Some observations concerning saccadic eye movements

Vliim Rer. Vol. 13, pp. K@9-1020. PrrppmonPress 1973. Printedin Great Britain.

SOME OBSERVATIONS CONCERNING MOVEMENTS

SACCADIC EYE

MELVIN K. KOMODA,~ LEONFEXINGER,LOUISJ.

PHILLIPS, ROBERT H. DUCKMAN and RICHARDA. YOUNG

The GraduateFacultyof the New School for SocialRcsearch,366 West12thStreet,New York,New York 10011,U.S.A. (Received12 June 1972;in revisedform 10 Novmzber 1972)

THE C~-~ARA~~ERIS~CS of saccadic eye movements may be investigated in order to make inferences about the efferent system that controls such eye movements. Many of the main characteristics of saccadic eye movements have been known since the classic work of DODGE and CLINE (1901). For example, if a target that an observer is to fixate suddenly moves to a new location, about 200-250 msec will elapse between the movement of the target and the beginning of the eye movement (STARK,Vossrus and YOUNG, 1962; WI~EELESS, BOYNTONand COHEN,1966). This delay, or latency, has been shown to be dependent upon the magnitude (BARTZ,1962; &SLOW, 1967; Wr-rrrn and EASON, 1962) and the accuracy required (LEUSHINA, 1965) of the saccade. These facts imply that there is a processing delay in the saccadic system. The system also appears to be all-ot-none in nature in that once a saccade is initiated, neither the velocity nor the predestined stopping point can be changed (BECKERand FUCHS,1969). A significant impetus for inferring the information input and processing characteristics of the saccadic control system was provided by WESTHEIMER (1954b). Observers in his experiment were asked to look at a target that was displaced to another point in the horizontal plane, remained at that position for a variable duration of time, and then returned to its original position. Westheimer called this pattern of target motion “pulse stimuli” and used various “pulse durations” (i.e. the time at the displaced position), some as short as 40 msec. His results can be best described by quoting him: “The response to a pulse stimulus alwu~~s consisted of (1) a saccadic movement bringing the eyes fully over to a position corresponding to the displaced stimulus (the reaction time for this movement is the typical reaction time for a saccadic movement), and (2) a return sweep in the form of a saccadic movement occurring [in reference to very brief pulses] 200 to 250 msec after the eyes have reached the new position. . . . This response pattern may be interpreted as indicating that each displacement of the pulse stimulus is being reacted to individually. Thus, the initial displacement produces a saccadic movement after the usual reaction time, even rough the stimulus may have returned to its original position before thejirst saccadic mouement has begun.” (p. 935, italics ours). ’ This researchwas supportedin part from NIH GrantNo. 16327whichwasawardedto Leon Festinger. ‘This researchwas conductedwhile the senior author was supportedby a PostdoctoralFellowship, 1 FO2 MH48972-01,awarded by the National Institute of Mental Health. ’ Ruwsts for reprints should be sent to MelvinK. Komoda, The GraduateFaculty,Departmentof Psychology,programin VisualPerception,NewSchoolfor social Research,66 West12thStreet,New York, New York 10011. loo9

1010

MELVINK. KOMODA

et al.

