i t m i s m a t c h e s t h e s e m a n t i c c o n t e x t e s t a b -l i sh ed b y t h e f i r s t wo rd ( e .g . , a l a rg e N4 0 0 i s e l i c i t ed b y t h es e c o n d w o r d i n pick le -rope b u t n o t i n shoe-foot) . B e c a u s et h e m e a n i n g o f a w o r d m u s t b e a c c e s s e d b e f o r e i t c a n b ec o m p a r e d w i t h a s e m a n t i c c o nt e x t , th e p r e s e n c e o f a n N 4 0 0p e a k f o r a m i s m a t c h i n g w o r d i n d i c a t e s t h a t t h e w o r d h a sb e e n i d e n t if i ed t o a p o i n t a t w h i c h s o m e e l e m e n t s o f t h ew o r d ' s m e a n i n g a r e a v a i l a b l e ( a l t h o u g h n o t n e c e s s a r i l y a l la s p e c t s o f i ts m e a n i n g ) . T h e r e f o r e , t h e p r e s e n c e o f a n o r m a lN 4 0 0 c o m p o n e n t a t t h e L a g 3 p o s i t io n w o u l d p r o v i d e s t r o n gev id en ce t h a t wo rd s p resen t ed d u r in g t h e a t t en t i o n a l b l i n kare fu l l y i d en t if i ed , ev en t h o u g h t h e o b se r v er can n o t accu -r a t e l y r e p o r t t h e m . H o w e v e r , i f i n f o r m a t i o n i s s u p p r e s s e db e f o r e p e r c e p t u a l p r o c e s s i n g i s c o m p l e t e d u r i n g t h e a t t e n -t i o n a l b l i n k , t h e N 4 0 0 s h o u l d b e s u p p r e s s e d f o r w o r d sp resen t ed d u r in g t h e b l i n k r e l a t i v e t o wo rd s p resen t edo u t s i d e o f t h e b l in k .A n R S V P p a r a d i g m w a s u s e d f o r t h is e x p e r i m e n t , w i t h ad ig i t ta rg e t a s T1 an d a wo rd t a rg e t a s T2 ( see F ig u re 5 A) . Toi s o l a te t h e N 4 0 0 c o m p o n e n t f o r t h e T 2 w o r d , w e a r t i fi c i al l ye s t a b l is h e d a s e m a n t i c c o n t e x t a t t h e b e g i n n i n g o f e a c h t r i ala n d c o m p a r e d t r ia l s o n w h i c h T 2 m a t c h e d t h is c o n t e x t w i t ht r i a l s o n wh ich T2 mi smatch ed t h i s co n t ex t . S p ec i f i ca l l y , ac o n t e x t w o r d w a s p r e s e n t e d i m m e d i a t e l y b e f o r e e a c h R S V Ps t r eam, an d t h e p a r t i c i p an t s were r eq u i r ed t o r ep o r t wh e th ert h e T 2 w o r d w i t h i n t h e R S V P s t r e a m w a s s e m a n t i c a l l yr e l a t e d o r u n re l a t e d t o t h is c o n t e x t w o r d . T h e N 4 0 0 w a s t h e nm e a s u r e d f r o m d i f f e r e n c e w a v e s i n w h i c h r e l a t e d T 2 t r i a l swere su b t r ac t ed f ro m u n re l a t ed T2 t r i a l s . I t i s imp o r t an t t h a tt h e o n l y d i f f e r e n c e b e t w e e n t h e s e t w o t y p e s o f t r ia l s w a s t h es e m a n t i c r e l a ti o n s h ip b e t w e e n t h e c o n t e x t w o r d a n d t h e T 2w o r d , a n d a n y d e v i a t i o n f r o m z e r o i n t h e d i f fe r e n c e w a v e st h e r e f o r e d e p e n d e d o n t h e a c c u r a t e i d e n t i fi c a ti o n a n d s e m a n -t ic a n a l y s i s o f T 2 . B o t h t h e b e h a v i o r a l t a s k a n d t h e E R Pm e a s u r e m e n t s r e f l e c t e d t h e s a m e r e l a t e d v e r s u s u n r e l a t e dseman t i c d i sc r imin a t i o n . No te t h a t t h e u se o f d i f f e r en cew a v e s i n t h i s e x p e r i m e n t a l s o r e m o v e d t h e o v e r l a p p i n gs e n s o r y E R P s f r o m t h e p r e v i o u s a n d s u b s e q u e n t s t i m u l i ,w h i c h w e r e t h e s a m e o n r e l a t e d T 2 t r ia l s a n d u n r e l a te d T 2trials.Me t h o d

Participants. Fourteen individuals participated in this experi-ment. They were drawn from the same population used inExperiment 1 except that they were also required to be nativeEnglish speakers.Stimuli. As shown in Figure 5A, each trial began with thepresentation of a context w ord for a duration of 1,000 ms, followedby a b lank interval o f 1 ,000 ms. A stream of 20 seven-characterswings of letters or numbers was then presented at a ra te of 12 itemsper second. Either the 7th or the 10th string served as T1, and thisswing consisted of a randomly selected digit (2-9), repeated seven


