M 1969IVISIBILITY OF SINE-WAVE GRATINGS 617

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 59, NUMBER 5

Saccadic Suppression*t

WHITMAN RICHARDSDepartment of Psychology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

(Received 12 September 1968)

A shift of the peak of the Stiles-Crawford effect suggests that saccades shear the retina. This action appearsto lead to an increase of the retinal activity of a real-light background. Thus, thresholds following a saccadeare raised the most for test wavelengths which are most similar to the adapting-field wavelength. If theadapting field is eliminated, saccadic suppression is reduced. Saccades also affect the customary rises ofthresholds found near the onset and extinction of the adapting field. This effect is as if the retinal feedbackloop underlying adaptation is disrupted by the saccade.INDEX HEADING: Vision.

Vision is generally impaired during rapid eye move-ments. This impairment, however, is not due entirelyto optical smearing of the retinal image, for two reasons.First, the visual threshold has been shown to be alteredfor microsecond flashes',2 and, second, the maximalsuppression effect often occurs just prior to the eyemovement.'- 3 Thus, neural factors are apparently in-

* Supported under U. S. Air Force contract AFOSR-F44620-67-C0085, with supplementary funding to Professor H.-L. Teuberunder NASA and NIMH grants NsG 496 and MH 05673.

t Presented in part at the Optical Society of America Meetingin Pittsburgh, October, 1968 [J. Opt. Soc. Am. 58, 1559A (1968)].

'P. L. Latour, Vision Res. 2, 261 (1962).2B. L. Zuber and L. Stark, Exptl. Neurol. 16, 65 (1966).3 F. C. Volkmann, A. M. L. Schick, and L. A. Riggs, J. Opt. Soc.

Am. 58, 562 (1968).

volved in the visual suppression associated with sac-cadic eye movements.

Because of the presence of elevated thresholds priorto the actual eye movement, most investigators haveattributed the suppression effect to central factors.2 -4

For example, the expectancy of making an eye move-ment could lead to a central command coordinatedwith the command for eye movement, i.e., a "corollarydischarge,"' which would attenuate the incoming

4 P. L. Latour, "Cortical Control of Eye Movement," thesis,Institute for Perception RVO-TNO, Soesterberg, The Nether-lands (1966).

5 H.-L. Teuber, in Handbook of Physiology, Vol. III, Neurophysi-ology, J. Field, Ed. (American Physiological Society, Washington,D. C., 1960), p. 1595.

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blurred visual signal arising during the actual eyemovement.

Recently, however, this view has been questioned bythe observation that visual suppression may occurduring passive eye movement.7 Based upon this evi-dence, an alternate explanation of the suppressioneffect is proposed: that the origin of the effect is inthe retina, which is disrupted as a consequence of theeye movement.8 Because the neural effect of a lightflash is still residing in the retina several tens of milli-seconds following the flash, it is possible that an eyemovement might affect the threshold for a flash de-livered prior to the actual movement.

Consider the eye as a rigid sphere containing a gela-tinous material. If this sphere is subjected to high rotaryacceleration, then shearing forces will act with differentmagnitudes at different distances from the center ofthe sphere. Like a bowl of jelly suddenly made to rotate,in which the greatest forces occur at the wall of thecontainer, the retina will be subject to shear. The shearin the neighborhood of the vitreous-scleral boundarymight conceivably cause displacements within theretina itself which would lead to an increase of thethreshold for detecting the neural signal arising froma light flash. In order to decide whether or not theretina might be sheared during eye movement, measure-ments of the Stiles-Crawford effect were made bothbefore and following eye movements. Following anaffirmative result, further experiments were performedwhich suggest that eye movement affects adaptationmechanisms in the retina.

APPARATUS

A two-channel maxwellian-view system (2-mmpupil) was used to provide the test and backgroundfields, as shown in Fig. 1. An 110 diam adapting fieldwas provided by a tungsten source (SI) plus variousKodak Wratten filters (F). The 10 test field was formed

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FIG. 1. Schematic diagram of the apparatus: B, backgroundfield; CW, circular wedge; E, eye; F, Wratten filter; GG, groundglass; HS, horizontal slit; IR, infrared filter; L, lens; M, mono-chromator; P, prism; PM, photomultiplier; PT, phototransistor;S, tungsten source; SH, shutter; TF, test-field stop; VM, verniermicrometer; X, xenon arc.

