Neuropsychologia, Vol. 28, No. 10, pp. 1095 1116, 1990. 0028 3932 t;0 $3.00+//.00 Printed in Great Britain. t 1990 Pergamon Press pie

I N T E G R A T I O N OF VISUAL I N F O R M A T I O N A N D MOTOR O U T P U T IN R E A C H I N G A N D GRASPING: THE

C O N T R I B U T I O N S OF PERIPHERAL A N D CENTRAL VISION

BARBARA SIVAK and CHRISTINE L. MACKENZIE

Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada

(Received 21 August 1989: accepted 11 May 1990)

Abstract--This study examined the contributions made by peripheral and central vision to reaching and grasping. A specially designed contact lens system was used to restrict information to the peripheral retina. Modified goggles were used to restrict information to the central retina. A WATSMART motion analysis system was used to record and reconstruct three dimensional kinematic data. Analyses included an examination of peak kinematic values as well as a qualitative description of the trajectory profiles as related to transport and grasp components. With only peripheral vision, information related to size and shape of an object was inadequate, thus affecting the organization of both the transport and grasp components. With only central vision, information related to the location of an object was inadequate, affecting the organization of the transport but not the grasp component. Implications are discussed relevant to the current models of visuomotor control of reaching and grasping.

INTRODUCTION

VISION HAS an impor t an t role when reaching to grasp an object . Vision is impor t an t for processing in format ion abou t the spat ia l loca t ion of objects and object character is t ics such as shape, size, weight and texture. JEANNEROD [9, 10] identif ied t r anspor t and grasp componen t s ; the hand must be t r anspor t ed to the correct loca t ion and dur ing this t ime, the fingers must be pos tu red to g rasp the object . He suggested the existence of paral lel , i ndependen t v i suomoto r channels whereby the t r anspor t c o m p o n e n t is organized from informat ion arr iv ing via the v i suomoto r channel a b o u t extrinsic proper t ies of an object (e.g. the locat ion) , and grasp c o m p o n e n t is organized from informat ion arr iving from the v i suomoto r channel abou t intr insic proper t ies of an object (e.g. size and shape).

In add i t ion , visual in format ion has been shown to improve movemen t accuracy in both po in t ing and grasping tasks. Studies tha t have man ipu la t ed "what" is seen dur ing the movemen t have shown that vision of bo th the hand and target resulted in the greatest accuracy in an a iming task [4, 6, 11, 19] and in a grasping task [10], c o m p a r e d to vision of jus t the target or hand. In these cases, vision of bo th the hand and target a l lowed for the best visual e r ror informat ion . JEANNEROD [10] showed that wi thout vision of the hand dur ing the movemen t to the object or no vision of the hand or object after movemen t began, subjects undersho t the target by app rox 1-2 cm. PRABLANC et al. 1-19] have repor ted that vision of the hand in its s tar t ing pos i t ion before movemen t begins also makes a big difference in movement accuracy.

F u r t h e r m o r e , the impor t ance of "when" visual in format ion is avai lable dur ing the

1095

1096 B. SIVAK and C. L. MACKENZIE

movement has been shown to be relevant in determining movement accuracy. PAILLARD [ 16] and BEAUBATON and HAY [3] showed that subjects benefited most from visual feedback information provided towards the final phase of the movement. However, visual feedback provided at the beginning of the movement also improved accuracy over conditions when no visual feedback information was provided during the movement.

When reaching to grasp an object, visual information is normally provided by both central and peripheral vision. This needs to be experimentally addressed. Information from central and peripheral vision is used by the motor system to enable accurate movements. PAILLARD and BEAUBATON [18], PAILLARD f l 6-1 and PAILLARD and AMBLARD [17] acknowledged the important role of central and peripheral vision in the control of visually guided reaching to a target. They suggested that movement cues, mainly processed in the peripheral field, used to control the direction of the arm movement and central vision, sensitive to positional cues, are important in the late phase of the movement for accurate positioning of the hand on a target.

Decomposition of the visual field into its central and peripheral components suggests the following possible contributions to grasping an object. Individuals usually fixate the eyes and turn the head toward an object to be grasped. Before the start of the grasping movement, both the hand and arm are in peripheral vision and the object is in central vision. During the grasping movement, the hand and arm remain in peripheral vision. At the end of the grasping movement, the hand comes into central vision, along with the object, while the arm remains in peripheral vision. Thus, peripheral and central vision must provide specific information to the organization and control of the grasping movement.

The purpose of these experiments was to examine the specific contributions of peripheral and central vision to the organization and control of reaching and grasping. This was possible through a physical optical separation of peripheral and central visual fields. If central vision was eliminated how were the organization and control of reaching and grasping affected? Referred to as the condition with only peripheral vision, this was compared to a normal vision condition. Likewise, if peripheral vision was eliminated, how were the organization and control of reaching and grasping affected? Referred to as the condition with only central vision, this was compared to a normal vision condition.

Two separate experiments were conducted, one to eliminate central vision, and one to eliminate peripheral vision. It was decided not to combine these visual manipulations in one experiment for several reasons. The difference in the method of isolating peripheral vision and central vision did not lend itself to counterbalancing, thus creating the possibility of carry-over effects. For example, consultation with clinicians at the School of Optometry, University of Waterloo, led to the recommendation that subjects should not put their own contact lenses in the eye for at least 5 h after using the specially designed contact lenses to isolate only peripheral vision. As contact lens wearers were used as subjects in the visual condition that isolated peripheral vision, this experimental condition could not be followed by testing under the only central vision condition. Furthermore, the testing session would have been too lengthy and impractical.

In the following experiments, the WATSMART (Waterloo Spatial Motion Analysis and Recording Technique) three-dimensional analysis system was used. The study of trajectories of movement has been used by various researchers to make inferences about central nervous system operations [1,13, 21]. Systematic variance and invariances in these kinematic profiles over various experimental manipulations may be a reflection of neural mechanisms subserving motor planning and control processes.

