Perception & Psychophysics1977, Vol. 22 (2), 201-205

Delay of stimulus presentation after thesaccade in visual search

JONATHAN VAUGHAN and THOMAS M. GRAEFEHamilton College, Clinton, New York 14431

Delay of stimulus onset after each saccade in visual search decreased oculomotor and manualreaction times, with a greater effect occurring for the oculomotor response. The saccadicoculomotor reaction might have been facilitated in three ways: by the facilitation of reactionwith a foreperiod warning stimulus, by the attenuation of saccadic suppression effects due tothe stimulus onset delay, or by the use of a strategy of preprogramming fixation durations.The results support a model of visual search using preprogrammed control of visual fixationdurations.

To study how the processing of visual informationis related to the intermittent visual input produced bysequential fixations in search, one might simplymeasure the duration of a subject's fixations whilehe is engaged in a visual search task. However, thismeasurement confounds two factors that might con­tribute to the control of fixation duration: (1) thetime required for cognitive processing of a visualstimulus, and (2) the time required for the oculo­motor system to initiate the next successive saccade.Independent estimates of the time required for eachof these two factors have been obtained in situationsdevised to hold the other factor constant in someway. For example, the time required for processinghas been estimated independently of eye movementsby rapidly presenting a sequence of stimuli at a singleretinal location. Under these conditions, theminimum exposure time required for the correctidentification of each stimulus is 200 to 300 msec,very close to the range of fixation durations normallyobserved in skilled readers (Kolers & Katzman, 1966;Travers, 1973, 1975). Similarly, the time betweentwo saccades has been measured, and again about200 msec appears as the minimum time that mustelapse between saccades (Tinker, 1958; Westheimer,1954). Both perceptual and oculomotor factorsappear to influence the minimum fixation duration.

The present experiment was designed to clarifyhow these factors determine fixation duration insearch. A delay was imposed between the beginningof a fixation and the onset of the visual stimulusto be observed during that fixation. It was expectedthat processes that depend on or are limited by the

A portion of these data were reported at the April 1976 meetingsof the Eastern Psychological Association. This research wassupported by NIMH Grant MH 26303. The authors thank GeorgeA. Gescheider, Douglas J. Herrmann, and Daniel Zwerner fortheir helpful comments on an early draft, and Patti Cohan, AmyPost, and Lydia Ruffolo for conducting the experiments.

rate of information processing would be synchron­ized with stimulus onset, and thus be affected by theimposition of a delay, whereas processes primarilyunder other oculomotor influences would be moreaffected by the beginning of a fixation than bystimulus onset per se. For instance, if the timerequired for processing is the limiting factor in therate of visual search in this task, then impositionof stimulus onset delay should lengthen fixationdurations in direct proportion to the delay imposed,since processing cannot begin until the stimulus ispresented. Conversely, if it is only the rate at whichone saccade can follow another that imposes a lowerlimit on the duration of individual fixations, then thedelay imposed might have no effect on fixationduration.

METHOD

SubjectsSix subjects (three female) with normal vision (corrected

if necessary) served in the experiment. Half the subjects were paidvolunteers; the others, salaried laboratory staff.

ApparatusThe horizontal electrooculogram (EOG) was recorded using

Grass E4S silver-silver chloride electrodes applied with Grass EC2cream as close as possible to the external canthi. Electrode-to­electrode resistance was established at 3,000 ohms or less. An ear­clip electrode served as ground reference. The amplified (Grass,P-18 dc amplifier) EOG was recorded by a PDP-8/e computer.Subjects were lightly restrained in a headrest with a chin support.