6 E. B. Holt, Harvard Psychol. Stud. I, 3 (1903).W. Richards, J. Opt. Soc. Am. 58, 1159 (1968).

8 Note also the phosphene of quick eye motion reported by B.Nebel, Arch. Ophthalmol. 58, 236 (1957).

II'

FIG. 2. Stimulus configuration used for mostof the experiments.

by a xenon source (X) whose beam passed throughjamonochromator (M) with a 4.4-nm half-width, andthen through Wratten neutral-density filters (F),a circular inconel wedge (CW), and two field stops(TF 1, TF 2). At one focal point in this path was ahigh-speed shutter (SH) which could be triggered topresent the test flash for approximately 2 msec. Alsoin this same path was a prism (P 2) which could berotated by turning a vernier micrometer (VM). Rota-tion of this prism (P 2) caused the focal point of L 7 seenby the eye (E) to move laterally, causing the test-fieldstimulus to enter the observer's pupil eccentrically formeasurements of the Stiles-Crawford effect.

Eye positions were monitored using the methoddeveloped by Stark and members of his laboratory.9

The beam from the ir source (S2) had a second focalpoint near the plane of the test-field stop (TF1 ), whichthus created a slightly diverging, invisible beam afterpassing through lens L7. This ir beam illuminated theiris-scleral boundary with two horizontal lines of flux;the two lines were projected onto opposite sides of theiris. Lateral eye movements altered the amount ofreflecting surface of the eye illuminated by the lines.These changes in total flux reflected from each side ofthe eye were monitored by two LS-400 photo transistors(PT). To minimize the effects of fluctuations in fluxnot related to horizontal eye movements, the outputfrom these phototransistors was fed to a differentialamplifier and a Schmidt trigger, which, in turn, acti-vated an electronic timer. This timer provided avariable delay between the firing of the Schmidt triggerand the activation of the high-speed shutter in thepath of the test-field beam. A second timer in thiscircuit prevented the shutter from being activatedduring the following two seconds.

Figure 2 shows the stimulus configuration seenthrough a reduction tube by the subject, who held hishead in position with a bite board. The ir illuminant

9 L. Stark, G. Vossius, and L. R. Young, IRE Trans. HFE-3, 52(1962).

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was not visible. To the right of a black fixation spot(-° diam) was a 1° test field, which appeared as a 2-msec flash superimposed upon a background of 3 cd/m2

(color temperature 2500 K). The horizontal sides ofthe test field were in sharp focus at an effective accom-modation distance of 50 cm. The vertical boundariesof this test field were intentionally blurred, however,by placing the appropriate field stop (TF 2) beyondthe focal point of the objective. Thus, detection ofthe brief flash was based principally on the appearanceof its upper and lower horizontal borders, which are insharp focus, and not upon the vertical borders whosesharpness might be affected slightly by residual hori-zontal movements following a saccade.

PROCEDURE

The task of the subject was to make a voluntarysaccade to the region of the background field where thetest flash appeared and then, immediately to make areturn saccade, back to the fixation point. As can beseen from Fig. 2, the test flash appeared below a smallspot which served as a cue to the correct distance forthe first saccade. After some practice, both subjectscould make consistent saccades to the region of the testflash even though the cuing spot was located above theflash. The second return saccade back to the initialfixation position was always delayed by about 200msec. Thus, following the first saccade to the right, theeye continued to be oriented toward the position of thetest flash for another 200 msec or so before returning tothe initial fixation position. This delay is seen clearlyin Fig. 3 for the two observers who were used for thefollowing studies. After the first saccade, there is a deadtime of about 250 msec for WW and 150 msec for WR,before the return saccade. Note also that each observershows hardly any consistent overshooting following thefirst saccade.

The advantage of making two successive saccades isthat the dead time preceding the second saccade canbe used to present flashes with fairly reliable time inter-vals prior to a saccade. Thus, the first saccade triggersa delayed flash which will precede the second saccadeby an interval which depends upon the length of thedead time. This period of no movement was found to beroughly constant for each observer, varying by nomore than 20% during a session. As will be seen by theresults, this stationary period is sufficient to permit adecision whether the first or second saccade has prin-cipally altered the threshold. For long delays, to insurethat the test flash occurred prior to the second saccade,the eye movement record was displayed on a cathode-ray tube, and monitored by the experimenter.