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POSTPERCEPTUAL LOCUS OF SUPPRESSION 1663word and an unrelated word and appeared in both conditions of theexperiment.Procedure. In the dual-target condition, the participants wereinstructed to identify the number string and the red word when theyappeared i n the RSVP stream and to make 2 two-alternativeforced-choice responses at the end of each trial. These responsesindicated whether the number string was even or odd and whetherthe red word was semantically related or unrelated to the contextword presented at the beginning of that trial. T1 was equally likelyto be an even or an odd number and T2 was equally likely to berelated or unrelated to the context word. A single-target condi tionwas also used in this experiment in which the participants wereinstructed to perform only the T2 word task. Each participantreceived six 60-1rial blocks in the dual-target condition and six60-trial blocks in the single-target condition (in counterbalancedorder).Recording and analysis. The recording and general analyticprocedures were the same as those used in Experiment 1. Artifactsled to the rejection of a mean of 17% and a maximum of 24% of thetrials for a given participant.The averaged ERP waveforms were time locked to the onset ofthe T2 word. Difference waves were constructed by subtracting theERP waveforms elicited by T2 words that were related to thecontext word from the ERP waveforms elicited by T2 words thatwere unrelated to the context word. The N400 component wasmeasured from these difference waves as the mean amplitude300-500 ms poststimulus, relative to a 200-ms prestimulus base-line.4 Measurements were obtained at the frontal, central, andparietal electrodes (F3, Fz, F4, C3, Cz, C4, P3, Pz, and P4). As inExperiment 1, all behavioral and ERP analyses for T2 were l imitedto trials on which T1 was correctly discriminated.R e s u l t s

Figure 5. A: Example stimuli from Experiment 2. B: Meandiscrimination accuracy for the second target (T2) word as afunction of lag for the single- and dual-target conditions inExperiment 2.

times to create a seven-character string. The nontargets in thestream consisted of seven-character strings consisting of randomlyselected consonants. All character strings subtended 4.9 0.8* ofvisual angle, and all were presented in blue except for T2, whichwas presented in red. At the end of each trial, a question markappeared, signaling he participant to respond. Stimulus timing wasidentical to that in Experiment 1.T2 was a word of three to seven characters, drawn in red. Wordsless than seven characters long were flanked by Xs to create aseven-character string. The T2 word was always the first, third, orseventh string presented after T1 (i.e., Lag 1, Lag 3, or Lag 7). Onhalf the trials, the T2 word was semantically related to the contextword (e.g., doctor-nurse). On the other half, the T2 word was notsemantically related to the context word (e.g., doctor-chicken).Each related word pair was randomly selected from a pool of 360highly related word pairs (Postman & Keppel, 1970), and eachunrelated word pair was created by selecting words at random fromthese pairs. 3 Each word was presented twice (once per condition),once within a related word pair and once within an unrelated pair.The ordering of the word pairs was counterbalanced for eachparticipant so that a word used in a related pair in the dual-targetcondition would be used again in an unrelated pair in thesingle-target condition. Thus, each T2 word served as both a related

Behavior . Mean T 2 discrimination accuracy is plotted asa function of T2 lag in Figure 5. Asymptotic accuracy wassomewhat higher in the single-target condition than in thedual-target condition. Within the single-target condition,there appeared to be no effect of lag. In the dual-targetcondition, however, there was a substantial decrease inaccuracy for Lag 3 compared with Lags 1 and 7. Atwo-factor ANOVA was conducted with condition (singlevs. dual target) and lag (Lag 1, Lag 3, or Lag 7) as repeatedmeasures factors, and this analysis revealed a significantmain effect of condition, F(1, 13) = 138.51, p < .001, asignificant main effect of lag, F(2, 26) = 59.00, p < .001,and a significant Condi tion Lag interaction, F(2, 26) --65.71, p < .001. The mean accuracy for the T1 discr imina-

3 Because unrelated pairs were created by selecting words atrandom, it is possible that some of the word pairs created in thismanner were actually somewhat semantically related. However,such occurrences were rare (if they happened at all) and wouldhave simply decreased overall accuracy and N400 ampli tude onunrelated trials. Any such effects would have been small and wouldhave been equivalen t across the experimental conditions, leading tono impact on the conclusions drawn from this experiment.4 This measurement window was selected to minimize anypotential contributions from the P3 component, which typicallyfollows the N400 component in an experiment of this nature. TheN400 was also measured with wider windows, and the results wereessentially the same.


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1 6 6 4 V O G E L , L U C K , A N D S H A P IR Ot i o n w a s 9 3 % c o r r e c t , w i t h n o s i g n i f i c an t m a i n e f f e c t o f T 2l a g ( F < 1 ) .Electrophysiology. T h e d i f f e r en c e w a v e f o r m s f o r b o tht h e s i n g l e - a n d d u a l - t a r g e t c o n d i t i o n s a r e p lo t t e d a s af u n c t io n o f T 2 l a g i n F i g u r e 6 . T h e w a v e f o r m s c o n s i s t e dp r i m a r i l y o f a s i n g l e l a r g e d e f le c t i o n w i t h t h e u s u a l c h a r a c t e r -i s t ic s o f t h e N 4 0 0 c o m p o n e n t ( K u t a s & H i l l y a r d , 1 9 8 0 b ,1 9 8 3 ; K u t a s e t a l. , 1 9 8 8 ) . S p e c i f i c a l l y , i t w a s a n e g a t i v e -g o i n g d e f l e c t io n , p e a k i n g a t a p p r o x i m a t e l y 4 0 0 m s , w i t h ac e n t r o p a r i e t a l d i s t r i b u t i o n a n d s l i g h t l y g r e a t e r a m p l i t u d eo v e r th e r i g h t h e m i s p h e r e , e s p e c i a l l y a t th e a n t e r i o r e l e c -t r o d e s i t e s . 5