PERIPHERAL AND CENTRAL VISION 1097

EXPERIMENT 1

Reachin,q and ,qraspinq with only peripheral vision

The purpose of this experiment was to examine the contributions made by peripheral vision to reaching to grasp an object. We hypothesized that with only peripheral vision the grasp component and not the transport component would be affected compared to normal vision. JEANNEROD [-9] suggested that the grasp component is organized from visual information about the intrinsic properties (e.g. size) of an object. Since peripheral vision is lacking in high resolution vision, it is logical to predict that the grasp component would be affected compared to normal vision. In contrast, JEANNEROD [9] suggested that the transport component is organized from visual information about the extrinsic properties (e.g. location ) of an object. Since peripheral vision has a large field of view and thus is capable of providing location information and information from the moving limb, we predicted that the transport component would not be affected compared to normal vision. The task required subjects to reach and grasp a wooden dowel placed in front of them with only a peripheral vision or full normal vision.

Method

Subjects. Six, right-handed university students were tested; all were experienced contact lens wearers. Subjects were instructed to wear their own corrective glasses over the experimental lenses if near vision needed correction.

Method ~['isolatinq peripheral vision. A special contact lens system was developed using a piggyback contact lens design in which a partially painted hard lens was positioned on top of a special soft lens carrier (Fig. 1 ; for details see [20]). The hard lenses were painted black in the center so that when lenses were inserted in both eyes, no visual information was available through the center 10 of the visual field. Field size reduction was measured using a standard clinical tangent screen and procedure for individuals with central field impairment. Approximately l 0 of the central field remained as a scotoma and therefore could not be used to provide visual information even when the eyes or head moved.

Apparatus and experimental set-up. The apparatus consisted of a wooden dowel 2.5 cm in diameter and I 1.5 cm in length. The starting position for the hand was on a table in front of the body midline of the seated subject. The distance between the start position and the dowel was 30 cm. Movements were in a forward direction in a sagital plane corresponding to the body midline.

Procedure and experimental desi.qn. The subject's head was supported in a chin rest to stabilize the head in a straight ahead position. All visual conditions were conducted using binocular vision. However, in the only peripheral vision condition, if subjects looked straight ahead they could not see the dowel. Turning their eyes enabled subjects to see the dowel situated in front of them. Subjects turned the eyes approx 25 30' to position the dowel within the peripheral visual field of both eyes. Visual fixation of the dowel was controlled by the subject. Whether subjects looked to the left or to the right was counterbalanced among subjects by instructions from the experimenter. Pilot testing had revealed that whether both the head and eyes were turned to the left or right or whether the head remained in a straight ahead position and the eyes turned did not affect the transport or grasp components, for the midline reaches used in this experiment.

Subjects were shown the apparatus before testing began. Subjects started each trial with the forearm and medial surface of the hand resting on the table. The tips of the index finger and thumb, touching in a relaxed manner, were positioned 2 cm above the starting position. Due to the resting posture and the location and shape of the object to be grasped, it was assumed that very little motion about the wrist occurred during the movement. Subjects were instructed to grasp the dowel near the top using the pads of index finger and thumb and lift it offthe table. Subjects were instructed to proceed at their own pace on hearing the word "go". Only the preferred right hand was tested. The left hand rested in the lap.

The visual conditions included only peripheral vision (no central 10' of visual field, as restricted by the lenses) and normal vision (including both central and peripheral vision). Subjects participated in both conditions. The order of the visual conditions was counterbalanced. For each vision condition there were 16 trials, the first 4 of which were practice trials and not recorded. Of the remaining 12 trials, the last 8 trials free of problems were used in analyses of the data.

Re:ordinq system. A three camera WATSMART system was used for recording and analysis of the data. 1REDs (infra-red emitting diodes) served as markers and were positioned on the proximal lateral corner of the index linger nail. proximal medial corner of the thumb nail and wrist (on the skin above the distal end of the radius) of each subject, as well as on the top center of the dowel. The wrist |RED was used to indicate the transport component. The

1098 B. SWAK and C. L. MACKEYzlE

(a) T c ,TX 1 4 . 5 m m = .

S o f t L e n s

c a r r i e r

© 9 9 m m

Hard Lens

( b ) ,20 ,05 90 ~ 60

t 65 50 ~5 45 30 15

180 0

195 345

210 330

24O 255 27O ee5 3oo

Fig. 1. Exper iment 1. (a) Con tac t lens system used to restrict vision to per ipheral retina. These lenses were inserted in both eyes. (b) Typica l visual field m a p p e d for one subject when lenses were pos i t ioned in both eyes. Note that ha tched area indicates that approx 1 0 of central vision remained as a

scotoma.

subject, as well as on the top center of the dowel. The wrist IR ED was used to indicate the t r anspor t componen t . The index finger and t h u m b I R E D s gave a measure of the aper ture between the index finger and t h u m b to indicate the grasp componen t . The two-d imens iona l pos i t ion of the I R E D s over t ime was sampled at a frequency of 200 Hz by each of three cameras . The angu la r p lacement of camera 1 and 2 was 4 4 , cameras 2 and 3 was 60 ~ and cameras 3 and I was 104" to each other. The dis tances from the center of the ca l ibra t ion cube to cameras 1, 2 and 3 were 177 cm, 172 cm and 179 cm respectively. S t rob ing of the I R E D s and sampl ing rate were control led by the I B M - P C and W A T S M A R T unit. Er ror of the W A T S M A R T da ta as measured with the ca l ib ra t ion cube was shown to be less than 1 mm. In addi t ion , s tat ic ca lcula t ion of known dis tances between two I R E D s was accurate to within 1 mm.

For each sampl ing point the best two sets of two d imens iona l co-ord ina tes were reconstructed after da ta col lect ion to provide three d imens iona l co-ord ina tes of the pos i t ion of the IREDs . The da ta were ro ta ted to define X, Y and Z axes as follows: X axis as movemen t in the forward direct ion; Y axis as o r thogona l devia t ion from the forward direct ion (posit ive Y to the left); and Z axis as the movement vert ical ly (posit ive Z is up). To smooth and minimize d is tor t ion of the recons t ruc ted 3-D posi t ion data , a second order But te rwor th filter p rog ram with a dual pass was used. This e l imina ted phase lag. A residual analys is of s ignal to noise ra t io indicated 4 Hz as the op t ima l frequency to filter the data . Thus , the da ta were filtered at a frequency of 4 Hz.