All threshold settings were made by the observer,who manipulated the circular neutral-density wedge(CW) which was placed in the path of the test flash.Observer WR was accustomed to this method, butWW, a naive observer who previously had no experience

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FIG. 3. Averages of 25 double saccades made by the two ob-servers. Upper trace: WR; Lower trace: WW. Note the absenceof any consistent overshooting once the eye reaches the targetarea, and the subsequent stability of the eye for 150 or 200 msecprior to the beginning of the return saccade.

making such judgments, needed several practice ses-sions before he could make consistent settings. Thesepractice sessions were run before beginning the entireseries of experiments.

SACCADES AND THE STILES-CRAWFORD EFFECT

- The hypothesis under examination is that saccadicsuppression takes place in the retina, and is an indirectresult of shear in the retina due to eye acceleration.As previously discussed,7 the suppression prior to thebeginning of eye movement may be due to delays ofphototransduction and transmission that occur in theneural chain preceding the site of suppression. Suchdelays would require the stimulus to be presented priorto the eye movement in order for the delayed neuralactivity to occur when the neural components of theretina are subject to shear.

Even though it is clear that acceleration of the eye-ball must result in shear in the retina during rapid eyemovements, apparently there has not been any directconfirmation of this. The Stiles-Crawford effect isone method that can be used to measure the extent ofshear in the retina. If the sclera is moving faster thanthe vitreous, then the resultant shear should cause thephotoreceptors to be tilted away from the direction ofmovement. Thus, the optimal path of entry for thephotoreceptors will change by the angle of tilt. Todemonstrate this tilting, the Stiles-Crawford effectimmediately following a rapid eye movement may becompared with similar measurements made withoutpreceding eye movements.

Figure 4 shows the results of two such sets of measure-ments made using a 2-mm exit pupil, with a naturalpupil diameter of about 6.5 mm. Each point indicatesthe mean relative sensitivity (reciprocal of the threshold

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FIG. 4. The change of the Stiles-Crawford effect following asaccade. The circles show the customary attenuation versuseccentric entry; the crosses show the new sensitivities 40 msecfollowing the beginning of a 50 saccade. Note the slight temporaldisplacement of the curve, as well as the reduced sensitivity.This shift of the curve is also shown more clearly by the decliningratios between the crosses and circles, plotted below with tri-angles. (Observer WW.)

luminance) estimated from four separate thresholddeterminations made on different days. The approxi-mate standard error of these means is given on thefigure. The data were collected using a haphazardsequence, except that at any given eccentricity eachthreshold for steady fixation was always followed (orwas preceded by) a similar threshold determination forthe same 1-deg flash delivered 40 msec after the begin-ning of the saccade. The circles indicate the relativesensitivities obtained with steady fixation of the back-ground field (equivalent luminance of 3 cd/m2 ) atthe position where the 575-mm test flash was presented.The crosses show how the visual sensitivity is reducedwhen the test flash is presented 40 msec after a saccadefrom the left fixation mark to the same position on thebackground field. As expected because of the saccadicsuppression effect, the sensitivity following the saccadeis attenuated by about 0.5 log units, even though theeye was then stationary. However, Fig. 4 also showsthat if one smooth curve is to fit both sets of points,then this curve must be displaced toward the temporalside of the pupil in order to fit the crosses. This shift isalso shown in the lower portion of the figure, where theratios between the crosses and circles are compared ateach eccentricity.

The average shift in the Stiles-Crawford effect fol-lowing a 5° saccade is about 0.6 mm for this observer,corresponding to about a 2° angular tilt on the retina.As can be.seen from Fig. 3 for observer WW, it is un-likely that this effect of the prior eye movement is dueto the eye overshooting the target. The simplest ex-planation for this shift following an eye movement isthat the photoreceptors have been tilted slightly.According to the data, the direction of tilt suggests

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FIG. 5. Thresholds obtained for a 1° foveal test flash for steadyfixation (solid circles) and for fixation 40 msec following the be-ginning of a 50 saccade (crosses). As the background luminanceis decreased, the effect of the preceding saccade on the thresholdis reduced. However, the change of the logarithm of the equivalentbackground luminance following the saccade appears to beindependent of the luminance level. (Observer WR.)

that 15 msec after the eye is stationary, the position ofthe retina still trails that of the sclera.