A s s h o w n i n F i g u r e 6 , t h e m e a n N 4 0 0 a m p l i t u d e w a sg e n e r a l l y s m a l l e r i n t h e d u a l - t a r g e t c o n d i t i o n t h a n i n t h es i n g l e - t a r g e t c o n d i t i o n . H o w e v e r , t h e r e w a s n o o b v i o u se f f e ct o f la g f o r e i th e r c o n d i ti o n . A t h r e e - w a y A N O V A o nN 4 0 0 a m p l i t u d e w i t h t h e f a c t o r s o f c o n d i t i o n , l a g , a n de l e c t r o d e s i te y i e l d e d a s i g n i f ic a n t m a i n e f f e c t o f c o n d i t i o n ,F ( 1 , 1 3 ) = 8 . 7 0 , p < . 0 1 , c o r r e s p o n d i n g t o t h e g e n e r a l l yg r e a t e r N 4 0 0 a m p l i t u d e s o b s e r v e d i n t h e s i n g l e - t a rg e t c o n d i -t i o n . I n a d d i t i o n , t h i s d i f f e r e n c e w a s g r e a t e r a t s i t e s w h e r et h e N 4 0 0 w a s l a r g e r , w h i c h l e d t o a s i g n i f i c a n t i n t e r a c t i o nb e t w e e n l a g a n d e l e c t r o d e s it e , F ( 8 , 1 0 4 ) = 8 . 9 5 , p < . 0 0 1 .H o w e v e r , t h e m a i n e f f e c t o f la g w a s n o t s i g n if i c a n t ( F < 1 ),n o r w e r e th e L a g X C o n d i t i o n i n t e r a c ti o n , F ( 2 , 2 6 ) = 1 . 02 ,p > . 3 5 , t h e L a g X E l e c t r o d e S i t e i n t e r a c t i o n , F ( 1 6 , 2 0 8 ) =1 . 22 , p > . 3 0 , a n d t h e t h r e e - w a y i n t e r a c t i o n a m o n g l a g ,

Dual-TargetCondit ion

F r o n t a l W . _ F . ~ - , , , ~ ,

C e ntra , , . , . , . , i

P a r i e t a l . - * ~ . - , ~ - . ~ - . . r , , . ~ .1

Single-TargetCondit ion

- 5 1 . t V YL a g 1' ' ' : i , ' , l i, . . . . . . . . . . L a g 3

i I ' 2 0 0 4 0 0 6 0 0 . . . . . . L a g 7+ s V T i m e ( m s )

Figure 6 . G r a n d a v e r a g e e v e n t - r e l a t e d p o t e n t i a l d i f f e r e n c e w a v e -f o r m s f r o m E x p e r i m e n t 2 , f o r m e d b y s u b t r a c t i n g r e l a t e d T 2 t r i a l sf r o m u n r e l a t e d T 2 t r i a l s . T h e s e w a v e f o r m s w e r e r e c o r d e d a tm i d l i n e e l e c t r o d e s i t e s a n d w e r e a v e r a g e d a c r o s s p a r t i c i p a n t s .N e g a t i v e is p lo t t e d u p w a r d . T 2 = s e c o n d t a rg e t .

c o n d i t i o n , a n d e l e c t r o d e s i te ( F < 1 ). T h u s , t h e r e w a s n oe v i d e n c e o f N 4 0 0 s u p p r e s s i o n d u r i n g t h e a t te n t i o n a l b l i n kp e r i o d .D i s c u s s i o n