The computer generated stimuli on a Tektronix oscilloscopewith P31 phosphor (32 msec delay time) located 27 cm in front ofthe subject. The two stimulus locations were located 7.5 degon either side of the center of the oscilloscope screen, and werepresented on a background of about 0.6 cd/m-. Two stimuli weremade up of 12 dots from a 4 by 6 dot matrix, forming thecharacters "X" or "0." Stimuli subtended an angle of 0.6 deghorizontally and 1.0 deg vertically, and their brightness was ad­justed for comfortable visibility without persistence on the oscillo­scope screen. The subject operated a rnicroswitch with the thumbof the preferred hand. Red and green pilot lights mounted belowthe oscilloscope provided performance feedback after each trial.

201

202 VAUGHAN AND GRAEFE

Note-The first fixation of each trial and all error trials havebeen excluded. All times are milliseconds.

Table 1Mean Fixation Duration, Oculomotor Latency, and

Reaction Time at Each Stimulus Onset Delay

Figure 1. (a) The stimulus display (left) and apparatus used.(b) The sequence of stimuli presented in a typical trial. A fixationon a nontarget stimulus on the left was followed by a fixationon a target stimulus on the right. Stimulus onset delay was 90 rnsecand stimulus duration was 200 msec. Note that the oculomotorlatency and reaction time are measured from stimulus onset, andthat oculomotor latency plus stimulus onset delay equals fixationduration.

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REACTION 1TIME

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Onset Fixation Oculomotor ReactionDelay Duration Latency Time

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120 394 274 373150 415 265 378

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p < .001] were significant using a conservative testfor interactions in repeated measures designs (Winer,1962, p. 323). Given this analysis, Table 1 and Fig­ure 2 may be interpreted as follows: Stimulus dura­tion and stimulus onset delay affected oculomotorlatency and reaction time; the effects were greaterfor oculomotor latency than for reaction time. Fig­ure 3 shows the oculomotor latency for each in­dividual subject for the two most extreme durations,100 and 300 msec. The data of five subjects showincreasing divergence between the 100- and 300-mseccurves as stimulus onset delay increases. The singleexception (subject D.H.) is also the fastest subjectin the group, with minimum oculomotor latenciesof 200 msec; it is plausible that this subject was moreaffected by a physiological "floor" effect than theothers.

RESULTS

ProcedureFigure IA shows the stimulus display as it appeared to the

subject, and the apparatus that was used. Each trial began with thescreen blank except for a central fixation point. To begin a trial,the subject pressed the microswitch and stared at the center fixa­tion point. This fixation was used by the computer to recalibratethe recording received from the subject at the beginning of eachtrial (necessary because of drift in the EOG signal, particularlyin the early part of each session). After a fixation of 450 msec,the center fixation point was extinguished and two other fixationpoints were presented at the stimulus locations. The subject thensearched for the target stimulus (which was "0" throughout), byalternately fixating the two stimulus locations.

In detail, the presentation of the stimuli during each fixationwas as follows (see Figure 1B): After the beginning of a fixa­tion on one side, the screen was blank for the period of stimulusonset delay (0 to 150msec). Then the stimulus appeared at thelocation of the subject's fixation for the stimulus duration (100to 300 msec). Following stimulus offset, if the subject was stilllooking at that location, the stimulus was replaced by a maskingnoise pattern of 12 randomly placed dots. The presentation ofthe stimulus or mask was terminated when the subject looked awayfrom it.

If the stimulus presented during a fixation was the nontarget("X") stimulus, the subject continued his search by making asaccade to the other side; his oculomotor latency was recordedfrom the onset of that nontarget stimulus to that saccade. Ifthe stimulus presented was the target ("0") stimulus, the subjectreleased the microswitch as quickly as possible, and manualreaction time was recorded form the onset of that target stimulusto the release of the microswitch. An error was recorded on a trialif the subject released the rnicroswitch before the target onset, ormade any additional saccades after the target onset; errors werereported to the subject by illumination of the red pilot light forI sec, while correct responses were reported by illuminationof the green light for 0.5 sec.