EFFECT OF BACKGROUND LUMINANCE

In order to investigate the visual effects of saccadicsuppression in more detail, it seemed worthwhile toexamine the magnitude of suppression as a function ofthe luminance of the background. If central factorswere the principal source of brightness attenuationassociated with saccades, then the suppression effectmight reasonably be expected to be more dependentupon brightness than upon background luminance.On the other hand, the luminance of the adaptingfield, rather than brightness, might be the more im-portant variable if suppression occurred in the retina.

Figures 5 and 6 show the dependence of the suppres-sion effect upon background luminance. The blackcircles show the average thresholds for the 1-deg testflash of 575 nm for steady fixation. The crosses showthe thresholds also for a stationary eye, but 40 msecfollowing the onset of a 5° saccade.

The threshold curves obtained following the saccade(crosses) cannot be approximated by merely verticallyshifting the customary threshold curves (circles).

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Instead, the threshold curves for the eye movementcondition are displaced laterally toward lower levels ofluminance of the background. Thus, the effect of theeye movement appears to be to amplify the equivalentbackground level'0 rather than to attenuate the testflash. For the naive observer WW, this increase ofequivalent background level is almost tenfold.

It is conceivable, but unlikely, that such an amplifi-cation of equivalent background luminance could bebrought about by central factors. The principal dif-ficulty is to conceive of a central effect which increasesthe brightness of background, but not that of the teststimulus. However, retinal mechanisms associated withadaptation are already known to be present whichmight bring about similar changes of equivalent back-grounds without affecting the test-flash activity.1"-"

EFFECT OF DELAY

As luminance level is reduced, the photoreceptordelays increase.'4 Thus, another possible interpretationof the reduced attenuation near absolute threshold is

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FIG. 6. Same as Figure 5. (Observer WW.)

1 B. H. Crawford, Proc. Roy. Soc. (London) B134, 283 (1947).01 H. B. Barlow and J. M. B. Sparrock, Science 144, 1309 (1964).12 W. A. H. Rushton, Proc. Roy. Soc. (London) B162, 20 (1965).13 J. E. Dowling, Science 155, 273 (1967).

' 14 R. M. Chapman, Vision Res. 2, 89 (1962); R. A. Cone, J. Gen.Physiol. 47, 1089 (1964).

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FIG. 7. Change of thresholds associated with a 50 saccade fortwo test-field wavelengths. Circles: 460 nm; crosses: 580 nm.Time scale is with respect to the beginning of the first saccade,indicated by the left-hand arrow at the top of the figure. Theright-hand arrow indicates the approximate beginning of thereturn movement. (Observer WW.)

that the test flash has been presented too late. Becauseof the increased receptor latency at low luminancelevels, the test stimulus should be presented well beforethe saccade for the optimum effect. This possible ex-planation for Figs. 5 and 6 has not yet been explicitlytested as a function of luminance level. However,evidence will be presented later which suggests thatincreased photoreceptor delays do not cause the princi-pal effect shown in these two figures.

Nevertheless, it is true that photoreceptor delays willinfluence the temporal relationships needed betweenthe saccade and the test flash for optimal suppression.This effect is seen in Fig. 7 for test flashes of two dif-ferent wavelengths, 580 and 460 nm. For this observer(WW), the 580-nm test flash is maximally attenuatedfor about 50 msec following the beginning of the firstsaccade and also just prior to the beginning of the secondsaccade."5 The attenuation of the 460-nm flash followsa similar time course, except perhaps displaced 40 msecearlier, with maximal suppression now clearly prior tothe beginning of the saccade. The shift of the 4 60-nmcurve with respect to that obtained with 580-nmflashes suggests that the blue-sensitive process has aslightly longer latency under our test conditions usinga 3 cd/m2 background (color temperature 2500 K).

In addition, it is also clear from Fig. 7 that theshorter-wavelength test flash is always suppressed lessthan the 5 80-nm flash. One possible explanation forthis effect is that the thresholds obtained following thesaccade were not foveal but slightly parafoveal owingto imperfect eye movements which either exceeded orfell short of the true position of the test flash. However,measurements made with parafoveal test flashes of460 nm yielded similarly reduced attenuation, suggest-ing a true wavelength dependency for the saccadicsuppression effect.