Such a finding has very clear implications and accordingly led YOUNGand STARK(1962) to propose that the saccadic control system is a sampled-data system, In their model, the retinal error information between the position of the eyes and the target is sensed by an “impulse” sampler once every 200 msec. Provided that a saccade had not previously occurred, the onset of the sample is coincident with the occurrence of information about error. Once the sample has occurred, the saccadic system becomes refractory, presumably while processing the error information, and cannot accept any new information for another 200 msec. Clearly, such a model is consistent with the observations made by WESTHEIMER (1954b). The YOUNGand STARK(1962) model, however, was quickly brought into question by WHEELESS, BOYNTONand COHEN (1966). They measured eye movements in response to targets that were displaced briefly six degree to one side, then, after a variable duration, moved to a position six degrees to the other side of the original target position. These “pulse-step” stimuli were randomly presented in a sequence of simple displacements of the target. Unlike WES~HEIMER (1954b), WHEELESS et al. report that observers did not always respond to the initial displacement of their pulse-step stimuli. Rather, the probability of occurrence of an eye movement to the pulse was dependent upon the duration of the pulse. If the pulse had a duration of 50 msec, the pulse was responded to only 8 per cent of the time. Ninety-two per cent of the time, observers simply responded to the final position of the target. At the pulse durations of 100 and 200 msec, the pulse was responded to 23 and 68 per cent of the time, respectively. Hence, the saccadic control system does not remain refractory for 200 msec as YOUNG and STARKsuggest, but is able to input and process new information within 200 msec to modify the saccade. By measuring the latencies of eye movements, WHEELERS et al. (1966) attempted to estimate the time consumed by the saccadic system to modify a saccade on the basis of new information. In those instances when observers did not respond to the pulse, but only responded to the target’s final position, they found the latency of eye movements to be approximately 320 msec. The latency of these eye movements were measured from the end of the pulse and, hence, represents the time between the occurrence of the new information and the subsequent saccade. Since the latency of eye movements to a simple step displacement of the target was found to be 284 msec, WHEELERS et al.(1966) suggest that the saccadic control system utilizes about 40 msec to “cancel” the saccade to the initial pulse. There are also other data which tend to show that the saccadic control system is not a sampled-data system with a sampling period of 200 msec. BECKERand Fucw (1969) used an interesting technique to show this. For very large target displacements (e.g. more than 20 degrees of visual angle), the eye usually makes two successive saccades; the first covering most of the distance and the second with a relatively short latency covering the remainder. When BECKER and FUCHScaused the target to move in the interval between such saccades, they observed that 50 per cent of the second saccades took account of the changed target position. In some cases, the system took account of a change that occurred only 60 msec before the second saccade was made. HORROCK~ and STARK(1964) showed that the control system is able to use information available just 80 msec prior to the execution of a saccade. In their study, the latencies and accuracy of eye movements to predictable target displacements were measured. When target displacements are regular with respect to both position and timing, observers are able to reduce their latencies greatly from the usual 200 to 250 msec, frequently even anticipating the target motion. Horrocks and Stark showed that, while there is still a relatively

Some ObservationsConcerningSaccadic Eye Movements

1011

large error for eye movements that occur up to 80 msec after the tar@ has moved, the errors were down to virtually zero for movements that occurred more than 80 msec after the target was moved. Further evidence that the saccadic control system is not a sampled-data system comes from an entirely different approach. ROBNON (1965), studying the relationship between saccadic and smooth pursuit eye movements, recorded eye movement responses to a target that was displaced in one direction horizontally, then moved from the displaced position with a constant velocity in the opposite direction. He observed that if the target was back at the original fixation point within 1SO-200 msec, the eye did not execute a saccadic movement but only a smooth pursuit movement. The occurrence of only a smooth pursuit movement was proper as the occurrence of a saccade could only have moved the eye away from the target. Thus, Robinson’s data imply that the saccadic control system was continuously monitoring the changing retinal position of the smoothly-moving target. The more recent data, then, do not support the YOUNGand STARK(1962) model of the saccadic’control system, However, a sampled-data model of the saccadic control system can be retained if it is assumed that the impulse sampler has its onset stochastically distributed in time (FUCHS, 1971), or if the sampler has a fmite, but varying, time width rather than the infinitesimal time width of an impulse sampler (BECKERand Fucrrs, 1969; ROBINSON,1968). Such a modified sampled-data model would account for the data reported by WHEELet al. (1966), by assuming that eye movement only to the target’s final position took place if a sampling occurred after the target reached its final position, or if both the initial and final displacements of the target occurred during a single sampling. On the other hand, the recent literature is not inconsistent with a description of the saccadic system as one that inputs and processes information continuously. One simple view of such a system is that the usual latency of a saccadic response to a target displacement is due to neural trans~ssion time plus some amount of computation time. If, during the period of computation, new information arrives, the computation can be altered and the efferent program that is finally issued will be appropriate to the new information. This simple view of the efferent control system for saccadic eye movements has several implications. First assuming that it takes 35 msec for the afferent information to travel from the retina to the central nervous system and another 35 msec for the subsequent efferent command to reach the extraocular muscles, it is clear that the last usable input to the efferent system must occur 70 msec before a saccade takes place. This 70 msec “refractory” period for the saccadic system is not too different from the data reported by BECKERand Fucrrs (1969) and HORROCKSand STARK(1964). Second, if new information arrives during the ongoing computation for a saccade and is used to change the computation, one reasonable expectation is that this change would increase the computation time and, hence, increase the latency of the resulting eye movement. This increase in latency, due to recomputation of the efferent command, might well be the 40 msec “cancellation” time reported by WHEELESSet al. (1966). In these respects, the proposed description of the saccadic system fits, with the recent findings, more nicely than the sampled-data models. The following data were collected in an attempt to see if the notion of continuous information input into the saccadic system is at all tenable by replicating the procedures of WESTHEIMER (1954b) and WEEELESSet al. (1966), the findings of whom have never been reported as confirmed. In addition, by presenting two other pulse-step target patterns, the experiment WBSdesigned to obtain a better estimate of the time utilized by the saccadic control system to recompute new efferent commands.