The start and end of the movement were opera t iona l ly defined by the use of a contac t b reak ing system. This

PERIPHERAL AND CENTRAL VISION 1099

consisted of a metal disk (2 cm dia., glued to the medial surface of the subject's hand) that made contact with a rectangular metal plate (3 cm x 2.5 cm) placed by the starting position. Lifting the hand broke contact and defined the start of the movement. A similar metal disk placed on the bottom of the dowel made contact with a similar rectangular metal plate. The end of the movement was defined when the contact was broken at the dowel lift. Contact breaking signals were recorded on a WATSCOPE channel concurrent with the recording on the WATSMART system. Movement time, defined as the time between breaking the start and end contacts, was used as one dependent measure. As well, trajectories were windowed and normalized in time, based on the start and end defined by the contact breaking system.

Data analyses. After data were filtered, differentiated and windowed, analyses involved examination of peak kinematic values for both the transport and grasp components as well as a qualitative, descriptive analysis of the trajectory profiles. For the transport component (wrist IRED), velocity was derived from the filtered displacement data from each of the X, Y and Z axes using the central finite difference method. Each of these X, Y and Z velocities was then squared and the square root of their sum provided a measure of the resultant velocity (or speed along the path of the trajectory) for any given point in time. The resultant velocity data were differentiated to obtain the rate of change of speed along the path of the movement; for simplicity, these will be referred to as "acceleration data". For the grasp component, the distance between the thumb and index finger IREDs was calculated from the filtered displacement data over the time course of each trial.

The process of normalizing profiles (velocity, acceleration and aperture) was obtained by scaling each trial in the temporal domain to 100 points after velocity and accelaration were derived. This procedure permitted comparisons to be made between curves across experimental conditions. That is, systematic patterns in the time normalized profiles can be used to infer underlying organization and control processes. If across experimental conditions, temporal scaling of curves related to the transport and grasp components does not result in curves of the same shape, this may be interpreted as evidence for different organization and control processes [14].

Transport component. Dependent measures were derived from kinematic peaks for the wrist IRED. We report the times (msec) to the kinematic peaks and normalized time (per cent MT) after the kinematic peaks. The dependent measures included: peak speed along the path of the movement (highest point on the resultant velocity curve), time spent to reach peak speed (msec), per cent of movement time spent in deceleration {from peak speed to the end of the movement, from time normalized profiles), peak acceleration along the path of the trajectory (highesl point on the differentiated resultant velocity profile), time spent to peak acceleration (msec), per cent of movemenl time spent after peak acceleration to the end of the movement, peak deceleration along the path of the movement (lowest point on the differentiated resultant velocity profile), time spent to peak deceleration (msec), and per cent of movement time spent after peak deceleration to the end of the movement. These measures were computed for each trial. Means and SDs were computed over 8 trials in each experimental condition and for every subject.

Grasp component. For grip aperture (distance between the thumb and index finger IREDs), the following dependent measures were derived: maximum aperture (largest distance between thumb and index finger IREDs), time to reach maximum aperture (msec), per cent of movement time spent after maximum aperture (from time normalized profiles). Mean and SD values were computed over 8 trials for each dependent measure in each experimental condition and for every subject.

Results

T a b l e s 1 a n d 2 c o n t a i n a s u m m a r y o f all d e p e n d e n t m e a s u r e s for the t r a n s p o r t a n d g r a s p

c o m p o n e n t s for t h e t w o v i sua l c o n d i t i o n s r e spec t ive ly .

Movement time. A r e p e a t e d m e a s u r e s a n a l y s i s of v a r i a n c e of m o v e m e n t t ime r evea l ed t h a t

s u b j e c t s m o v e d s l o w e r w h e n t h e y h a d p e r i p h e r a l v i s ion t h a n w i th n o r m a l v i s ion ,

F ( 1, 5) = 13.1, P < 0.01. M o v e m e n t t i m e va lues r a n g e d f r o m 1502 m s e c to 1797 msec for on ly

p e r i p h e r a l v i s i o n a n d f r o m 1018 m s e c to 1205 for n o r m a l v i s ion . I t is s u g g e s t e d t h a t s u b j e c t s

m o v e d s l o w e r b e c a u s e t h e y were c a u t i o u s as a r e su l t of t he u n n a t u r a l v i sua l s i t u a t i o n c r e a t e d

w i th p e r i p h e r a l v i s ion .

Transport component . A n a l y s e s of v a r i a n c e of p e a k ve loc i ty va lues r evea l ed t h a t sub j ec t s

r e a c h e d a l o w e r p e a k s p e e d w i t h o n l y p e r i p h e r a l v i s ion (424 m m / s e c ) t h a n wi th n o r m a l

v i s ion (533 m m / s e c ) , F (1, 5 ) = 8 . 1 3 , P < 0 . 0 5 (Fig . 2(a)) . Also , sub j ec t s s h o w e d a t e n d e n c y to

r e a c h p e a k s p e e d l a t e r w i t h o n l y p e r i p h e r a l v i s ion (574 m s e c ) t h a n n o r m a l v i s ion (440 msec) ,

F ( 1 , 5 ) = 4 . 2 3 , P < 0 . 0 7 .

C o n s i s t e n t w i th t he ve loc i ty a n a l y s i s , a n a l y s i s of p e a k d e c e l e r a t i o n r e v e a l e d t h a t sub j ec t s

s h o w e d a s m a l l e r p e a k d e c e l e r a t i o n w i th o n l y p e r i p h e r a l v i s ion ( - 1608 m m / s e c / s e c ) t h a n

w i th n o r m a l v i s ion ( - 2 5 5 3 m m / s e c / s e c ) , F ( 1 , 5 ) = 11.3, P < 0 . 0 5 (Fig. 2(b)) . Also , sub j ec t s

1100 B. SIVAK and C. L. MACKENZXE

Table 1. Experiment l : summary table of means and mean within subject SD (in parentheses) for all dependent measures and experimental conditions for the

transport component (wrist IRED)

Peripheral vision Normal vision

Movement time (msec)

Peak velocity (mm/sec)

Time to peak velocity (msec)

Time after peak velocity (%)

Peak acceleration (mm/sec/sec5

Time to peak acceleration (msec)

Time after peak acceleration (%)

Peak deceleration (mm/sec/sec)

Time to peak deceleration (msec)

Time after peak deceleration (%)

1667.7 1156.4 (296.1) (87.2) 423.8 533.1 (55.0) (51.9) 573.6 440.2

(153.6) (54.4) 64.1 61.6 (6.3) (4. I )

2689.5 3375.0 (746.1) [919.1) 246.7 193.7 (95.3) (58.3) 85.1 84.1 (5.9) (5.1)