16 Note also the characteristic secondary decrease of the thres-hold near 180 to 200 msec equivalent to 50 or 70 msec prior tothe beginning of the return saccade. This decrease has also beenreported by others.- 4 We propose that the peak and valley pre-ceding the eye movement reflect neural disinhibition or the Broca-Sulzer phenomenon, whereas those decreases that occur after asingle eye movement are due to retinal (or vitreous) oscillations.

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WAVELENGTH DEPENDENCIES

Figure 8 presents further data showin!of the suppression effect at short wavelenobservers, the test flashes were delivererlowing the saccade, when the eye was sFig. 3). Maximal suppression occurs fcnear 575 nm.

We might infer from this result thatgreen-sensitive processes are suppressedblue-sensitive process. Thus, suppressiincreased still more if the relative sensired- and green-sensitive systems are rnwavelengths by depressing the activitysensitive process with a blue adapting field

The triangles in Fig. 9 show that thisobserved. The background field of 471 nhas decreased the suppression at longershifting the maximum effect toward;470 nm-the wavelength of the backgrcfirm that the 471-nm background decrepression effect of a long wavelength, anolrun using a 645-nm background. The sqishow increased suppression at longer waa marked decrease of suppression at shorlThe effect of the background, thereforopposite of what would be predicted if lthe suppressed receptors had been decibackground field. Rather, the greatestpression occurs when the test wavelengtlthe same as the wavelengths comprisiground.

This finding of the dependence of sacsion upon the spectral composition of tisuggests again that the principal effectments is upon the neural activity associbackground. This hypothesis was previouas an explanation for the displaced thresFigs. 5 and 6. Now, the results of Fig. 8,a background color temperature of 250with the two curves of Fig. 9, demo,portance of the background activity mThus, the greater the background ac

0

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FIG. 8. Saccadic-suppression effect for two subjiof wavelength of the test stimulus. Crosses: WBackground color temperature is 2500 K.

the decreasegths. For both1 40 msec fol-tationary (see)r test flashes

the red- andmore than theon might betivities of thetised at short* of, the blue-

increase is notm at 3 cd/M 2

wavelengths,i region near)und. To con-eased the sup-

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FIG. 9. Saccadic suppression for WR as a function of wave-length for two differently colored adapting fields. Triangles:dominant wavelength of background is 471 nm; squares: back-ground dominant wavelength is 645 nm.

given wavelength, the greater willsuppression at these wavelengths.

be the resultant

TRANSIENT BACKGROUND

'ner series was If saccadic suppression is due to an increase of thelares in Fig. 9 background activity, then elimination of the back-velengths and ground should eliminate the suppression effect asso-t wavelengths. 'ciated with rapid eye movements. In order to test thise, is Just the hypothesis, a background of 645 nm at 50 cd/m2 was,he actvetd y of presented for only 1.0 sec in a total cycle of 3.0 sec, and:reased by the thresholds were obtained at instants close to the light-saccadic sup- ing and extinction of the background.:is are roughly Under these conditions, when the background was

ing the back- off for most of the inter-cycle interval, an additionalluminous fixation mark was needed. This extra fixation

cadic suppres- mark was provided by a pin point of light introducedhe background at the right-most edge of the background field (whoseof eye move- configuration had been altered to 7.5° vertical by 110

,ated with the horizontal). The observer's task was to make a singlesly mentioned 4.50 saccade from the region of the test flash to thishold curves in luminous fixation point. As before, the beginning ofobtained with this saccade then triggered the test flash after a 40-0 K, together msec delay. Thus, for the following measurements, all;trate the im- thresholds were obtained for test flashes presentedore forcefully- approximately 40 to the left of the fovea. The test-tivity at any flash wavelength was 580 nm.

As for all of the previous measurements, thresholdswere obtained for steady fixation of the luminous mark,as well as 40 msec following the beginning of a saccade

x-x to the same fixation point. For this series, however,thresholds were to be obtained at various delays withrespect to the onset and extinction of the 645-nmbackground field. For the steady fixation position,these temporal positions with respect to the backgroundwere obtained easily by merely resetting a timer whichdelayed the test flash. In the case in which the flash

4 6 was triggered by the saccade, however, correct tem-640 680 poral positioning of the test flash was more difficult.