1012

MELEE K. &3MDDA et al.

METHOD Observers Three females, ages 19-23, with normal visual acuity and good binocular coordination served as observers. They were naive with respect to the purposes of the experiment and were paid for their participation. Apparatus

The target was a spot on an oscilloscope face located 55.5 cm from the observer. The position of the spot was controlled by a logic system and paper tape reader. By the suitable programming of a paper tape., the spot could be presented at any one of nine horizontal positions, each separated by one degree of visual angle, or off the oscilloscope face. The spot could be preacn&d for durations which wem integer multiples of onesixtieth of a second. The transition time from one position to the next was less than 1 msec. Eye movements to the target displacements were measured using a non-ntacting eye-tracker developed by CORNand CRANK(1967). The eye-tracker operates by measuring the distance between the 5rst and fourth Purkinje reflections. The obsen&s head was held ilxed by using a bitsboard and head-rest, and movements of the left eye wererecordedwith the right eye occluded. The eye movement records had a noise level of about 6 min of arc. Both eye position and target position were simultaneously recorded using a Grass Model 7 polygraph. The paper was run at 50 mm/sac, allowing the measurement of eye movement iatencies to within f 5 msec. Procedure Observers were presented with the following sequencea of target displacements: 1. Simple step. This was a simple displacement of the target either one or two degrees and either to the

left or to the right. 2. Puth-retum. This was a replication of the Westheimer (1954b) target pattern. The tarmt was briefly displaced and then returned to its original position. The initial displacemen t was either one or two degrees, either to the left or to the right with pulse durations of 50,100,150 or 200 msec. 3. Pulse-oucr return. This was a replication of the WnaaLaas et ul. (1966) target pattern. Following an initial displacement of either one or two degrees in either direction in the horizontai plane, the target moved to a position an equal distance to the other side of the original target position. The pulse durations were the same as that for the Pulse-return stimulus. 4. Pulse-partial return. The target was briefly displaced two degrees to the left or to the right and then moved one degree back towards the original position. The pulse durations were the same as in the previous stimuli. 5. Pubforward. The target was briefly displaced and then moved an equal distance in the same direction as the initial displacement. Again, the initial displacements were either one or two degrees. to the left or to the right, with the same pulse durations as in the previous stimuli. With the different stimulus parameters, there were 60 different sequences of target displacements. These 60 sequences were randomly presented to the observer in 5ve blocks of 12 trials each. The start of a trial was signalled by the disPppsurrn ce of the target, then reappeamnce of the target at the center fixation point. The observer was instructed to 6xate the target as quickly as possibk, than look at the tar@ wheneverand wherever it moved. Tbe time between the appearance of the targetand the initial displacement of the target was ra&omly varied between 2,3,4 and 5 sec. Hence, the observer was unaware as to precisely when or how the target would move. Two observers completed ten replications of the 60 target sequences. One obsorver terminated the experiment early and was al&e to cornpI& only five replicatio& of & target sequences. Observers completed two replications each day in a 2 hr session. There was a break of about 2 min between each block of 12 trials and a break of about 10 min between each replication of the 60 target sequences.

