WILLIAM P. BANKSt and DAN A. KANEPomona College, Claremont, California 91711

Discontinuity of seen motion reducesthe visual motion aftereffect*

Would a motion-picture film of a rotating spiral induce a spiral aftereffect?This question was studied in two experiments in which Ss viewed an animatedfilm of circles collapsing to a point. The rate of apparent motion of thecollapsing circles and the discontinuity of motion-the length of jump betweensuccessively projected circles-were varied independently. A visual aftereffectlike the spiral aftereffect was created. The aftereffect increased in strength andduration with the rate of motion, but at all rates of motion it declined asdiscontinuity of motion increased. The results are taken as evidence that motionaftereffects are caused by selective fatigue of small, directionally sensitivemotion-receptive fields.

A visual motion aftereffect can beinduced by viewing a fixed point as apattern (e.g., a band of stripes) movesuniformly by in a constant direction inthe visual field. Virtually any object orpattern seen in the area of the visualfield where the inducing motion fellwill be subject to the aftereffect. Theaftereffect is a paradoxical sort ofmotion without displacement that is ina direction opposite to that of theinducing motion. When a rotatingspiral is used for the inducing motion,the aftereffect is termed the spiralaftereffect, but it is probably based onthe same mechanisms as linear motionaftereffects. The spiral aftereffectmoves in a direction opposite to theapparent motion of the rotating spiralthat induced it. If the moving spiralseemed to collapse inward, theaftereffect is an expanding motion,and vice versa. (Holland, 1965, has abrief but comprehensive review ofresearch on the spiral aftereffect.)

A number of researchers have calledattention to the fact that movementaftereffects can be explained elegantlyin terms of motion-receptive units inthe human visual system (Sutherland,1961; Barlow & Hill, 1963; Sekuler &Pantle, 1967). According to thisexplanation, there is a large populationof motion detectors in the visualsystem, each serving a small angle ofvisual field and responding best tomotion in a single ("preferred")

*The authors thank w. R. Garner forlending his Perceptiscope and DigiBitsprogramming equipment for the secondexperiment. R. L. Gottwald and G. Felfoldyfor assistance with the apparatus, W. Kotefffor programming the CALCOMP plotter.and C. K. Peck, A. Pantle, and D. Berger foradvice in the preparation of this article. APomona College research grant and summerfellowship to W.P.B. supported most of thiswork.

tRequests for reprints should be sent toWilliam P. Banks. Department ofPsychology, Pomona College, Claremont.California 91711.

direction. Different units have variouspreferred directions, and any area ofmoderate size is supplied withdetectors for every possible directionof motion. When the inducing motionpasses across the visual field, it triggersonly those detectors responsive tomotion in that direction. As theinducing motion is repeated, the unitswhich respond become fatigued. Whenthe motion stops, their firing rates arereduced and remain depressed relativeto those of their nonfatiguedneighbors for a short period of time. Aprocess central to these units isassumed to compare the outputs ofunits with various preferred directionsand report motion in the directionwhose motion-receptive units are mostactive. Thus, for as long as the unitsfatigued by the inducing motion fireless often than the unaffected units,the comparative process will signalmotion in a direction opposite to thatof the inducing pattern, superimposingsignals of motion on stationarypatterns within the area where theinducing motion fell.

This account of the motionaftereffect has a number of predictionsabout the parameters of the aftereffectin man beyond those (such asinspection duration, speed of inducingmotion, and so forth) usually studied.The prediction examined here i& thatan inducing motion which is notsmooth but has small discrete stepswill produce less aftereffect as the sizeof the steps is increased, even if thediscontinuity of motipn is notperceptible. This prediction followsfrom the e v i d e e c e thatmotion-receptive fields "btend smallvisual anlies (cf. Hubel &; Wi1!sel,1962; Michael, 1968). Progressivelyincreasing the discontinuity of motionshould cause more and more of thefields to be skipped, not to befatigued, and therefore not tocontribute to a motion aftereffect.

The present experiments used an

inducing motion which was notcontinuous but moved in smalldiscrete steps. Animated films ofcircles with successively decreasingradii were used to induce anaftereffect of the sort a rotating spiralcreates. Five films were made byphotographing computer-generated(CALCOMP) circles in an animationframe in such a way that each film hada different size of step between radiiof successively photographed circles.Five concentric circles, evenly spaced,were photographed on each frame sothat the appearance of these films,when viewed on a projection screen,was of five circles collapsing insuccession to their common center.