Our final method was to use an auditory click, whichectsrscleafunction served as a cue to make the saccade. The experimenter

then adjusted the click delay in order to control the

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average temporal position of the flash with respectto the lighting of the background. Because the latencybetween the click and the saccade was variable, theexperimenter had to track the temporal position of thetest flash on a cathode-ray tube, attempting to keepthe delivery of the flash within zi25 msec of the de-sired temporal position. Threshold judgments weremade only if the flash appeared within this tolerance.As before, all threshold settings were made by theobserver WR, who manipulated the circular neutraldensity wedge placed in the path of the test flash.

Figure 10 shows the thresholds obtained for boththe steady fixation (circles) and the saccadic con-dition (crosses). As expected from previous deter-minations,"0

1617 the thresholds obtained during steadyfixation are slightly elevated near the lighting andextinction of the background field. These transienteffects are believed to reflect the underlying "on" and"off" bursts of neural activity."7', 8 The thresholdsobtained following the saccade (crosses) do not showthese transients. Instead, the threshold following asaccade appears to continue to rise beyond the peakof the on burst and then to reach a plateau that re-mains flat until the extinction of the background light.At this time, the saccadic threshold then falls moresharply than the thresholds for the steady fixationcondition, leading to an enhanced detection of the testflash close to the time of extinction of the background.As expected from the previous data (Figs. 5-9), thesaccadic suppression effect is minimal in the absenceof the background.

DISCUSSION

The results of Fig. 10 further reinforce the hypothesisthat the visual suppression associated with saccades isdue to an amplification of the background activity fol-lowing an eye movement. Such an increase mustalmost certainly be retinal.11 1 2 However, a furtherinference may be drawn from the behavior of thetransients at the beginning and extinction of thebackground. Figure 10 suggests that the saccade haseliminated the neural component that customarilylowers the threshold, by counteracting the initial surgeof activity associated with the first appearance of alight. Thus, the effect of the saccade is as if a feedbackloop has been broken, perhaps the synaptic loop sug-gested by Dowling" which may underlie the mechan-isms of adaptation and lateral inhibition. Disruptionof this same loop could also be responsible for the sharp

16 H. D. Baker, J. Opt. Soc. Am. 39, 172 (1949); 53, 98 (1963).17 R. M. Boynton, Arch. Ophthalmol. 60, 800 (1958).18 F. Ratliff, H. K. Hartline, and W. H. Miller, J. Opt. Soc.

Am. 53, 110 (1963).

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FIG. 10. Comparison of thresholds obtained with steady fixation(circles) and 40 msec following a 4.50 saccade (crosses). Test-field wavelength: 580 nm; background wavelength: 645 nm.The background was on for 1.0 sec every 3.0 sec.

decrease of threshold at the instant of extinction of thebackground field, providing the residual activity inthis loop originated from the presence of the real back-ground field. If this explanation is valid, then theeffects of saccades are being felt at the level of thebipolar and amacrine cells of the retina. In addition,we can also predict that rapid eye movements shouldalter the increment thresholds measured according toWestheimer's technique." Thus, if the effect of thesaccade is to eliminate (or reduce) lateral interactionsin the retina, then a preceding saccade should cause theincremental threshold for a small brief flash to remainat its maximum value as the diameter of a backgroundfield is further increased. The effect of the saccade onspatial interactions should be analogous to the temporaleffects seen in Figure 10.

Finally, if the customary increases of thresholds atthe lighting and extinction of an adapting field arecorrelates of "on" and "off" bursts, then elimination ofthese transients by the saccade also provides informa-tion about the site of saccadic suppression. If the effectof the saccade occurred at the second or a later "off"burst, then the activity of the first such burst wouldstill mask the test-flash activity, causing the thresholdto increase in this period. The elimination of a subse-quent, higher-order transient could not recover thesignal which had already been masked. Hence, thesaccadic effect must have occurred at or prior to thesite of the first "off" burst. Because these are found inthe optic nerve, the site of saccadic suppression mustbe in the retina.

'9 G. Westheimer, J.- Physiol. (London) 190, 139 (1967).

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