I n E x p e r i m e n t 2 , w e o b s e r v e d a n o r m a l a t t e nt i on a l b l i n kp a t t e r n f o r T 2 a c c u r a c y . T h a t i s , T 2 a c c u r a c y w a s r e l a t i v e l yh i g h a t L a g s 1 a n d 7 , w i t h a s u b s t a n t i a l d e c r e a s e a t L a g 3 i nt h e d u a l - t a r g e t c o n d i t i o n . H o w e v e r , d e s p i t e t h i s s u b s t a n t i a li m p a i r m e n t i n b e h a v i o ra l p e r f o r m a n c e , t h e r e w a s n o s u p p r es -s i o n o f th e N 4 0 0 c o m p o n e n t a t L a g 3 . T h i s f i n d in g p r o v i d e ss t r o n g e v i d e n c e t h a t t h e T 2 w o r d w a s i d e n t i f i e d t o t h e p o i n to f m e a n i n g e x t r a c t i o n 6 d u r i n g t h e a t t e n t i o n a l b l i n k p e r i o db e c a u s e n o N 4 0 0 a c t iv i t y w o u l d h a v e b e e n p o s s i b l e i n th eu n r e l a t e d - m i n u s - r e l a t e d d i f f e r e n c e w a v e s u n l e s s t h e m e a n -i n g o f th e T 2 w o r d h a d b e e n e x t r a c t e d . T h e a t t e n t i o n a l b l i n kt h e r e f o r e a p p e a r s t o r e f le c t a l o s s o f i n f o r m a t i o n a f t e rs t i m u l u s i d e n t if i c a t io n i s c o m p l e t e , p e r h a p s a t t h e s t a g e o fw o r k i n g m e m o r y .S e v e r a l a l t e r n a t iv e e x p l a n a t i o n s m u s t b e c o n s i d e r e d b e -f o r e a c c e p t i n g t h i s c o n c l u s i o n . F i r s t, i t i s p o s s i b l e t h a t t h ed i f f e r e n c e w a v e s u s e d t o m e a s u r e t h e N 4 0 0 c o m p o n e n t d i dn o t a c t u a l l y r e fl e c t t h e N 4 0 0 c o m p o n e n t b u t i n s t e a d r e f l e c t e da d if f e re n t c o m p o n e n t s u c h a s th e P 3 w a v e o r s o m ec o m b i n a t i o n o f c o m p o n e n t s . H o w e v e r , o u r c o n c l u s io n s d on o t i n a n y w a y d e p e n d o n e s t a b l i s h i n g t h a t t h e a c t i v i t yr e c o r d e d i n th i s e x p e r i m e n t i s t h e s a m e a s th e N 4 0 0 a c t i v i t yo b s e r v e d i n p r i o r e x p e r i m e n t s . T h e s i m p l e f a c t t h a t t h e r ew a s a d i f f e r e n c e b e t w e e n r e l a t e d T 2 t r i a l s a n d u n r e l a t e d T 2t r i a l s d u r i n g t h e a t t e n t i o n a l b l i n k p e r i o d i s s u f f i c i e n t t oi n d i c a t e t h a t t h e T 2 w o r d s w e r e i d e n t i f i e d t o t h e p o i n t o fm e a n i n g e x t r a c t io n . N o t e , h o w e v e r , t h a t th e p o l a ri t y , t i m i n g ,a n d s c a l p d i s t ri b u t i o n o f th e a c t i v i t y o b s e r v e d i n t h i se x p e r i m e n t a r e c o n s i s t e n t w i t h t h e h y p o t h e s i s t h a t t hi sa c t iv i t y c o n si s te d p r i m a r i l y o f t he s a m e N 4 0 0 c o m p o n e n tt h a t h a s b e e n o b s e r v e d p r e v i o u s l y ( K u t a s & H i l l y a r d , 1 9 8 0 b ;K u t a s & H i l l y a r d , 1 9 8 3 ; K u t a s e t a l. , 1 9 8 8 ). W e t h e r e f o r ec o n t i n u e t o re f e r t o t h i s a c t i v it y a s t h e N 4 0 0 c o m p o n e n t .

A s e c o n d a l t e r n a t i v e e x p l a n a t i o n f o r o u r r e s u l ts i s th a t t h ea b s e n c e o f N 4 0 0 s u p p r e s s i o n d u r i n g t h e a t t e n ti o n a l b l i n kr e f l ec t s a l a c k o f s e n s i t i v it y o r p o w e r . S e v e r a l p i e c e s o fe v i d e n c e a r g u e a g a i n s t t h i s p o s s i b i l i t y , h o w e v e r . F i r s t , t h eN 4 0 0 w a s s i g n i f i c a n t l y s m a l l e r i n t h e d u a l - t a r g e t c o n d i t i o nt h a n i n t h e s i n g l e - ta r g e t c o n d i t i o n , w h i c h m a y r e f l e c t a

5 T h e l a t e r al i z a t io n o f t h e N 4 0 0 w a s t e s t e d s t a t i s t ic a l l y i n af o u r - w a y A N O V A w i t h t h e f a c t o r s o f c o n d i t i o n ( s i n g l e t a r g e t o rdua l ta rge t ) , lag (1 , 3 , o r 7 ) , an te r io r -pos te r io r e lec t rode pos i t ion( f ron ta l , cen t ra l , o r pa r ie ta l ) , and hemisphere ( le f t o r r igh t ) .A l t h o u g h t h e m a i n e f f e c t o f h e m i s p h e r e d i d n o t a p p r o a c h s i g n i f i -c a n c e , t h e i n t e r a c t io n b e t w e e n a n t e r i o r - p o s t e r i o r e l e c t r o d e p o s i t i o nand hemisp here was s igni f ican t , F ( 2 , 26 ) = 7 .22 , p < .02 .6 T h e p r e s e n c e o f a n N 4 0 0 d o e s n o t n e c e s s a r i l y i m p l y t h a t aw o r d ' s m e a n i n g h a s b e e n f u l l y c o m p u t e d . F o r e x a m p l e , i t i sp o s s i b l e t h a t t h e N 4 0 0 r e f le c t s s o m e a s p e c t o f l e x i c a l a c c e s s t h a tp r o c e e d s m o r e q u i c k l y o r e f f o r t l e s s ly f o r w o r d s t h a t h a v e a l r e a d yb e e n p r i m e d . H o w e v e r , t h e p r e s e n c e o f a n N 4 0 0 d o e s i n d i c a te t h a ta w o r d h a s b e e n p r o c e s s e d t o a l e v e l a t w h i c h m e a n i n g - b a s e dp r i m i n g p l a y s a r o l e .