- 1679.7 -2553.2 (446.8 (602.95 768.6 584.0

(137.9) (78.4) 51.0 48.8 (6.3) (6.0)

Table 2. Experiment 1: summary table of means and mean within subject SD (in parentheses) for all dependent measures and experimental conditions for the grasp

component (index and thumb IREDs)

Peripheral vision Normal vision

Maximum aperture (mm)

Time to maximum aperture (msec)

Time after maximum aperture (%)

Aperture 0 displacement wrist (mm)

Time 0 displacement wrist (%)

Aperture at thumb contact (mm)

Time at thumb contact (%)

Aperture at index contact (mm)

Time at index contact (%)

116.5 85.4 (7.6) (7.2)

806.5 624.1 (192.2) 178.7)

51.6 44.4 (6.45 (7.25 96.0 55.2 (5.1) (4.2) 72.3 90.6 (4.8) (3.6) 74.2 54.1 (3.9) (3.4) 80.5 93.2 (4.15 (4.6) 63.9 51.8 (4.35 (3.0) 89.8 94.6 (4.1) (3.5)

showed a tendency to reach peak deceleration later with only peripheral vision than normal vision, P < 0.07.

Peak displacement on the Y axis as a function of time was used as an indication of deviation from the straight line trajectory, i.e. deviation from sagital plane movement. This

PERIPHERAL A N D CENTRAL VISION 1 t01

analysis did not reveal any significant differences between the vision conditions. Subjects did not deviate more from the primary axis of the movement using only peripheral vision compared to normal vision.

Aside from the analyses of variance of peak values, an examination of the profiles revealed differences in the shape of the profiles after peak deceleration. With only peripheral vision, subjects appear to decelerate rapidly and maintain a near 0 acceleration (Fig. 2(b)). The start of this period of near 0 acceleration coincides with the time on the velocity profile when velocity is approaching base value (Fig. 2(a)). A possible explanation for this observation is that the arm may have been in the vicinity of the dowel before dowel lift-off indicated the end of the movement. This is important because it suggests that, with only peripheral vision, the transport component may have been completed before the grasp component. In contrast, with normal vision, an examination of the profiles showed a more gradual deceleration process (Fig. 2(a,b)). This suggests that the arm may have been moving forward throughout the entire movement time. That is, with normal vision, the transport and grasp components would have been completed at about the same time.

Grasp component. Analyses of variance of the peak values associated with the grasp component showed that subjects opened their hand wider with only peripheral vision (116 mm) than under normal vision (86 mm), F (1, 5)= 12.7, P<0.01. This was character- istic of all subjects. Not only did subjects show a wider maximum aperture with only peripheral vision compared to normal vision but the time normalized profiles revealed that the time spent after maximum aperture was longer for only peripheral vision (52%) than for normal vision (44%), F (1, 5)=8.9, P<0 .05 (Fig. 2(c)).

Co-ordination of the transport and grasp components. Further analyses corroborated that the transport and grasp components were co-ordinated differently with only peripheral vision compared to normal vision. The straight line distance between the wrist IRED and dowel IRED while the arm was travelling to the dowel was computed. Working backwards from the end of the trial, the time when the distance between these IREDs changed by 2 mm between consecutive frames was operationally defined as the time when the arm had moved as much as it was going to move. This provided a measure of the per cent of movement time when the distance between the wrist and dowel did not change (D on Fig. 3(b)). The outcome of this analysis (performed for every trial, vision condition and subject) indicated that the wrist reached a stationary position relatively earlier for only peripheral vision than for normal vision (Table 3 and Fig. 3(b)). For only peripheral vision, the wrist stopped moving when approx 72% of the movement is completed. In contrast, with normal vision, the wrist stopped moving when approx 91% of the movement had been completed. This was true for all subjects.

To understand what the hand was doing when the arm was travelling to the dowel, the size of the aperture was noted when the wrist stopped moving. These apertures showed that the size of the aperture was wider with only peripheral vision than with normal vision for all subjects (Table 3, Fig. 3). This suggests that subjects may have been using a different strategy to complete the actual grasp with only peripheral vision than with normal vision; subjects may be relying on tactile information for enclosing the hand. In contrast, with normal vision, a small aperture at the time that the wrist stops moving suggests that the hand is enclosing while the arm is moving.

To investigate these possibilities the data were examined further (Table 4, Fig. 4). The distances between the thumb IRED and dowel IRED and between the index finger IRED and the dowel IRED were used as an indication of when contact with the dowel was made.

I I02 B. SIVAK and C. L. MAcKENzIE

E E

7 0 0 0

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5 0 0 0

4 0 0 0

3 0 0 I

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I I I I I I I I I I I0 20 30 4 0 50 6 0 70 8 0 90 lOO

NormaLized time (%)

Fig. 2. Experiment 1. Typical mean curves for (aj velocity, (bj acceleration and (cj aperture for one subject (A.W.R.) for only peripheral vision (PV) and normal vision (NV). For illustrative purposes

mean curves are presented. However, all data analyses are based on individual curves.

PERIPHERAL AND CENTRAL VISION 1103

Table 3. Experiment 1: mean aperture size (mm) and mean per cent of movement (MT) when distance between wrist IRED and dowel IRED is

constant for each subject and vision condition

Subject

When dowel-wrist distance is constant Peripheral vision Normal vision

Aperture size % MT Aperture size % MT

A.P.R. 101.6 61 61.2 85 N.H.R. 79.8 81 55.9 94 A.W.R. 90.1 69 56.3 95 E.H.R. 130.2 61 54.0 88 J.C.R. 98.9 82 53.1 93 L.R.R. 78.8 80 50.9 89

Mean 96.5 72.3 55.2 90.6

-g

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<

1200

I 0 0 0

80.0

60 C

40 O

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Peripheral vision

(a )

I I I i I ] i I i i

Normal vision

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o

4000

350.0

3000

250.0

200.0

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I0 20 30 40 50 60 70 80 90 I00

NormaLized time (%)

Fig. 3. Experiment 1. Mean curves for (a) aperture size and (b) distance between wrist IRE[) and dowel IRED for one subject (A.W.R.). The curves on the left illustrate mean trials for only peripheral vision and on the right, normal vision. When wrist dowel distance is constant (D), the size of the aperture is (D). Note that the size of the aperture is larger for only peripheral vision than for normal

vision when wrist dowel distance is constant.