RESULTS

AND DISCUSSION

For each observer, both the magnitudes and the latencies of the saccadic responses to the pulse-step stimuli were measured from the polygraph recordings. Table 1 shows the percentage of responses to the pulse and the mean latencies of the saccadcs together with the 95 per cent confidence interval for each mean. In the computation of the results the data were averaged over observers, direction and magnitude of displacement of the pulse. The results for the magnitudes of the saccadic responses are not presented as the accuracy of the eye movements did not show any systematic variation.

1013

Some Observations Concerning Saaadic Eye Movements TABJX 1.

PERCENTAGE

Stimulus-pulse duration

OF RESWNSES

n

Pulse-rctum 50 100 150 200

101

Pulse-over return 50 100 150 200

0 17 79 100

Pubfomard

50 100 150 200

Pubpartiaf

lf

return

150 200

3 ::

10

TXE

PuLS!Z AND

MEAN

Response to pulse % L,t

SA~XDIC

umm

No response to pulse

n

Lf

100 84 16 3

-

240 213 f 8 234 f 5 241 &6

170 179 f 10 193 f 6 196k 6

0 :‘8

213 f 7 232 f 5 248&S

146; 25 171 f 5 181 f 6

;:

258 21s k 7 235 f 6 248*16

14 3

23

36 45

;:

3 19 75 93

2

I#uLBEONDS*

L

3 18 83 97

97

m

102 85 22 3

261 17 252 + 7 254 f 17 253

209 171 f 12 189 f 10 195 * 9

97 27

216 f 5 187 f6 154 f 15 163

214 f 8 293

211 f22 203

37 51

208zk8

236&6 239 f 6

202 f 14 205 f 11

15 4

?w 98

7

*The plus or minus value entered after each mean iadicntes the limits of the 95 per cent confidence irlterval for that mean. t The mean latency for a sacxadc to a simple displacement is 245 f 6 msdc, n = 104.

The percentage of responses to the pulse The first results to be considered are the percentage of cases, at each pulse duration, in which the observer responded to the pulse, or initial displacement, of the target. These results are also presented in Fig 1, and bear directly on the replicability of the findings of WESTHEIMER (1954b) and WHEELES et al. (1966). The solid line in Fig. 1 represents the average percentage of responses across the four pulse-step stimuli. As Fig. 1 shows, the percentage of responses to the pulse is similar for the four pulse-step stimuli at each pulse duration and increases as the pulse duration increases. Although similar at the shorter pulse durations, the present results do show a higher percentage of responses at the pulse duration of 200 msec than that reported by WHEELESet al. This difference in results may be due to the fact that the observers in the study by Wunax.~ et at. had a longer mean latency for an eye movement to a simple step displacement of the target, namely, 284 msec as compared to 250 msec in the present study. The results, however, do not confirm the report by WESHEIMER(1954b) that observers always responded to the pulse irrespective of the duration of the pulse. Rather, as in the case of the other pulse-step stimuli, the number of responses to the pulse using Pulse-return stimuli (the target pattern used by WESTHEXMER) increases with increasing pulse durations. Our findings seem to support the view that the efferent control system for saccadic eye movements continuously engages in computation. Further evidence which supports this view may be seen in the latencies associated with the eye movements.

1014

MELW K. K~MODAet al.

100 -

% 40 -

0 0 0

60 zo-

//

PR Pof3 l PF 0 PPR

FIQ. 1. The pacsatrroeof mapoasesto the puke, with PR rcpmscnting Pubrewm; POR, Pub-over mum; PF, Pulse-fomard;and PPR, Pulse-partial rctumstimuli.