In Experiment 1, Ss viewed all fivefilms at the same projector speed, andit was predicted that the aftereffectwould become shorter as discontinuityincreased. In Experiment 2, bothdiscontinuity and speed of motionwere varied, and it was predicted thatincreasing the discontinuity woulddecrease the aftereffect at all speeds ofmotion.

EXPERIMENT 1Method

Seven Ss, Pomona College students,viewed all five films at a distance of72 in. (184 cm) from a white paperscreen on which the films werefront-projected at 24 frames/sec. Fromthe Ss' perspective, the stimuli wereellipses with an eccentricity of about1.1. Along the minor axis, the largestcircle subtended 8 deg 24 min of theSs' field, the width of line was 1 min1 sec of arc, the smallest jump (ofwhich the others are multiples) was2 min 30 sec, and the mean rate ofmotion of the edges was 1 deg2 min/sec. Ss viewed the screenthrough dark-adaptation goggles(Polaroid nonpolarizing red,XDA8FAP) to reduce the brightnessof the image. [Use of these gogglesalso eliminates the somewhat remotepossibility of rod-cone interactionsaccounting for the aftereffect (Granit,1928 ).] Measured through the goggles,the luminance of the field was about10 ml. Ss adapted to the goggles for5 min and then had the aftereffectdemonstrated with the film with leastdiacon.tinuity.

During experimental trials, Ss staredfixedly at the center of the projecteddisplay for jan inducing time of 2 minand. then shifted their llaze to a sheetof Circles from the CALCOMP outputplaced 5 ft (160 cm) in front of them.The aftereffect was thus seen as anexpansion of the circles on theinspection figure. They timed theaftereffect with a stopwatch. Becausethe aftereffect seemed to wax andwane, they were instructed to let thewatch run while they saw the

Perception & Psychophysics, 1972, Vol. 12 (IB) Copyright 1972, Psychonomic Society, Austin, Texas 69

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Fig. 2. Mean duration of motionaftereffect as a function of inducingspeed for four levels of discontinuityof inducing motion. Length of jumpbet ween successive steps ofdiscontinuous motion is in parenthesesbelow each film designation. Viewingtime was 30 sec. Dashed lines connectpoints of equal apparent inducingspeed.

aftereffect varied with speed for eachfilm, and Fig. 3 shows how the judgedstrength of aftereffect varied. Theduration of aftereffect for 1 deg2 min/sec estimated from Fig. 2 is, ateach level of discontinuity, about halfthat found in Experiment 1 (whereinducing speed was 1 deg 2 min/sec).This difference between theexperiments probably results from thefact that the inducing time was 2 minin Experiment 1 and 30 sec inExperiment 2, but other differencesbetween the two experimentalprocedures (i.e., recovery period,luminance, goggles) may havecontributed to the differences induration of aftereffect.

The dashed lines in these figuresconnect points for which the apparentspeeds of the inducing stimuli areequal. These equal apparent speedcontours were derived from a previousscaling experiment in which the samefour Ss gave magnitude estimations ofthe apparent speed of each film atvarious projector speeds. If theaftereffect functions in Figs. 2 and 3were plotted as a function of apparentinducing speed, these contours wouldbe vertical. Since the contours arequite steep, rectifying the aftereffectfunctions according to the apparentinducing speed contours (Le., usingapparent rather than actual speed asthe abscissa) would not reduce theeffect of discontinuity very much.

An analysis of variance wasperformed only on the duration

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32 min of are, line thickness was1 min, and the smallest jump was2 min 52 sec. No goggles were used,and the luminance of the image wasabout 8 mL. Ss warmed up with twoadaptation periods of 1 min to thesecond film (discontinuity 5 min43 sec), moving at 1 deg 12 min/sec.During experimental trials, each S waspresented every combination of filmand speed for 30 sec in an irregularorder counterbalanced over Ss. Aftereach inducing stimulus was viewed forthe 30 sec, the projector was stopped,leaving a frame of stationary circlesprojected on the screen. Ss gavemagnitude estimations of the strengthof the aftereffect (Sekuler & Pantle,1967, used magnitude estimationsimilarly, but had Ss judge theapparent velocity of the aftereffect)immediately and timed its durationwith a stopwatch. "Strength" wasdefined for the Ss as the phenomenalclarity and "noticeableness" of theaftereffect. These judgments areprobably correlated with apparentvelocity of the aftereffect, but theaftereffects of some of the stimuliwere so weak that apparent velocityjudgments would have been difficultto make. Two minutes were allowedfor recovery from the aftereffect ofeach film before the next was shown,and a 7-min rest period was given eachS in the middle of the series.