1104 B. SIVAK and C. L. MAcKENzIE

This time was operationally defined as the time when the distance did not change by more than 2 ram. The time when the index finger-dowel distance (or the thumb~lowel distance) was constant was used as an indication of when the index finger or thumb made contact with the dowel.

Several points are of interest. First, with peripheral vision, either the thumb or the index finger made contact with the dowel before the other digit. Figure 4 shows one subject who contacted the dowel first with the thumb. In contrast, with normal vision, both digits contacted the dowel at approximately the same time. Second, with only peripheral vision, no consistent pattern was seen as to whether the index finger or thumb contacted the dowel first. These results confirm that, with peripheral vision, subjects were depending more on tactile information for the actual grasp.

Table 4. Experiment l: mean aperture size (mm) and per cent of movement time (MT) when (a) distance between thumb IRED and dowel IRED and (b) distance between index finger IRED and dorsal IRED are

constant for all subjects

Subject

Thumb dowel distance Index dowel distance Peripheral vision Normal vision Peripheral vision Normal vision

Aperture Aperture Aperture Aperture size % MT size % MT size % MT size %, MT

A.PR. 84.5 71 46.3 98 56.9 97 47.6 97 N.H.R. 52.3 95 50.8 96 58.1 90 51.3 95 A.W.R. 86.5 74 59.4 95 57.1 96 46.3 97 E.H.R. 92.6 71 63.5 89 59.5 95 58.0 97 J .C.R. 70.4 87 51.5 94 61.0 96 50.3 90 L.R.R. 59.2 85 53.5 95 90.9 65 57.2 92

Discussion

This study examined the contributions made by peripheral vision (in the absence of central vision) to reaching and grasping~ As a consequence of the experimental procedure to isolate peripheral vision, high resolution vision (mediated by the central regions of the retina) was excluded. Subjects reached to grasp a dowel that they saw with only peripheral vision.

With respect to the transport component, a paradox exists in the results. Analyses based on time normalized peak values showed that, although subjects tended to move slower with only peripheral vision compared to normal vision, the transport component was scaled similarly for both vision conditions. This suggests that the transport component was organized similarly for both only peripheral vision and normal vision. However, visual examination of the time normalized wrist acceleration curves for only peripheral vision and normal vision indicated a difference between the shape of the curves (Fig. 2(b)). This difference was particularly evident in the portion of the curve after peak deceleration for all subjects. Further analyses related to this shape difference revealed that the arm stopped movement toward the dowel earlier with only peripheral vision with than normal vision. It is suggested that subjects adopted this movement strategy to allow for tactile information from either the thumb or index finger making contact with the dowel to help in completion of the actual grasp. Thus, from visual inspection of the curves, it is apparent that the organization of the transport component was affected with only peripheral vision compared to normal vision. This suggests a limitation in inferring underlying movement organization based only on the normalized time analysis of peak values.

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Fig. 4. Experiment l. Mean curves for (a) aperture size, (b) distance between thumb IRED and dowel IRED and (c) distance between index finger IRED and dowel IRED for one subject (A.W.R.). The curves on the left illustrate mean trails for only peripheral vision and on the right, normal vision. When thumb~lowel distance is constant (T), the size of the aperture is (T). When index finger dowel distance is constant (it, the size of the aperture is (I). Note, the larger aperture size when the thumb

makes contact with the dowel with only peripheral vision compared to normal vision.

Futhermore, we suggest that the organization of the transport component was affected with only peripheral vision because the grasp component was affected. According to JEANNEROD [9], the transport component is organized from information about the location of an object (extrinsic properties) through a visuomotor channel independent from the

1106 B. SIVAK and C. L. MACKENZIE

visuomotor channel used to organize the grasp component. The fact that the organization of the transport component could be affected by what the hand is doing suggests that there may be a communication link between the visuomotor channels for the organization of the transport and the grasp components. Furthermore, it suggests that the visual information about intrinsic properties of an object may also provide important information to the organization of the transport component.

As predicted, the organization of the grasp component was affected by the visual manipulation. As well as opening the hand wider during the reaching movement, subjects also opened the hand earlier and spent relatively more time from the widest opening to the end of the movement with only peripheral vision compared to normal vision. JEANNEROD [9] has suggested that the grasp component is organized from visual information about intrinsic properties of an object. For the index finger and thumb precision grasp used by subjects in this study good visual acuity is probably required. Therefore, the grasp component may have been affected because high resolution vision was inadequate with only peripheral vision compared to normal vision.

As WING et al. [22] have demonstrated, subjects opened the hand wider to ensure end point accuracy under conditions that required faster movements than normal or movements in the dark. As subjects in this experiment did not see the dowel clearly, they adopted a similar strategy. The increase in the relative time spent after maximum aperture with only peripheral vision over normal vision may be a reflection of finger adjustments after dowel contact.

With normal vision, the opening of the hand and the start of enclosing the hand take place while the hand is in peripheral vision. If peripheral vision is important for the opening and closing of the hand, then the grasp component should not have been affected by the lack of adequate visual information about the characteristics of the dowel. Depressed visual acuity related to the intrinsic features of the dowel, appeared to affect the organization of the grasp component. The peripheral visual information of finger position before movement begins and during the movement appears irrelevant to the organization of the grasp component. Visual information about object characteristics obtained both before and during the movement is what is important for the organization of the grasp component.

With respect to the co-ordination between the transport and grasp components, subjects depended more on tactile information from dowel contact with only peripheral vision than with normal vision. The evidence for this is two-fold. First, when the arm had stopped moving forward, the size of the aperture was larger for peripheral vision compared to normal vision. In fact, the size of the aperture when the arm stopped moving with normal vision was almost as small as the final aperture. This suggested that subjects were adjusting the size of the aperture while the arm was moving forward with normal vision but less so with only peripheral vision. Second, subjects appeared to be making contact with either the thumb or index finger while the aperture was relatively wide with only peripheral vision in comparison to normal vision. This initial contact triggered the actual grasp. JOHANSSON and WESTI,INCI [12] found that the minimum response latencies of the distal hand muscles to tactile stimuli in the adult was approx 60-80 msec. As the time from contact with the dowel by either the thumb or the index finger to the end of the movement exceeded 80 msec in the only peripheral vision condition for five out of six subjects, this explanation is plausible. In normal vision~ this did not seem to be the case.