A complete session comprised 240 trials during which thenumber of nontarget stimuli preceding the target was varied from2 to 6 (there were 48 trials with each number); the stimulus onsetdelay was varied from 0 to 150 msec in steps of 30 msec, and thestimulus duration was selected from the values 100, ISO, 200,and 300 msec. Values for these variables were combined in acounterbalanced order; the values were the same within each trial.Each subject received practice until accuracy was 90070 or above;then five additional sessions were conducted.

Overall mean fixation duration, oculomotorlatency, and reaction time are shown in Table 1 foreach stimulus onset delay.

Figure 2 shows the effect of stimulus onset delayon oculomotor latency and reaction time for the fourstimulus durations used. Both oculomotor latencyand reaction time were facilitated by a precedingdelay, but the effect was greater for the oculomotorlatency than for reaction time. A 2 by 4 by 6 analysisof variance showed significant main effects for theresponse measure [reaction time vs. oculomotorlatency: F(l,5) = 44.26; p < .002], stimulus dura­tion [F(3,15) = 16.08; p < .001], and stimulus onsetdelay [F(5,25) = 189.68; p < .0001]. The two-wayinteractions of Response Measure by Stimulus Dura­tion [F(l,5) = 16.74; p < .01] and ResponseMeasure by Stimulus Onset Delay [F(l,20) = 46.88;

DELAY OF STIMULUS PRESENTATION 203

Foreperiod EffectsIn the paradigm used here, the saccade serves to

and nontarget stimuli. Saccadic suppression should,then, equally effect the perception of both target andnontarget stimuli. Thus, if the presentation of thestimulus is delayed until the time of suppression iscompleted, we should find quantitatively similareffects of the delay of stimulus onset for both oculo­motor latency and reaction time. However, the effectof stimulus onset delay was larger for oculomotorlatency than for reaction time, and extended (foroculomotor latency) far beyond the 30 to 40 msecafter saccade onset during which saccadic sup­pression has typically been observed psycho­physically (e.g., Volkmann, Schick, & Riggs, 1968).Thus the steep slope of the oculomotor latency curvebeyond the 30-msec delay cannot be attributed tosaccadic suppression during a stage common to thedetermination of oculomotor latency and reactiontime.

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Figure 2. Oculomotor latency and reaction time at each stimulusonset delay, for each stimulus duration used. The dashed line hasa slope of - 1. The first fixation of each trial and all error trialshave been excluded.

Because each number of nontarget stimuliappeared in an equal number of trials, the condi­tional probability of a target stimulus varied over thecourse of each trial; however, this did not appear toaffect the overall appearance of the oculomotorlatency function (Figure 4). Though the functionwas shifted downwards for fixations when the prob­ability of a target stimulus was zero, all curves haveabout the same shape, regardless of the ordinalnumber of the fixation within a trial or the condi­tional probability of the target on that fixation.

DISCUSSION

The major effect observed is the steeper slope foroculomotor latency than for reaction time, as stimu­lus onset delay is increased. This result appears torule out the possibility that the reaction time andoculomotor latency functions are determined by acommon mechanism, either saccadic suppression orwarning stimulus foreperiod effects.

Saccadic SuppressionSaccadic suppression influences ought to occur

only in the earliest stages of processing a stimulus.These initial stages of processing (perception andencoding) are common to the fixations of both target

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Figure 3. Oculomotor latency for each subject, at the 100- and3OO-msec stimulus durations.

204 VAUGHAN ANDGRAEFE

Figure 4. Oculomotor latency and reaction time for stimuliappearing on the first through sixth fixations of each trial. Theprobability of a target after zero or one nontarget stimuli is 0.0;after 2, 3, 4, 5, or 6 nootargets, it is 0.20, 0.25, 0.33, 0.5, or 1.0,respectiYely.