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POSTPERCEFTUAL LOCUS OF SUPPRESSION 16 65g e n e r a l i m p a i r m e n t i n p r o c e s s i n g a s s o c i a t e d w i t h p e r f o r m -i n g t w o t a s k s a t t h e s a m e t i m e . T h i s r e s u l t i n d i c a t e s t h a t t h eN 4 0 0 c o m p o n e n t i s s e n s i t i v e t o a t l e a s t s o m e c o g n i t i v em a n i p u l a t i o n s , a n d t h e f a c t t h a t t h i s d i f f e r e n c e w a s s i g n i f i -c a n t a t t h e . 0 1 l e v e l d e m o n s t r a t e s t h a t t h e s i g n a l - t o - n o i s er a t io o f t h e E R P w a v e f o r m s w a s r e a s o n a b l y h i g h . I na d d i t i o n , p r e v i o u s s t u d i e s o f s p a t ia l a t t e n t io n h a v e s h o w nt h a t th e N 4 0 0 i s s u p p r e s s e d f o r w o r d s p r e s e n t e d a t i g n o r e dl o c a t io n s ( M c C a r t h y & N o b r e , 1 9 9 3 ; O t t e n , R u g g , & D o y l e ,1 9 9 3 ) , w h i c h a l s o i n d i c a t e s t h a t t h e N 4 0 0 i s s e n s i t i v e t os o m e a t t e n t io n a i m a n i p u l a t i o n s . H o w e v e r , i t i s p o s s i b l e t h a tt h e a t te n t i o n al b l i n k p r o d u c e d a m o d e s t a m o u n t o f p e r c e p -t u a l d e g r a d a t i o n t h a t w a s s u f f i c i e n t t o p r o d u c e a s i g n i f i c a n td e c r e m e n t i n a c c u r a c y b u t w a s n o t s u f f ic i e n t t o y i e ld as i g n i fi c a n t d e c r e a s e i n N 4 0 0 a m p l i t u d e. F o r e x a m p l e , t h e r ec o u l d b e a n o n l i n e a r r e l a t i o n s h i p b e t w e e n N 4 0 0 a m p l i t u d ea n d p e r c e p t u a l d i s c r i m i n a b i l i t y s u c h t h a t a c o n s t a n t N 4 0 0a m p l i t u d e i s o b s e r v e d u n l e s s t h e p e r c e p t u a l d e g r a d a t i o n i sn e a r l y c o m p l e t e . T o o u r k n o w l e d g e , t h e r e a r e n o d a t a i n th el i t e ra t u r e t h a t q u a n t i f y t h e r e l a t i o n s h i p b e t w e e n N 4 0 0 a m p l i -t u d e a n d p e r c e p t u a l q u a l i t y u n d e r c o n d i t i o n s l i k e t h o s e o ft h e p r e s e n t e x p e r i m e n t . T h e r e f o r e , t h i s a l t e r n a t i v e e x p l a n a -t i o n o f o u r r e s u l t s i s d i f f i c u lt t o e v a l u a t e w i t h o u t a d d i t i o n a ld a t a. E x p e r i m e n t 3 w a s d e s i g n e d t o p r o v i d e t h e n e c e s s a r yd a t a .

E x p e r i m e n t 3T o q u a n t i f y t h e re l a t io n s h i p b e t w e e n p e r c e p t u a l d e g r a d a -

t i o n a n d N 4 0 0 a m p l i t u d e , w e u s e d t h e s i n g l e - t a r g e t c o n d i -t i o n f r o m E x p e r i m e n t 2 a n d a d d e d v a r y i n g i n te n s it ie s o fv i s u a l n o i s e t o t h e T 2 w o r d s t i m u l u s . W e p r e d i c t e d t h a ta d d i n g v i s u a l n o i s e t o t h e w o r d w o u l d l e a d t o a s u b s t a n ti a ld e c l i n e i n b o t h b e h a v i o r a l a c c u r a c y a n d N 4 0 0 a m p l i tu d e . B ys h o w i n g t h a t N 4 0 0 a m p l i t u d e i s r e d u c e d w h e n t h e s t i m u l u si s p e rc e p t u a l l y d e g r a d e d , w e s o u g h t t o b u tt r e ss o u r c o n c l u -s i o n t h a t t h e r o b u s t N 4 0 0 o b s e r v e d d u r i n g t h e a t t e n t i o n a lb l i n k i n E x p e r i m e n t 2 c a n b e i n t e rp r e t ed a s e v i d e n c e f o ru n i m p a i r e d p e r c e p t io n .

T h e s i n g l e -t a r g e t c o n d i t io n w a s u s e d i n s t ea d o f th ed u a l - t a r g e t c o n d i t i o n f o r t h i s e x p e r i m e n t s o t h a t w e c o u l da s s e ss t h e e f f e c t s o f p e r c e p t u a l d e g r a d a t i o n w i t h o u t a n ya d d i t i o n a l e f f e c t s o f a tt e n t i o n a i s u p p r e s s i o n . T h i s n e c e s s i -t a t ed a sm a l l c h a n g e i n t e r m i n o l o g y : B e c a u s e t h e " T 2 w o r d "w a s t h e o n l y t a r g e t u s e d i n t h i s e x p e r im e n t , w e s i m p l y r e f e rt o it a s th e " t a r g e t w o r d . "

MethodTen individuals drawn f ro m the same popula t ion used inExp erim ent 2 participated in this experiment. Th e stimuli, proce-dure , and recording protocol were ident ica l to those used in thes ingle-target condi t ion of Exper im ent 1 wi th the fol low ing excep-t ions . The target word w as accom panied by a visual noise ar ray thatcons i s t ed o f 210 do t s , e ach o f w h ich m easu red app rox im a te ly0.07* 0 .07 . The do ts were the same red hue as the target wordand were rando mly dist r ibuted across the same 4 .9 X 0.8 rectangle that conta ined the target word. The am ount of perceptualdegradat ion was var ied by us ing three di f ferent levels of v isual