What were the differences between only peripheral and normal vision that may have contributed to the effects on the transport and grasp components and their co-ordination?

PERIPHERAL A N D CENTRAL VISION 1107

With only peripheral vision, subjects had lenses inserted in both eyes, whereas with normal vision subjects did not wear any lenses. However, since all subjects were experienced contact lens wearers, it should not have been a new experience to have lenses inserted in the eyes. Therefore it is difficult to attribute the differences in the transport and grasp components between the vision conditions to this reason.

Furthermore, with only peripheral vision, the head was stabilized in a straight ahead position, whereas the eyes were turned to the left or right in order for subjects to use peripheral vision to see the dowel. In contrast, with normal vision, the head and eyes were positioned straight ahead. Some evidence suggests that the position of the eye in the orbit may provide important information toward endpoint accuracy when vision of the limb is unavailable (e.g. [l 5]). Since the hand was always in view in the present study, it is doubtful whether, for the peripheral vision condition, the effect on the transport and grasp components can be attributed to the position of the eyes in the orbit.

The most obvious difference between the vision conditions was the lack of high resolution vision with only peripheral vision compared to normal vision. The peripheral retina is incapable of mediating high resolution vision. It is important to remember that this and other fundamental processing differences may be correlated with the anatomical and functional organization of the peripheral retina (e.g. [5]). At this time, we suggest that our results on the transport and grasp components with only peripheral vision were due to the lack of high resolution vision normally mediated by central regions of the retina. Similar results might be obtained with defocusing lenses and central vision. In addition, for grasping tasks that rely less on acuity information (e.g. large accepted area for finger placement, collective grasp types, etc), the present results may not be achieved. Further experiments should be done to address these important issues.

In summary, contrary to what had been predicted, both the grasp aml transport components were affected with only peripheral vision compared to normal vision. For the index finger and thumb precision-type grasp used in this study, it appears that the organization of the grasp component relied heavily on visual acuity information about intrinsic object characteristics normally mediated via central regions of the retina. The transport component was modified to accommodate the changes in the organization of the grasp component.

EXPERIMENT 2

Reaching and gra~sping with only central vision

The purpose of this study was to investigate the contribution made by central vision to reaching and grasping. We predicted that with only central vision, the transport and not the grasp component would be affected compared to normal vision. JEANNEROD [_9] suggested that the transport component is organized from visual information about the extrinsic properties (location) of an object. Since with only central vision a large field of view is lacking, therefore limiting the amount of information about the location of an object, it was hypothesized that the transport component would be affected compared to normal vision. JEANNEROD [9] suggested that the grasp component is organized from visual information about the intrinsic properties (e.g. size) of an object. Since central vision is not lacking in high resolution vision (e.g. [_5]), it was hypothesized that the grasp component would not be affected compared to normal vision. As a complement to Experiment 1, subjects reached to grasp a dowel using only central vision or normal vision.

1108 B. SIVAK and C. L. MACKENZIE

Method

Subjects. Six right-handed university students were tested, different from those in Experiment 1. If near vision needed correction, subjects were permitted to wear their own contact lenses throughout the experiment.

Method ofisolatin 9 central vision. A goggle system was developed that allowed for 10" of binocular central vision (Fig. 5). This system used swimming goggles that were painted black on the sides to eliminate light. The front of the goggles contained slightly depressed circles into which were fitted circular black opaque disks. Each disk had a 0.5 mm aperture that was displaced 4 mm medially from the center of the disk. Extensive pilot testing of the size and position of the aperture in the disks ensured that the desired visual manipulat ion was achieved with these goggles and disks, i.e. central 10 of binocular vision. Further adjustments to ensure visual binocularity were done by way of adjustments of the goggles strap. We made sure each subject saw one and not two images when wearing the goggles.

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Field size reduction was measured for each subject using a standard clinical tangent screen and procedure for measuring visual fields. Approximatley 10" of central vision was seen when wearing the goggles (Fig. 5).

Apparatus, recordiny system, procedure, experimental design and data analysis. Experiment 2 used the same apparatus, recording system, procedure and data analysis as Experiment 1. Two experimental conditions were used: only central vision (central I0 ~ of visual field) and normal vision (including central and peripheral vision). Subjects participated in both conditions, with the order of vision conditions counterbalanced. Subjects reached to grasp a dowel placed 30 cm in front of them with either only central vision or normal vision.

Resul ts

T a b l e s 5 a n d 6 c o n t a i n a s u m m a r y o f a l l d e p e n d e n t m e a s u r e s fo r t h e t r a n s p o r t c o m p o n e n t

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

PERIPHERAL A N D CENTRAL VISION 1109

Table 5. Experiment 2: summary table of means and mean within subject SD (in parentheses) for all dependent measures and experimental conditions for the

transport component (wrist IRED)

Central vision Normal vision

Movement time (msec)

Peak velocity (mm/sec)

Time to peak velocity (msec}

Time after peak velocity (%)

Peak acceleration (mm/sec/sec)

Time to peak acceleration (msec)

Time after peak acceleration (%)

Peak deceleration (mm/sec/sec)

Time to peak deceleration (msec)

Time after peak deceleration (%)

1233.8 872.9 (183.8) (79.7) 578.8 722.7 (55.8) (43.4) 39l .5 409.3 (54.6) (57.7) 67.2 55.6 (4.2) (5.3)

3227.5 3528.6 (528.1) (485.4} 156.6 177.1 (45.7) (55.7} 87.2 79.6 (4.1) {5.5)

- 1926.1 2837.2 (356.1) (420.8)

580.7 523.9 (62.1} (54.9) 52.1 38.5 (4.7) (5.1)

Table 6. Experiment 2: summary table of means and mean within subject SD (in parentheses) for all dependent measures and experimental conditions for the grasp

component (index and thumb IREDs)

Central vision Normal vision

Maximum aperture {ram)

Time to maximum aperture (msec)

Time after maximum aperture (%)

Aperture 0 displacement wrist (mm}

Time 0 displacement wrist (%/

Aperture at thumb contact (mm)

Time at thumb contact (%)

Aperture at index contact (mm)

Time at index contact (%)

91.9 88.3 (5.7) (5.4)

640.1 498.4 (93.5) (75.6) 48.1 43.9 (6.2) (6.3} 55.7 56.4 (3.5) (3.2) 92.6 92.8 (3.2) (2.8) 55.3 56.1 (3.8) (3.3) 94.6 95.1 (2.8) (3.1) 55.4 51.0 (4.1) (2.9) 95.3 94.1 (3.8) (4.2}

Movement time. S u b j e c t s m o v e d s l o w e r w h e n t hey u sed o n l y c e n t r a l v i s ion c o m p a r e d to

n o r m a l v i s ion , F ( l , 5 ) = 15.5, P < 0 . 0 1 . M o v e m e n t t i m e va lues r a n g e d f r o m 1136 m s e c to

1405 m s e c for o n l y c e n t r a l v i s i o n a n d f r o m 844 m s e c to 923 m s e c for n o r m a l v i s ion .