Latency of eye movements to both pulse and step Whenever the eye responds to both the pulse and the step, two latencies can be calculated. One latency (LJ is the time from the initial displacement of the target to the beginning of the saccade to that displacement. The second latency (LJ is the time from the end of the first saccade to the beginning of the saccade which brings the eye to the target’s linal position. The L,, and L, latencies for each pulse-step stimulus are also presented in Fig. 2 for ease of comparison. (The data obtained for the pulse duration of 50 msec are not plotted as each observer made very few responses to the initial displacement at that pulse duration.) The dotted line parallel to the abscissa in Fig. 2 indicates the magnitude of the latencies of eye movements to a single displacement of the target. The high degree of similarity of the LD latencies for all pulse-step stimuli is evident from Fig. 2(a). Such results were expected since the observer did not know which pulse-step stimulus would be presented. We did not expect the increase in the h latencies with increasing pulse durations. However, this increase is probably due to a statistical sampling artifact because responses to pulses with short durations may occur more often for a saccade that would, by chance, have had a shorter latency anyway. The experimental design requires, in principle, that the L, latencies be similar, and no differences were indeed found. The L, latencies, then, are the first results in which differences in the four pulse-step stimuli may be attributed to the characteristics of the saccadic control system. As a comparison between Figs. 2(a) and 2(b) shows, the I,, latencies are shorter than L, latencies for each pulse-step stimulus at all pulse durations. This difference between the latencies presents a difficulty for the sampled-data models. In none of these models is there any suggestion that the second sample is in any way different from the sample that resulted in the initial sacoade. Since the second sample cannot occur until the first saccade has been completed, such models would have to predict that the L, latencies should be equal to the L, later&s. To preserve a sampled-data model in the face of these results requires the kind of specific assumption that is proposed by BECKERand Fuchs (1969), namely, that when sampling occurs in immediate succession in the saccadk system, the time width of the second sample is shorter than the first. While they provide no rationale for such a proposal, it can

Some Observations ChUXhng Saaxdic Eye Movements

101s

2 l

160

$

PR

0 POR 8 PF 0 PPR

140

160 t

Pulse

duration, msec

@) (a) FIG. 2. Mean late&es of the first sxxade &J and second saccade (L,) when observers responded to both target displacements. with PR representing Pulse-return; POR, Pulse-over return; PF, Pulse-forward;and PPR, Pulse-partial return stimuli.

account for the shorter L, latencies from a sampled-data model point of view. However, the fact that the L, latencies are as much as 60 msec shorter than the L,, latencies may imply that there is continuous information input into the saccadic system and that the computation of the second saccade began before the computation of the first saccade had been completed. When the differences between the L, latencies for different pulse-step stimuli and pulse durations are considered, the results do not even appear to be in accord with the specific assumption proposed by BECKERand FUCH.T(1969). A sampled-data model of the saccadic control system must assume that the sampling parameters are only dependent upon the characteristics of the system and independent of the stimulus parameters. Yet, as can be seen in Fig. 2 (b), L, latencies vary with the duration of the pulse and with the particular pulse-step stimuli presented. It is difficult to imagine how the sampled-data model can be modified so that it provides an explanation of the present results while retaining the property that the sampling characteristics are independent of the stimulus parameters. The findings that the I+ latencies vary with pulse duration and the pulse-step type also provide difficulties for the notion that there is continuous information input into the saccadic system, although for different reasons. Thus far, the saccadic system has been simply viewed as one which engages in the computation of saccades continuously. However, if such were the case, then the second saccade should follow the first saccade with a delay equal to the pulse duration of the stimulus presented. Clearly, from Fig. 2(b), the L, latencies are, on the average, 70 msec longer than the pulse duration, 40 msec longer than the pulse duration, and approximately equal to the pulse duration for pulses with durations of 100,150 and 200 msec, respectively. In other words, the time required by the saccadic control system to process the second saccade, increares as the pulse duration decreases. This finding seems to imply that the saccadic control system, while engaging in computation continuously, cannot compute sac&es for two target displacements simultaneously with equal efficiency if a

1016

MELVINK. ibMODA et uf.