signal the subject that the stimulus is about to bepresented as well as to expose new information onthe retina. Warning stimuli have been observed todecrease disjunctive reaction time in a number ofsituations, presumably by facilitating attentionalor preparative processes. For instance, Bertelson andTisseyre (1969) presented a warning click or flasho to 750 msec before a visual signal calling for achoice reaction. Both auditory and visual warningsignals facilitated reaction time to the visualstimulus, as in the reaction time data of the presentexperiment. We might suppose, then, that the effectof stimulus onset delay on oculomotor latency issimply such a foreperiod effect where the saccadeserves as a warning stimulus. However, the magni­tude of the facilitation of oculomotor latency bystimulus onset delay in the present experiment ismuch larger than that observed for other reactiontime tasks, which have involved finger movementswith auditory or visual warning stimuli. In several

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studies, the slope relating reaction time to the dura­tion of the warning foreperiod ranges from - 0.16to -0.26 over a 0-to-150-msec range (Bertelson,1967; Bertelson & Tisseyre, 1968, 1969; Davis &Green, 1969). Of particular interest is a study bySaslow (1967), who measured saccadic reaction timeto a displaced target preceded by a visual warningstimulus. The slope of reaction time as a functionof foreperiod duration was - 0.22 over the O-to­150-msec range. By comparison, the average slope ofoculomotor latency in the present data over thisrange was over twice these values (about - 0.60),with a slope of nearly - 1 for stimulus onset delaysof 60 msec or less. Thus, the effect observed onoculomotor latency is much greater than knownwarning stimulus effects. 1

The foreperiod effect on reaction time (that is, theshape of the reaction time function) appears to beindependent of target stimulus probability, whichincreases as the trial progresses (Figure 4). Similarly,the shape of the oculomotor latency fixation is un­changed as nontarget probability decreases, thoughboth oculomotor latency and reaction time tend to befaster as stimulus probability (nontarget or target,respectively) increases. Oculomotor latencies becomelonger as the probability of a nontarget stimulusdecreases over the course of a trial, while reactiontimes decrease as the probability of a target increases.Nevertheless, the essential observation of the experi­ment (the near - 1 slope of the oculomotor latencyfunction at the shorter stimulus onset delays) is notchanged by this expectation effect.

Preprogramming of Fixation DurationsIn a visual search task such as this one, the sub­

ject might use a serial information processingstrategy, with several stages executed one after theother. The subject would first perceive the stimuluspresented, then encode it, and finally decide whetherto produce a nontarget response or a target response.In such a model, the stages prior to response selectionwould be common to both target and nontarget fixa­tions. Within this model, it is difficult to see howstimulus onset delay might have different effects onoculomotor latency and reaction time.

Alternatively, the subject might use a strategyin which the control of fixation duration occurs in.parallel with, and independent of, the processeswhich identify the stimuli and initiate target report­ing responses. In this parallel model, the function ofthe fixations in the search task is to present to thesubject the visual stimuli necessary to perform on thetask. This might be done without stimulus control ofthe duration of each fixation, unless the stimulusfixated required an extraordinarily long processingtime. Suppose that in each trial of the search task thesubject initiated a series of fixations of roughly

constant, predetermined duration. If this predeter­mined duration were greater than the maximumamount of time (including the stimulus onset delay,if any) required for processing a stimulus, the infor­mation processing requirements of the stimuluswould have no effects on the duration of fixations,and the function relating the oculomotor latency tostimulus onset delay would have a slope of - 1. If,on the other hand, the predetermined duration ofthe fixations were less than that required to processthe stimulus, then the oculomotor latency functionwould have a slope of zero. In this case, the nextsaccade could only be initiated when the informationpresented in the preceding fixation has beenprocessed, regardless of the delay. By the foregoinganalysis, the slope of the oculomotor latency curveought to approach - 1 if the fixation duration timeexceeded the time required for processing (includingdelay), and approach 0 if the processing time weregreater than the preprogrammed fixation duration.Now, both stimulus duration and stimulus onsetdelay varied, and this ought to affect processingtime.