noise luminance: d im (3 .7 cd/m2), med ium (5.3 cd/m2) , and br ight(7.2 cd/m2). Trials w ithou t visual noise w ere also included.The RSVP s t reams in th is exper iment were exact ly l ike thoseused in Exper im ent 2 . Thus , each t r ia l con ta ined a digi t s t ring eventhough this i tem was ne ver re levant. In addit ion, there w as a lag of1 , 3, or 7 betwe en the digi t s t r ing and the target wo rd, but th is lagwas i r relevant for the present purposes and the data were thereforecol lapsed across lags.The di f ferent levels of v isual noise were equiprobable an d wererandom ly in termixed w i thin tr ial b locks . E ach par t ic ipant receivedeight b locks o f 60 t r ia ls, wh ich yie lded the sam e num ber o f tr ialsper condi t ion as in Ex per iment 2 . As in Exper irnent 2 , thepar tic ipants were required to repor t whether the target wo rd wassemant ical ly re la ted o r unrela ted to the con text word for that t r ia l .Rela ted and u nrela ted target words we re equiprobable . Ar t i fac ts ledto t he re j ec ti on o f a m ean o f 15% and a m ax im um o f 23% o f t hetrials for a g iven participant.Results and Discussion

T h e b e h a v i o r a l a n d e l e c t r o p h y s i o l o g i c a l r e s u l t s f r o mE x p e r i m e n t 3 a r e s u m m a r i z e d i n F i g u r e 7 . A s t h e l u m i n a n c eo f t h e v i s u a l n o i s e i n c r e a s e d , a c c u r a c y i n r e p o r t i n g t h es e m a n t i c r e l a t i o n s h i p b e t w e e n t h e t a r g e t w o r d a n d t h ec o n t e x t w o r d d e c r e a s e d . N o t e t h a t th e a c c u r a c y d i f f e re n c eb e t w e e n t h e n o - n o i s e a n d b r i g h t - n o i s e t ri a ls w a s c o m p a r a b l et o th e a c c u r a c y d i f fe r e n c e b e t w e e n L a g s 1 a n d 3 i nE x p e r i m e n t 2 . T h e a c c u r a c y v a l u e s w e r e a n a l y z e d i n a nA N O V A w i t h a s i n g l e f a c t o r o f n o i s e i n t en s it y . T h i s a n a l y s i si n d i c a te d t h a t t h e e f f e c t s o f v i s u a l n o i s e w e r e h i g h l ys ign i f i can t , F (3 , 27 ) = 29 . 66 , p < . 001 .

T h e N 4 0 0 c o m p o n e n t o b s e r v e d i n t hi s e x p e ri m e n t w a sh i g h l y s i m i l a r t o t h e N 4 0 0 c o m p o n e n t o b s e r v e d i n E x p e r i -m e n t 2 i n t e r m s o f l a t e n c y a n d s c a l p d i s tr i b u t i o n . H o w e v e r ,N 4 0 0 a m p l i t u d e w a s m u c h m o r e c l o s e l y r e l a te d t o b e h a v -i o r a l a c c u r a c y i n t h i s e x p e r i m e n t . S p e c i f i c a l ly , a s t h e i n t e n -s i ty o f t h e v i s u a l n o i se i n c r e a s e d , N 4 0 0 a m p l i t u d e d e c l i n e di n a m a n n e r t h a t c l o s e l y p a r a l l e l e d t h e e f f e c t s o f v i s u a l n o i s eo n a c c u r a c y . S t a t is t i c a l s u p p o r t f o r t h e e f f e c t o f v i s u a l n o i s eo n N 4 0 0 a m p l it u d e w a s o b t a i n e d in a t w o - w a y A N O V A w i tht h e f a c t o r s o f n o i s e i n t e n s i t y a n d e l e c t r o d e s i te , w h i c hy i e l d e d a s i g n i f i c a n t m a i n e f f e c t o f n o i s e i n t e n si t y , F ( 3 , 2 7 ) =

- 6 -~ ' - 5 -(9 -4-10~ . - 3 -EZ

.2 --1 -0 -

None

-9 0" ', ,, " ~ A cc ura cy - 8 0

~ ' I ~ ~ - 7 0 ;

N , O O /Ampl i tude -5 0i i iD im Med ium BdghtNo i s e L e v e l

T200 400 600

+ S la V - L T i m e ( m s )i N o N o i s e. . . . . . . . . D i m N o i s e. . . . . . M ed i u m N o i s e. . . . . . . . . B r i g h t N o i s e

Figure7. Com par ison of N40 0 ampl itude (averaged across scalps i tes) and behaviora l accuracy as a funct ion of v isual noiseintens ity in Exper imen t 3 . The grand averag e event-re la ted poten-t ia l waveforms a t the r ight of the f igure were recorded a t themidline central electrode site (Cz). Negative is plotted upward.