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ResultsFigure 2 shows how the duration of

Fig. 3. Mean judgment of strengthof motion aftereffect as a function ofspeed for four levels of discontinuityof inducing motion. Parameters anddashed lines are the same as in Fig. 2.

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EXPERIMENT 2Method

In this experiment, only the fourfilms with the least discontinuity wereused. Four new Ss viewed these on ahigh-contrast Kodak screen at anumber of different projector speeds.Viewing distance was 96 in. (246 em),The largest circle subtended 9 deg

5

aftereffect and to stop it when theydid not. When the aftereffect hadfinally dissipated, the E recorded thestopwatch reading and waited 5 min toallow recovery from the aftereffectbefore showing the next inducingstimulus. Initially, five Ss adaptedonce to each film in an ordercounterbalanced for position. Theposition of the film in the inspectionseries seemed to have no effect, anddata from two additional Ss who sawthe films in random order were added.

ResultsThe mean duration of aftereffect

for the seven Ss is plotted as afunction of discontinuity in Fig. 1.The predicted reduction in duration ofaftereffect is strong and significant,with a F(4,24) of 41.6 (p < .001).

It waa observed that the morediscontinuous circles appear to bemoving more slowly than the morecontinuous ones when shown at thesame projector speed. The secondexperiment, in which speed as well asdiscontinuity of inducing motion wasvaried, eliminates the possibility thatthese apparent speed differencesaccount for the differences in durationof aftereffect.

70 Perception &: Psychophysics, 1972, Vol. 12 (IB)

measures (the strength judgments arehighly correlated with duration). Datafrom two combinations ofdiscontinuity and speed (Film 1 at0.2 deg/sec and Film 2 at 0.3 degrsec)were not included in the analysis sothat an orthogonal design could beused. The remaining data were castinto a three-factor 4 (discontinuity) by3 (speed) by 4 (Sa) design. The maineffects of discontinuity and speedwere tested against pooled error termsbecause neither factor interactedsignificantly with Sa. The main effectsof both discontinuity and speed weresignificant beyond the .01 level, withan F(3,27) of 23.74 and an F(2,24) of19.72, relpectively. (Use of anunpooled Sa by Treatment error termresults in the aame level of significanceof the main effects.)

DISCUSSIONAs discontinuity of motion

increases, the aftereffect decreases.This fact is taken as evidence for amotion-detecting system that gathersinformation from smallmotion-receptive fields, but there areother ways of interpreting the results,some of them compatible with thereceptive field notion.

One intriguing interpretation issuggested by Gregory's (1964. 1966)hypothesis of two motion-detectingsystems in the visual apparatus. One ofthese systems, the positional system,detects motion by comparingsuccessive positions of an object or bycomparing the position of one objectwith another or with the background.This system would, presumably, beresponsible for the phi phenomenonand some illusions of relative motion.The other system hypothesized byGregory is the velocity system. Thissystem gives direct signals of motion,probably by interpreting localdisplacements of contours on theretina. As Gregory has pointed out,motion aftereffects seem to be adisturbance of the velocity system andnot the positional system, since theaftereffect has illusory motion but notillusory displacement. Aninterpretation of the present study interms of the two systems of motiondetection is straightforward: Changesin discontinuity of motion affect thetwo systems differentially. Thevelocity system seems to be greatlyaffected by discontinuity because theaftereffect, mediated by the velocitysystem, drops off rapidly asdiscontinuity increases. The positionalsystem seems less sensitive todiscontinuity of motion because theapparent motion of the inspectiondisplay, presumably mediated largelyby the positional system, is not muchaffected by discontinuity of motionwithin the limits of discontinuity used

here. It seems reasonable to conclude,further, that the velocity systemdepends on receptive fields to detectmotion and that the positional systemuses a different mechanism.

Lichtenstein (1963) used aningenious system to presentdiscontinuously moving lines on theface of an oscilloscope. He studiedsensitivity to rather than theaftereffect of discontinuous motion,but some of his observations are ofinterest here. First, he foundmove men t sensitivity to beproportional to the "space-timedensity of the stimuli producing themotion." Figurel 2 and 3 show thatspeed and discontinuity can be tradedfor each other and thus that theaftereffect may also be proportional tothe space-time density of the inducingstimulus. Considerably more preciseand complete functions would.however, be necessary before thisconclusion could be drawn with anyconfidence. Second, Lichtensteinfound that with high stimulusvelocities and peripheral viewing, theoscilloscope traces could sometimesnot be seen clearly but had aconsiderable apparent rate. On thebasis of this and other observations, hesuggested that motion may beperceived without a perception of athing moving. Such a possibility isexpected if motion is detected byreceptive fields. A field could betriggered by motion whether or not amoving object was perceived.