The time required for the processing of a stimulusought to increase if the stimulus quality were im­paired by decreasing its duration (if it is followed bya mask). Furthermore, the time available for process­ing in a fixation of constant duration would be less ifthe stimulus onset delay were greater. In either ofthese cases, it is more likely that the informationprocessing requirement would exceed a predeter­mined fixation duration. Thus, we would predict thatthe slope of the function relating oculomotor latencyand stimulus onset delay ought to be flatter whenthe stimulus duration was short, or the delay waslong. Just these trends are evident in Figure 2. Itremains to be seen whether other manipulations ofthe stimulus that ought to increase processing timewill show similar effects on this function-for in­stance, stimulus degradation, or increase in thenumber of stimulus alternatives. Finally, the modelproposed here depends on the subject's estimationof the time required for the processing of thestimulus. This estimation could be made moreaccurately in a blocked (constant-delay) design thanin a variable-delay design like the present one, inwhich the delay was different for each trial. Whetherthis prediction is borne out will depend on furtherexperimentation.

In summary, the relation between saccade­synchronized stimulus onset delay and the oculo-

DELAY OF STIMULUS PRESENTATION 205

motor patterns of subjects suggests that under someconditions the duration of fixations in search maybe independent of the visual information presentedin each of those fixations.

REI"ERENCE NOTE

1. Graefe. T. M.• & Vaughan. J. Saccadic and manual reaction­time to stimuli initiated by eye or finger movements. Unpublishedmanuscript. 1977. (Available from Jonathan Vaughan. PsychologyDepartment. Hamilton College, Clinton. N. Y. 13323)

REIo'ERENCES

BERTELSON. P. The time course of preparation. Quarterly JournalofExperimental Psychology. 1967. 19, 272-279.

BERTELSON, P., & TISSEYRE, F. The time-course of preparationwith regular and irregular foreperiods. Quarterly Journal ofExperimental Psychology. 1968, 20. 297-300.

BERTELSON. P., & T!SSEYRE, F. The time-course of preparation:Confirmatory results with visual and auditory warning signals.Acta Psychologica: Attention and Performance II, 1969, 30,145-154.

DAVIS, R., & GREEN, F. A. Intersensory differences in theeffect of warning signals on reaction time. Acta Psychologica:Attention and Performance II, 1969, 30, 155-167.

KOLERS. P. A., & KATZMAN, M. T. Naming sequentially presentedletters and words. Language and Speech, 1966, 9, 84-95.

SASLOW. M. G. Effects of components of displacement-step stimuliupon latency for saccadic eye movement. Journal of the OpticalSociety ofAmerica, 1967, 57, 1024-1029.

TiNKER, M. A. Recent studies of eye movements in reading.Psychological Bulletin. 1958, 55, 215-231.

TRAVERS, J. R. The effects of forced serial processing on identifi­cation of words and random letter strings. Cognitive Psychology,1973, 5, 109-137.

TRAVERS. J. R. Forced serial processing of words and letter strings:A reexamination. Perception & Psychophysics, 1975. 18,447-452.

VOLKMANN. F. C., SCHICK, A. M. L., & RIGGS, L. A. Time courseof visual inhibition during voluntary saccades. Journal of theOptical Society ofA merica, 1968, 58,562-569.

WESTHEIMER, G. Mechanism of saccadic eye movements. A.M.A.Archives of Opthalmology, 1954, 52, 710-724.

WINER, B. J. Statistical principles in experimental design. NewYork: McGraw-Hill, 1962.

NOTE

1. A control experiment (Graefe & Vaughan, Note I) failedto detect any difference between finger and oculomotor responsesin a reaction-time paradigm, observing a slope of about -0.25over the 0-to-150-msec range in both cases. Thus, while a warning­stimulus effect might account for the reaction-time data of thepresent experiment, it does not plausibly account for the differ­ence between reaction time and oculomotor latency.

(Received for publication August 23,1976;revision accepted May 24,1977.)