Transport component. C o n s i s t e n t w i t h t he m o v e m e n t t ime resu l t s , a n a l y s e s of v a r i a n c e

r e v e a l e d t h a t s u b j e c t s s h o w e d a l o w e r p e a k speed , F ( 1 , 5 ) = 2 1 . 4 , a n d l o w e r peak

1110 B, SIVAK and C. L. MACKENZIE

deceleration, F ( I , 5 )= 18.3, with only central vision compared to normal vision, both at P<0.01 (Fig. 6(a)). For the time normalized data, subjects consistently spent a greater per cent of movement time after peak acceleration, F (1, 5) = 11.9, peak speed, F (1, 5) = 21.4 and peak deceleration, F (1, 5) = 33.6, with only central vision compared to normal vision, all at P < 0.01 (Fig. 6(b)). All subjects showed this effect. There were no significant effects for the time to peak acceleration, peak velocity or peak deceleration. This suggests that the increase in movement time seen in the only central vision condition compared to normal vision was occurring after peak deceleration. To verify this, an analysis of variance of the time spent after peak deceleration was performed. This analysis confirmed that subjects spent significantly more time after peak deceleration with only central vision (653 msec) compared to normal vision (343 msec), F (1, 5)=26.11, P<0.005.

Analysis of peak displacement on the Y axis revealed no significant differences between only central vision and normal vision. This implies that subjects did not deviate more from the primary axis of movement with only central vision when they could not see their arm moving to the dowel as when they could see their arm with normal vision.

In addition to the results of the kinematic peak analyses, a description of the trajectory profiles between the vision conditions is warranted. Examination of the velocity and the acceleration (Fig. 6(a,b)) profiles showed differences in the deceleration phase of the movement between the vision conditions. With only central vision, the area of the curve representing the time spent in deceleration after 0 crossing in Fig. 6(b) has a semblance to two phases. These two deceleration phases are less distinct with normal vision. With only central vision, the first phase represents a large deceleration (similar to the normal vision deceleration) and the second phase represents a smaller near constant deceleration as the dowel is approached. The second phase is much longer than the first phase for only the central vision condition. Furthermore, with central vision, the second phase indicates a constant deceleration in contrast to normal vision where there does not appear to be two distinct phases of deceleration. All subjects showed similar differences between these two phases in deceleration with only central vision and normal vision.

Grasp component. Analyses of variance of maximum aperture values revealed that the time spent prior to maximum aperture was longer for only central vision compared to normal vision, F ( 1, 5 ) = 9.4, P < 0.05. Recall that movement time was longer with only central vision compared to normal vision. However, the time normalized data did not reveal any significant effects, nor were there any differences in the size of the maximum aperture between only central vision (Fig. 6(c)).

Co-ordination of the transport and grasp components. As for Experiment 1, further analyses examining the size of the aperture and the per cent of movement time when the distance between the wrist IRED and dowel IRED was constant for only central vision and normal vision were performed. In contrast to the results of Experiment 1, there was no difference in the size of the aperture when the arm had stopped moving between only central vision and normal vision (Table 7, Fig. 7). For all subjects, for both only central vision and normal vision, the per cent of movement time when the difference between the wrist and dowel became constant was on the average 93%. This suggests that subjects were adjusting their aperture similarly during the approach to the dowel for both vision conditions.

Discussion

This experiment investigated the contribution of central vision to reaching and grasping. The visual manipulation was such that with the goggles on, the dowel, but not the arm and

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two-phases are less distinct with normal vision.

1112 B. SIVAK and C. L. MACKENZIE

Table 7. Experiment 2: mean aperture size (mm) and mean per cent of movement (MT) when distance between wrist IRED and dowel IRED is

constant for each subject and vision condition

Subject

When wrist dowel distance is constant Central vision Normal vision

Aperture size % MT Aperture size °/(7 MT

C.DR. 54.3 94 58.3 93 J.R.R. 59.2 95 60.1 96 M.E.R. 58.7 95 60.3 92 A.SR. 52.8 91 55.4 89 H .C.R. 53.7 89 52.0 94 A.P.R. 55.6 92 52.2 93

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when wrist dowel distance is constant.

PERIPHERAL AND CENTRAL VISION 1113

hand were visible at the start of the movement. Nor were the hand and arm visible throughout the reaching movement. The hand came into view with only central vision when it approached the dowel at the end of the movement. The dowel was always visible with the goggles on.

According to JEANNEROD [-9, 10], the transport component was organized from the visuomotor channel relaying information about the location of the dowel. The time normalized data suggest that the organization of the transport component was affected by the visual manipulation. Peak velocity, peak acceleration and peak deceleration occurred relatively earlier with only central vision than with normal vision. Furthermore, pilot observation of subjects asked to reach for the dowel as fast as possible showed that they consistently undershot the dowel. That is, they always judged the dowel to be closer than it was. The experimental data together with these observations suggest that without a large field of view provided normally by peripheral vision, subjects could not estimate the distance of the dowel accurately. The parameters of the transport component were organized based on information that suggested a nearer target than in reality. Further support for this suggestion is provided by the timing data for the transport component. These data illustrate that the time spent to peak acceleration, peak velocity and peak deceleration were not affected by the visual manipulation.