It was hoped that the form of thediscontinuity effect in the presentstudy could be used to estimate theaverage size and other properties ofthe motion-receptive fields.Unfortunately, these estimates cannotbe made without a precise model ofthe properties and arrangement of thefields, and no such model presentlyexists (but see Thomas, 1970). Theresults do seem to indicate that thefields are fairly small, since theaftereffect all but diaappears by thetime discontinuity has reached 10 minof arc. This small estimated size of thefields might seem to be in conflictwith the fact that motion-responsiveunits in physiological preparationsrarely serve less than .5-1 deg of arc. Itshould be pointed out. however. that afield represented even at the retinalganglion level must contain manysmaller units; otherwise, the responsewould not be maintained continuouslyover the field. Thus, the discontinuityeffect would seem to be largely aresult of the smaller units withinreceptive fields being "skipped" andnot being fatigued by thediscontinuous motion. Barlow andLevick (1965) have proposed a modelof the neural organization within themotion receptive fields of the rabbit's

retina that relies on interconnectionsamong smaller subunits. Theirexperimental estimates of the size ofthese subunits placed them at between15 and 30 min of visual angle. Theresults of the present experimentswould be in agreement withapproximately the same estimate ofthe size of subunits in human receptivefields.

Finally, it must be stated that thequestion of where motion-receptiveunits are situated in the visual systemis not answered by this research. norwas it asked in the first place. Thisstudy demonstrates what might betermed a granularity in motionperception, or, at least, in onecomponent of it. The effect ofdiscontinuity was discussed in terms ofmotion-receptive units of cella beingskipped, but the units could beanywhere. The cells are surely retinal,but the neural organization thatcombines them to form a unit mayormay not involve structures deeper thanthe retina. It might, however, bepossible to use the discontinuity effectto discover whether or not the units

, were retinal. If thecollapsing circles ofthis experiment were presentedbinocularly to alternate eyes, such thatthe first circle in the series waspresented to the left eye, the next tothe right, the next to the left, and soon, and if the discontinuity effectwere the same all that withpresentation of all circles to a singleeye, the units would have to becentral. If the aftereffect in such asituation were the same as that foundwith twice the discontinuity presentedmonocularly. the units would have tobe retinal, or at least specific to eachretina.

REFERENCESBARLOW, H. B., .. HILL, R. M. Evidence

for a physiololical explanation of thewaterfall phenomenon and figuralafter-effects. Nature, 1963, 200,1345-1347.

BARLOW, H. B., .. LEVICK, W. R. Themechanism of directionally selective unitsin rabbit's retina. Journal of Physiology,1965, 178, 477-504.

GRANIT, R. On inhibition in theafter-effect of seen movement. BritishJournal of Psychology, 1928, 19,147-157.

GREGORY, R. L. Human perception.British Medical Bulletin. 1964. 2Q, 21-26.

GREGORY. R. L. Eye and brain. NewYork: McGraw-Hill, 1966.

HOLLAND, H. C. The spiral after-effect.New York: Pergamon Press, 1965.

HUBEL, D. H., .. WIESEL, T. N. Receptivefields. binocular interaction, andfunctional architecture in the eat's visualcortex. Journal of Physiology, 1962, 160,106-154.

LICHTENSTEIN, M. Spatio-temporalfactors In cessation of smooth apparentmotion. Journal of the Optical Society ofAmerica, 1963,53, 304-306.

MICHAEL, C. R. Receptive fields of singleoptic nerve fibers in a mammal with anall-cone retina. Journal ofNeurophysiology, 1968. 31, 257-267.

Perception & Psychophysics, 1972, Vol. 12 (lB) 71

SEKULER. R. W.• 8< PANTLE. A. A modelfor after-effects of seen movement.Vision Research. 1967. 7. 427-439.

SUTHERLAND. N S. Figural after-effects

and apparent size. Quarterly Journal ofExperimental Psychology. 1961. 13.222-228.

THOMAS. J. P. Model of the function of

receptive fields in human vrsion.Psychological Review. 1970. 77.121-134.

(Accepted for publication February 15,1972.)

72 Perception &. Psychophysics, 1972, Vol. 12 (lB)