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16 66 V O G EL, LU C K , A N D SH A PIR O8 . 0 7, p < . 00 5 . B e c a u s e N 4 0 0 a m p l i t u d e w a s g r e a t e s t at th em i d l i n e c e n t r a l a n d p a r i e t a l e l e c t r o d e s , t h e r e w a s a l s o as i g n i f i c a n t m a i n e f f e c t o f e le c t r o d e s i t e , F ( 8 , 7 2 ) = 1 0 . 7 6 ,p < . 0 0 1 , a n d a si g n i f i c a n t i n t e r a c t i o n b e t w e e n n o i s ei n t e n s i t y a n d e l e c t r o d e s i te , F ( 2 4 , 2 1 6 ) = 3 . 6 7 , p < . 0 2 .T h e s e r e s u l t s d e m o n s t r a t e t h a t t h e N 4 0 0 c o m p o n e n t i sh i g h l y s e n s it i v e t o m a n i p u l a t i o n s o f p e r c e p t u a l q u a l i ty .T h u s , t h e l a c k o f a d e c r e m e n t i n N 4 0 0 a m p l i t u d e d u r i n g t h ea t t e n t i o n a l b l i n k i n E x p e r i m e n t 2 c a n b e t a k e n a s s t r o n ge v i d e n c e f o r a p o s t p e r c e p t u a l e f f e c t o f a tt e n t i o n .

e l ic i t a m u c h l a r g e r P 3 c o m p o n e n t t h a n t h e f r e q u e n ta l te r n a ti v e , a n d t h e P 3 c o m p o n e n t w a s t h e r e f o r e i s o la t e d b ym e a n s o f d i f fe r e n c e w a v e s i n w h i c h t h e E R P w a v e f o r me l i c i t e d b y t h e f r e q u e n t a l t e r n a t i v e w a s s u b t r a c t e d f r o m t h eE R P w a v e f o r m e l i c i t e d b y t h e i n f r e q u e n t a l t e r n a t i v e . T h i ss u b t r a c ti o n p r o c e d u r e a l s o e l i m i n a te d t h e o v e r l a p p i n g s e n -s o r y r e s p o n s e s e l i c i t e d b y t h e p r e c e d i n g a n d s u b s e q u e n ts t i m u l i, a s i n t h e p r e c e d i n g e x p e r i m e n t s .M e ~ o d

E x p e r i m e n t 4B y d e m o n s t r a t i n g t h a t s t i m u l i a r e f u l l y i d e n t i f i e d d u r i n g

t h e a t t e n t i o n a l b l i n k , t h e p r e c e d i n g e x p e r i m e n t s p r o v i d e d al o w e r b o u n d o n t h e s t a g e o f p r o c e s s in g i n f lu e n c e d b y t h ea t t e n t i o n a l b li n k . I n o t h e r w o r d s , p r o c e s s i n g i s u n i m p a i r e d a tl e a s t t o th e s t a g e o f w o r d r e c o g n i t i o n d u r i n g t h e a t t e n ti o n a lb l in k . T h e g o a l o f E x p e r i m e n t 4 w a s t o p r o v i d e a n u p p e rb o u n d b y i d e n t i f y i n g a s ta g e o f p r o c e s s i n g a t w h i c h p r o c e s s -i n g i s i m p a i r e d d u r i n g t h e b l i n k i n t e r v a l. I n p a r t i c u la r , w ee x a m i n e d t h e P 3 c o m p o n e n t , w h i c h h a s b e e n h y p o t h e s i z e dt o r e fl e c t w o r k i n g m e m o r y p r o c e s s e s.

T h e P 3 w a v e i s a l a rg e p o s i ti v e c o m p o n e n t t h a t t y p i c a l l yp e a k s @ m s p o s t s t im u l u s a n d i s l a r g e st o v e r c e n t ra la n d p a r i e t a l m i d l i n e s i t e s . T h e a m p l i t u d e o f t h e P 3 i sm o d u l a t e d b y t h e f r e q u e n c y o f a t ar g e t c a te g o r y , w i thi n f r e q u e n t t a r g e t s e l i c i t i n g l a r g e r a m p l i t u d e s t h a n f r e q u e n tt a r g e t s ( D o n c h i n , 1 9 8 1 ) . F o r e x a m p l e , i f a n o b s e r v e r i sr e q u i r e d t o d i s c r i m i n a t e b e t w e e n m a l e a n d f e m a l e n a m e sw i t hi n a s e q u e n ce c o m p o s e d o f 8 5 % m a l e n a m e s a n d 1 5 %f e m a l e n a m e s , t h e f e m a l e n a m e s w i l l e l i c i t a m u c h l a r g e r P 3c o m p o n e n t t h a n w i l l t h e m a l e n a m e s , T h e f a c t t h a t t h e P 3c o m p o n e n t is s e n s i t iv e t o t h e p r o b a b i l i t y o f a t a s k - d e f i n e ds t i m u l u s c a t e g o r y i m p l i e s t h a t i t is e l i c it e d a f t e r t h e s t i m u l u sh a s b e e n c a t e g o r i z e d a n d t h e r e f o r e r e f le c t s a p o s t p e r c e p t u a lp r o c e s s . T h e P 3 w a v e i s a l so s e n s i ti v e t o p e r c e p tu a l m a n i p u -l a t i o n s , s u c h a s c h a n g e s i n s t i m u l u s d i s c r i m i n a b i l i t y ( e . g . ,M c C a r t h y & D o n c h i n , 1 9 8 3 ) , b u t t h i s i s s e n s i b l e g i v e n t h a tt h e q u a l i t y o f t h e i n p u t t o p o s t p e r c e p t u a l p r o c e s s e s w i l li n f l u e n c e t h o s e p r o c e s s e s .