response is to be made to both displacements. The finding (that the time, required by the saccadic control system to compute and program a saccade, increases as the pulse duration decreases) can be explained if it is assumed that it is the computation of the second saccade that suffers from the system’s limited efficiency. Hence, the earlier the information about the second displacement enters the system, the longer is the time under which the computation of the saccade is being done with less than maximum efficiency. The results presented in Fig. 2(b) show that L, latencies also vary with the pulse-step stimulus presented. However, it is not clear from Fig. 2(b) whether the differences are due to the type of pulse-step stimulus or to the magnitude of the step (i.e. the second displacement

Stimulus-pulse duration Pulse-return 50 100 E

n

L

-

-

1

-

-

:f 51

Pulse-over return 50 100 2 Pulse-forward 50 100 150 200 Pulse-partial return 50 100 150 200

Magnitude of second dispkccmtnt 2”

4”

n

L

I” n

170 176 k 11

2 7

184* 192 f 7IO

;

L

170 184 f

201 20045 810

0

121 zk 37

0

163i39

-

-

50 3(:

162 175 if9 8

:SO

187zt9 179*7

-

-

-

-

0 9 38 47

170 4 24 178 rfr.11 193 rt 13

-

-

-

-

-

-

3 :; 46

3 14 ::

15

209 171 f 13 199 * 15 196 -fr 13

203 211 f22 202 f 14 2Os* I1

* The plus or minusvalue e&e& after each mean indicates the limits of the 95% confidence interval for that mean.

of the target) as the two factors are confounded in the experiment. That is, the magnitude of the step was either two or four degrees for Pulse-over return stimuli, either one or two degrees for both Pulse-return and pulse-forward stimuli, and onlyf one degree for Pulsepartial return stimuli. Thus, the L, Mencies were also analyzed according to the magnitude of the step. The results of the analysis, presented in Table 2, suggest that the L, latencies vary, not with the type of pulse-step stimulus, but with the magnitude of the step. This can be seen clearly in Fig. 3, in which the L, late&es in Tabk 2 have been averaged over the pulse-step stimuli for each magnitude of the step. The results shown in Fig. 3 suggest that

&me ObservationstZmcem&

5bdi~

EYCM~vemento

1017

160-

Pulse

FIG. 3. Mean latcnciesof the second

dumticn.msec

sacaxde(LJ as a function of the magnitudeof the target’s second displacement.

the computation time for the second saccade may not only be dependent upon when information about the second displacement enters the system, but also dependent upon the magnitude of the step. Perhaps, while the system is engaged in the computation of a saccade, it is less sensitive to subsequent target displacements. This line of reasoning might imply that the larger the second displacement, the more quickly the change is registered in the

saccadic control system.

The final results to be considered are the latencies of eye movements for those instances when the observer did not respond to the initial displacement of the target, but only to its final position. These results are of interest since they give an estimation of the time required by the saccadic control system to recompute a saccade on the basis of new information. The results are presented in Fig. 4, with the latency (L& representing the time that elapsed from the end of the pulse to the beginning of the eye movement. (The data obtained for the pulse duration of 200 msec are not presented as each observer made very few responses only to the final position of the target.) The results for the Pulse-return stimuh are, of course, not presented since a “response” to the final Position of the target was no eye movement at all. Again, the dotted line parallel to the abscissa in Fig. 4 indicates the magnitude of the latencies of eye movements to a simple step displacement of the target. The immediately striking feature of Fig. 4 is the clear difference in the LI latencies between the Pulse-over return stimuli, and the Pulse-forward and Pulse-partial return stimuli. If the Lc latencies for only the Pulse-over return stimuli (the target displacement sequence used by WHEELESS et al, [1966]) are considered, then the results are similar to the findings they report. That is, the latencies remah @9atively constant across different pulse durations and are longer than the latencies of eye movements to a simple step displacement of the target.

1018

MELVINK. KOIWDAet ol.

p

220

-

E G hi

209-

5 ICO-

160t-

140

c 50

loo Pulse

duration,

rnsec

FIG. 4. Mean latenciesof the saccade (L) when observers responded only to the target’s final position, with POR representing Pulse-over return; PF, Pulse-forward; PPR, Pulse-partial return stimuli.