It appears that subjects gauged the dowel to be closer but when they reached the judged position of the dowel, they had to move futher. Possibly these ajdustments were based on a comparison type of analysis where the position of the hand relative to the target was deemed incorrect, thus requiring a further forward movement of the arm. Possibly, they simply continued until anticipated tactile contact was achieved. Evidence for these possibilities is provided by the fact that the increase in movement time occurred after peak deceleration and by the two phases apparent in the deceleration profile. The two phases suggest that subjects planned to end the movement earlier than they did. That is, when subjects did not reach the dowel when they expected to, they continued to decelerate at a constant rate until tactile contact was made with the dowel (see Fig. 6(b)). It is important that these were not submovements, showing changes in acceleration and deceleration, hut rather a constant deceleration. This suggests to us that subjects were anticipating tactile contact, rather than making a series of corrections.

Consistent with the above is the subjective responses of the subjects. All subjects reported that the dowel appeared closer to them than it was. Surprising to us, however, was that subjects did not adapt to this illusion over the 16 repeated trials. This is in direct opposition to the findings of prism experiments which showed that subjects adapted to the prisms (e.g. [6 k 8]). However, the prism experiments have looked at accuracy as a dependent measure. In this study, subjects were just as accurate with only central vision as with normal vision: they always picked up the dowel. Following up the prism studies with 3-D kinematic analyses of the movements may reveal a different set of results from the previous end-point accuracy results.

Although subjects had difficulty gauging the distance to the dowel, they did not appear to have difficulty with the direction of the movement with only central vision compared to normal vision. The lack of a significant difference between the vision conditions in deviation from the midline plane attests to this statement. Although it has been shown that subjects can use information from the peripheral visual fields to adjust the direction of an arm movement [2] it is not clear that peripheral vision is necessary to assess the direction of movement, in the absence of peripheral vision, eye and head position may provide the necessary

1114 B. SIVAK and C. L. MacKeNziE

information. Experiment 2 results suggest that direction and distance are separable entities of location for grasping an object.

The grasp component was organized from visual information about dowel characteristics and relayed via the appropriate visuomotor channel [9, 10]. As predicted, the results showed that the grasp component was not affected by having only central vision compared to the normal vision. That is, there were no differences in the size of the maximum aperture and the relative time of when maximum aperture occurred between the vision conditions. This is important for two reasons. First, with only central vision, there was no visual information of the position of the fingers both before the movement began and during the movement as is present in a normal vision situation. This suggests that visual information of finger positions before movement begins and on-line visual feedback information of finger position while the hand is approaching the dowel are not necessary for the organization and control of the grasp component.

Second, the fact that the organization of the transport component was affected by only central vision and the organization of the grasp component was not, lends support to the notion that the grasp and transport components are organized through independent visuomotor channels as suggested by JEANNEROD [9] . The fact that the subjects gauged the dowel to be closer did not affect when or how much they opened the hand. Furthermore, the transport component showed a two phase deceleration profile with only central vision. The grasp component did not demonstrate these two phases, nor was it different with only central vision from normal vision. This means that subjects based the organization of the grasp component on visual information received from the size of the dowel (object characteristics) and did not need the visual information that the transport component needed for its organization.

What were the differences between only central vision and normal vision that may account for the differences in the transport component between the vision conditions? With normal vision, the eyes were free to move even though the head was in a straight ahead position. That is, with normal vision, subjects may have been relying on central vision to scan the environment, thus obtaining information of their surroundings. In contrast, with only central vision, the dowel was seen through two small apertures positioned in goggles. Thus, if the eyes moved without the head moving, the dowel would no longer be seen. One might argue that subjects could not use central vision to scan the environment. However, during pilot testing, when subjects wearing the goggles were allowed free head movements to scan their surroundings with central vision, the same undershooting was demonstrated as when the head was not free to move. Thus, it is unlikely that the obtained effects on the transport component can be attributed to whether or not subjects used central vision to scan the environment of the dowel.

The major difference between the vision conditions was the small visual field size with only central vision compared to normal vision. In addition to the lack of adequate environmental cues and therefore location information about the dowel with the reduced field size with only central vision, subjects also did not receive any on-line visual feedback information from the moving limb Although it has been suggested that peripheral vision is used to provide information necessary for directing the limb to proper location of an object [16 18], subjects in the present experiment had no difficulty with the direction, only the distance of the movement. Therefore, it appears that visual feedback information from the moving limb through peripheral vision does not control the direction of the arm movement although it might have some other important control function.

PERIPHERAL AND CF.NTRAL VISION 1 1 I 5

In summary, the organization of the transport component was affected compared to normal vision. It appears that the organization of the transport component relies on important environmental cues and information from the moving limb which are lacking with only central vision. This information appears to be unimportant for the organization of the grasp component. Central vision appears to provide adequate visual information for organization of the grasp component.

C O N C L U D I N G REMARKS

It has been established that, compared to normal vision, peripheral and central vision make specific contributions to reaching and grasping movements. Since peripheral vision lacks high resolution acuity, it seems that with only peripheral vision, information related to the intrinsic features of a dowel was poorly relayed thus affecting the organization and control of both the grasp and transport components and their co-ordination. Since central vision lacks a wide field of view, it seems that with central vision, information related to extrinsic properties of the dowel was inadequate, thus affecting the organization of the transport component and not the grasp component.

We can speculate on the role of peripheral vision and central vision in reaching to grasp an object. Peripheral vision provides important environmental information and visual information from the moving limb needed for the organization and control of the transport component. Central vision provides important visual information required for the organization and control of both the grasp and transport components.

Furthermore, as an outcome of this study, data pertinent to the concept of visuomotor channels were disclosed. The current view, that only visual information about the extrinsic properties of an object are used to organize the transport component, needs to be updated. The results from Experiment 1 suggests that both extrinsic and intrinsic properties about an object can affect the organization of the transport component. That is, processing related to intrinsic properties of an object such as the clarity of the visual image may affect the control parameter of the transport component as well as the grasp component. In contrast, from Experiment 2, the integrity of the grasp component was maintained even though the visual information obtained about the extrinsic properties of the dowel showed perceptual inaccuracies, as revealed by misreaching. That is, visual information about the extrinsic properties of an object does not appear to influence the organization of the grasp component. This does not dispute the existence of independent visuomotor channels but rather suggests that there may be a communication link between them. This communication link may only work one way. That is, when the grasp component is affected, it also affects the organization of the transport component. In contrast, when the organization of the transport component is affected, the organization of the grasp component is not.

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1116 B. SIVAK and C. L. MACKENZIE

6. ELLIOTT, D. and ALLARD, F. The utilization of visual feedback information during rapid pointing movements. Q. J. exp. Psychol. 37A, 407~425, 1985.

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