T h e m o s t w i d e l y a c c e p t ed t h e o r y o f th e P 3 c o m p o n e n tp r o p o s e s t h a t i t r e f l e ct s t h e u p d a t i n g o f in f o r m a t i o n i nw o r k i n g m e m o r y ( D o n c h i n , 1 9 8 1 ; D o n c h i n & C o l e s , 1 9 8 8 ).T h e r e a r e a l t e r n a t i v e h y p o t h e s e s a s w e l l ( e . g . , V e r l e g e r ,1 9 8 8 ), b u t v i r t u a ll y a l l th e o r i es o f t h e P 3 c o m p o n e n t w o u l dp o s i t t h a t th e P 3 w a v e i s p r e s e n t o n l y f o r s t im u l i t h a t h a v er e a c h e d t h e l ev e l o f w o r k i n g m e m o r y . T h e p r e s e n c e o f a nu n s u p p r e s s e d P 3 w a v e d u r i n g t h e a t t e n t i o n a l b l i n k w o u l dt h e r e f o r e i n d i c a t e t h a t t h e a t t e n t i o n a l b l i n k o c c u r s a f t e ri n f o r m a t i o n r e a c h e s w o r k i n g m e m o r y , I n c o n t r a st , a s u p p r e s -s i o n o f t h e P 3 w a v e w o u l d i n d i c a t e t h a t th e a t t e n t io n a l b l i n ko c c u r s a t o r b e f o r e t h e s t a g e o f w o r k i n g m e m o r y .

I n th i s e x p e ri m e n t , w e u s e d a n R S V P t a s k w i t h i n d i v id u -a l l y p r e s e n t e d l e t t e r a n d d i g i t s t i m u l i , a s i n E x p e r i m e n t 1 . T oi s o l a t e t h e p r o b a b i l i t y - s e n s i t i v e P 3 c o m p o n e n t , w e u s e d at w o - a l t e r n a ti v e f o r c e d - c h o i c e t a s k i n w h i c h o n e a l te r n a ti v eo c c u r r e d o n 8 5 % o f t ri a ls a n d t h e o t h e r a l t er n a ti v e o c c u r r e do n 1 5 % o f t r ia l s . T h e i n f r e q u e n t a l t e r n a t i v e w a s e x p e c t e d t o

Eight individuals drawn f r om the same pop ula t ion used inExp erime nt 1 participated in this experiment. Th e stimuli, proce-dure , and recording protocol were ident ica l to those used inExp erime nt 1 with the followin g exceptions. All characters ap -peared in black except for T2, which was w hi te . As in Exper iment1, the distractor i tems were randomly selected letters, and T1 wasthe sole digit in the RS V P stream. Th e participants we re required tomake a two-al ternat ive forced-choice response a t the end of thet r ia l , indicat ing whether the T1 digi t was odd or even (bothal ternat ives were eq ual ly l ikely to occur) . The T2 le tter w as theletter E o n 15% o f trims an d som e other letter, selected a t random ,on 85% of trials. A t the end of the trial , the participants wererequired to press a button if T2 was the infrequent letter E an d tomake no response to T2 i f i t was some other le t ter .7 Note thata l though the le t ter E served as the inf requent ca tegory, th is le t terwas actual ly more f requent than any individual le t ter in thefrequen t categor y; however, i t is the probab ili ty of the task-definedcateg ory rather than the actual stimulus tha t determines P3amplitude (see, e.g. , Courchesne, Hillyard, & Courchesne, 1977).R esponses t o bo th T1 and T2 w ere r equ i r ed i n t he dua l -target condi t ion, but only the T2 response was required in thesingle-target condition. No probe flashes were presented in thisexperiment.The ave raged ER P w ave fo rm s w ere t im e l ocked t o t he onse t o fthe T2 le t ter . D if ference waves w ere const ructed by subtracting theERP w aveform s e l ic i ted by the f requent T2 category f rom the ERPwav eforms e l ic i ted by the inf requent T2 category. The P3 com po-nen t w as m easu red f rom these d i f f e r ence w aves a s t he m eanampl itude 4 00 -80 0 m s posts timulus , re la t ive to a 200-ms pres t imu-lus baseline. Measurements we re obtained at the central andparietal electrodes, w here the P3 com pon ent wa s generally largest.A probabili ty-sensitive P2 co mp one nt wa s also presen t in thisexperiment, and th is com pon ent was measured as the meanampl i tude between 200 and 300 ms a t the f ronta l and cent ra le lect rode s i tes . A l l behaviora l and ERP analyses for T2 werelimited to trials o n which T1 wa s corr ectly discriminated. Artifac tsl ed t o the r e j ec ti on o f a m ean o f 14% and a m ax im um o f 26% o f t hetrials f or a give n participant.Resu l t s

Behavior. T 2 a c c u r a c y i s p l o t te d a s a f u n c t i o n o f l a g inF i g u r e 8 . A s i n E x p e r i m e n t s 1 a n d 2 , a c c u r a c y w a s s l ig h t l yl o w e r i n t h e d u a l - t a r g e t c o n d i t i o n t h a n i n t h e s i n g l e - t a r g e tc o n d i t i o n a t L a g s 1 a n d 7 , b u t it d r o p p e d s u b s t a n t i a l l y a t L a g3 i n t h e d u a l- t a rg e t c o n d i t io n . A t w o - f a c t o r A N O V A w a sp e r f o r m e d w i t h c o n d i t i o n ( s i n g l e v s . d u a l t a r g e t ) a n d l a g

7 This go/no -go procedure w as used to encou rage the par t ic i -pants to use a s t ra tegy of making a ca tego r ical E/not -E decis ionrather than s imply ident i fying the le t ter and performing thetask-defined catego rization at the end o f the trial .

8/14/2019 Electrophysiological Evidence for a Postperceptual Locus of Suppression During th

electrophysiological evidence for a postperceptual locus of suppression during the attentional blink

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