While WHEELESS et al. (1966) found a difference of 40 msec, Fig. 4 shows, at best, only a 16 msec difference between the Lr latencies and the latencies of eye movements to a simple step displacement of the target. Again, this difference in results may be due to the longer response times of observers in their study as indicated by the longer latency for an eye movement to a simple target displacement. The results, then, suggest that when presented with Pulse-over return stimuli, the saccadic control system appears to utilize a bit more time when it recomputes a saccade. When the results for the Pulse-forward and Pulse-partial return stimuli are considered, however, an entirely different picture of the saccadic control system emerges. Although there are differences in the late&es, for both types of pulse-step stimuli the LI latencies are shorter than the latencies of eye movements to a simple step displacement of the target, and become shorter as the duration of the pulse becomes longer. In direct contrast to the results for Pulse-over return stimuli, these results imply that the saccadic control system, rather than requiring additional time, actually saves time when it recomputes the saccade on the basis of new information. Moreover, the results also suggest that the farther along the saccadic control system is in the original computations, the greater is the savings when recomputation occurs. Indeed, for Pulse-forward and Pulse-partial return stimuli, the system appears to simply modify the partially completed original computation such that the resulting saccade is appropriate to the new information. The results, with respect to Lr latencies, clearly show that it is not possible to simply speak of a “cancellation time” as do WHEELEBer al. (1966), or even a recomputation time. The problem which arises, of course, is a description of the saccadic control system that can encompass the results for all three types of pulse-step stimuli. With the pulse duration we used, the target is invariably at its fhtal position before the eye has moved. Thus, the final position of the target is in the same direction as the initial

Some Observations Concexning Saccadic Eye Movements

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displacement for the Pulse-forward and Pulse-partial return stimuli, but in the opposite direction for Pulse-over return stimuli. This suggests that the saccadic control system may first compute only the direction and approximate magnitude of the saccade, then determine the precise magnitude of the saccade by refining the initial computation. For Pulse-forward and Pulse-partial return stimuli, then, when the system responds just to the final position of the target, much of the preliminary computation can be used in the recomputation of the saccade, saving the system some processing time. For the Pulse-over return stimuli, however, the system has to erase, or discard the preliminary computations and start new computations if the eye is to simply respond to the final position of the target. If the recomputation process also involves the cancelling of preliminary instructions already issued, then the system might consume more time when it responds to the final position of Pulse-over return stimuli than when it responds to a single displacement of the target. REFERENCES BARTZ,A. (1962). Eye movement latency, duration and responsetime as a function of angular displacement. J. exp. Psychol. 64,318-324.

Becrctx, W. and FUCRS,A. (1969). Further properties of the human saccadicsystem: eye movements and corraction saccades with and without visual fixation wints. Yision Res. 9.1247-1258. COOK, G. (1965). Control system study of the saccadi~ eye-mownent meckndsm. Sc.D. Thesis, M.I.T., Cambridge, Mass. CORNSWEET, T. N. and CRANE,H. D. (1967). Design of an optometer and two-dimensional eye tracker. NASA Tech. a-p., ContractNA2-3517. DOWE, R. and CLINE,T. (1901). Tha angle velocity of aye movements. Psychol. Rec. 8,125-157. Fucxs, A. F. (1971). The saccadic system. In Tire Control of Eye Mouements (edited by P. BACH-Y-RITA and C. C. C~LUN@pp. 343-362, Academic Press, New York. HORROCKS, A. and STARK, L. (1961). Experiments on error as a function of response time in horizontal eye movements. Q. Progr. Rep. Res. Lab. Electr. M.I.T. 72,267-269. HYDE, J. (1959). Some characteristics of voluntary human ocular movements in the horizontal plane. Am. J. Ophthal. 48,85-W. LEU~HXNA, L. I. (1965). On estimation of position of photostimulus and eye movements. Biofzika 10, 130436. ROBINSON, D. (1964). The mechanics of human saccadic eye movement. J. Physiol., Ltmd. 174,245-264. ROBINSON, D. (1%5). The mc&anics of human smooth pursuit eye movement. J. Physiol., Land. 180, 569-591. ROBINSON, D. (1968). The oculomotor control system: a review. Proc. IEEE 56, 1032-1049. !&SLOW, M. (1%7). Latency for saccadic eye movement. J. opt. Sot. Am. 57,1030-1033. STARK,L., Vossnr~, G. and YOUNG,L. (1962). Predictive control of eye tracking movements. IRE nuns. Hl?E3,52-57. Wm, G. (1954a). Mechanism of saccadic eye movements. Arch. Ophthul. !Q, 710-724. Wesnreace~, G. (1954b). Eye movementresponsesto a horizontally moving visual stimulus. Arch. Ophthol. s&932-941. Ww, L., JR., BONN, R. and COHEN,G. (1%6). Eye-movement responses to step and pulse-step stimuli. J. opt. Sot. Am. 56,956960. WHITE,C. and WN, R. (1962).Latency and duration of eye movementsin the horizontal plane. J. opt. Sot. Am. 52,210-213. YOUNG, L. and STARK,L. (1962). A sampled-data model for eye-tracking movements. Q. Progr. Rep. Res. Lab. Electr. M.I.T. 66, 370-384. Ah&act-Eye movements, made in response to a target that was displaced twice (pulscstep stimuli), were recorded for three obsezvers in order to ascertain the characte&tia of the saccadic control system. Five types of pulse-step stimuli were randomly presented to the observers. The results indicate that observers did not always reapond to the initial displacement of the target, but sometimes responded only to the target’s final position. The percentage of instances in which observers responded to the pulse ixmased as the duration of the pulse increased from 50 to 200 msec. In those instpnces when observem responded to both target displacements, the latency of the second saccade was shorter than that of the first. When V.R.13/6-B

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observers only responded to the target’s final position, the latency of the saccadc was shorter if the target’s final position was in the same direction than if it was in the opposite direction from its initial displacement. These results suggest that there is continuous input of information into the saccadic control system. R&am&-Qn enregistre sur trois observateurs les mouvements des yeux en reponse B une cible deplac& deux fois (stimuli B mouvement en echelon), atin dVtudier lea caracteristiques du systhnt de conMe des saccades. On constate que les observateurs ne ripondent pas toujours au dCplac.ement initial de la cible, mais seulement parfois a la position finale. Le pourcentage des reponses a I’eChelon augments quandladtmk de 1’8chelonpasse de56-266msec. Lorsque ks observateurs repondent aux deux d&placement de la cible, la latence est plus courte pour la seconde saccade que pour la premiere. Quand ks observateurs r&ondent seukment B la positiontinalede la cible, lalatence de lasaccade estplus courte quand lapositionfinaledelacible estdans la m&medirection que son deplacement initial, que dans le cas contraire. Ces r&&us suggerent qu’il y a entree continue dinformation dam k systime de contri3le des saccades. w-Augenbewegungen von drei Beobachtern, die als Antwort auf einen Impuisstufenreiz erfolgten, wurden aufgezeichnet, urn die Charakteristik des saccadischen Kontrollsyste.ms au ermitteln. Ftinf verschiedene Typen von Impulsstufenreizen wurden den Beobachtem in statistischer Reihenfofge dargaboten. Die Erg&mime zeigen, daas die Beobachter nicht immer auf die anf&ngbche Verscbiebung des Test&&ens, sondem manchmai mu auf die endgt@ige Lage reagieren. Der Prozentsatz der Reaktionen der Beobachter auf einen Impuls nahm zu, wenn die Dauer des Impulses von 50 auf 266 msec gesteigert wurde. In Fiilkn, in denen die Beobachtcr auf beide Testzeichenverschiibungen reagierten, war die Latenz der zweiten Saccade kibzer als die der ersten. Wenn die Beobachter nur auf die endgiiltige Lage des Testzeichens mgierten, war die L&cm der Saccade kUzcr, wormdie endgQltigcPosition in der gleichen Richtung lag, in der die erstc Verachkbung erfolgte, ala im Falk einer entgegongcsetzten zweiten L.ag&ndertig. Diese Ergebnisse lassen vemuten. dass die Information vom saccadischen Kontrollsystem kontinuierlich aufgenommen wird. Pemme-J&f4sme~

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