001-023 Distortion in the Perception of Real Movement.

7/30/2019 001-023 Distortion in the Perception of Real Movement.

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Journal of

Experimental Psychology

VO L. 34, No. 1 FEBRUARY, 1944

DISTORTION IN THE PERCEPTION OFREAL MOVEMENT

BY H. L. ANSBACHER

Brown University

I. INTRODUCTION

( I ) Description of the phenomenon.

(a) Historical considerations. In the course of his experimentingwith recurrent vision on the modified McDougall wheel of Kellogg(19) at Columbia University, 1937, Harold C. Brown changed theradial rotating light stimulus into a rotating arc of approximately 360

and discovered the following phenomenon:If one rotates such an arc at one revolution per second, it is seen

as a mere po int, provided one does no t follow it with the eyes. Eyefixation can be accomplished easily by looking at any convenient p oin t

in the set-up, e.g., the axis of the rotating disk. A t fast ro tationrates, of course, fusion results, and the arc is seen as a complete circle.Up to the limits of eight feet (the maximum he was able to use),

variations in distance did not affect the phenomenon. Likewise vari-ations in room illumination were not critical.

Brown has never published his observation . Its first publishedaccount is in an abstract of a paper read by the present writer (2).

(b) Systematic considerations. The phenomenon represents anaspect of the visual perception of real movem ent. Such percep tionmay occur in two ways:

(1) A moving object may be perceived as moving, by pursuing itwith one's eyes. In this way the picture of the object remains sta -tionary in reference to a specific portion of the retina, while the back-ground shifts.

(2) A moving object may be perceived as moving while the eyesremain fixed on some statio nary po int in the visual field. In this way,it is the picture of the object which shifts in reference to the retina,


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2 H. L. ANSBACHER

while the picture of the background remains station ary . Hereinafter,we shall refer to this kind of perception as FMP, for Fixation Move-

ment-Perception.The phenomenon on which this paper is based is concerned withone incidental aspect of FMP, namely, the distortion of the realstimulus.

( 2) Literature.

Three lines of research have been concerned with distortions inci-dental to FMP.

(a) The anorthoscopic approach. Zollner (30) described the phe-nomenon of the so-called anorthoscopic distorted figures as follows:Certain simple figures are moved behind a narrow slit of approxi-m ately two mm . width and eight cm. height. (1) W hen m oving at rela-tively high speed, they appear narrower than they actually are. ( Thefigures are perceived simultaneously.) Th is is the aspect of ourpresent concern. (2) W hen moving at relatively low speed, they ap -pear wider than they actually are. Th is situation is, according to H.H ec ht (17) , quite certain to have little relationship to the first. (Herethe figures are perceived successively.) (3) In both situations (1 and2) , the figures appear considerably wider than the slit behind whichthey move, the outstanding characteristic of the anorthoscopic ap-proach. For Helmholtz(18,p . 749) th e explanation of the distortionslay in involuntary pursuit eye movements: movements slower thanthe actual speed in situation 1 and faster tha n the actual speed insituation 2. Zollner disputed this explan ation, and Rothschild (23)dismissed it definitely.

Various other aspects of the anorthoscopic phenomenon werestudied by Vierordt (26), Gertz (16), and more recently by severalstudents of F. Schumann: Rothschild and Hecht, to whom we havejus t referred, Wenzel (28) , and Volk (27 ). T h e Schumann groupestablished that the perceived figure is essentially influenced by vari-ous attrib ute s of the slit, and other Gesta lt factors. For the dis-tortion as su ch ,"a psychophysical mechanism m ust be assumed whichtransforms a temporal phenomenon into a spatial one" (Hecht, 17)The principal characteristic of the Zollner set-up, however, that theseen figure is at all events wider than the slit, remained without asatisfactory theory (Koffka, 20, p. 1199).

Zollner's set-up was a simplification of the early stroboscope inwhich FM P distortion had been observed. Descriptions of thestroboscope and its many variations may be found in Boring (7, pp588 ff.) .


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DISTORTION IN PERCEPTION OF REAL MOVEMENT 3

Summary: FMP distortion was first observed with the anortho-scopic set-up, which, however, included too many inseparable vari-ables to have invited further research.

(b) Ehrensteiri's approach.Ehrenstein (9) studied FMP distor-tion in a set-up without a slit, where single figures were made to passin front of the observer's eye on an endless band . Here , of course,eye fixation became a real problem. Therefore, Eh renste in later (11)shielded part of the path of the band, leaving an opening of 10 to 30cm., wide enough to preclude the occurrence of the principal anortho-scopic characteristic.

Ehrenstein found that at optimal FMP speeds figures appearnarrowed in the direction of their movem ent. Among other Ges taltfactors, he (11) also found that (1) the more impressive parts of awhole Gestalt (Komplex) advance more slowly than the other parts,and (2) th a t change in configuration of moving figures may be due tointrafigural apparent movement and to the resulting simultaneouscon trast of movement. Earlier Ehrenstein (10) had found th a t figuresadapt themselves to the contour of the slit, confirming results of theSchumann group. His ultima te principle of explanation, th at FM Pdistortions are "complex qualities, jointly produced by the sense ofspace and the sense of movement," is almost identical with thatoffered by Hecht.

This explanation could be expressed in constancy terminology.Brunswik (Ansbacher, 1) would call F M P d istortion a compromiseobject between th e intended pole of width and the unintended pole oftime spent by the stimulus on its pa th . Th e work of Brown ( 8) onthe visual perception of velocity should be mentioned here, in whichhe used two m ovement fields which differed only with respect to the irsize and obtained different apparent velocities.

Summary: Ehrenstein studied FMP distortion in a set-up whichwas free from the Zollner anorthoscopic encumbrances, and offeredGestalt explanations.

(c) Frohlich's approach. When in FM P a vertical line is observedmoving behind an opening, it does not appear at the entrance edgebu t at a distance away , while it disappears at the exit edge. Th is

phenomenon was studied extnesively by Frohlich (13, 14, 15, andothers), who thought that the time difference between the actual andthe seen appearance of the moving line afforded a measurement of'sensation time.' While the observation remained uncontested,Frohlich's implications became the objects of a wide controversy.

Summary: Frohlich's studies and others in this connection showthat the entrance and conditions of entrance of a stimulus into thevisual field constitute phenomena in their own right.


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4 H. L. ANSBACHER

(3) Com parison of the present method with the previous ones.As against the various approaches discussed, the present one

affords the study of FM P d istortion free from all further concomitants . Specificically:(1) The anorthoscopic phenomenon is eliminated by not using a

narrow slit.(2) T he influence of a wide slit on the seen figure is eliminated by

not using any shield.(3) Eye fixation offers no difficulty.(4) The fact th a t the rotating stimulus remains continually within

the visual field(a) eliminates the psychological factors of the beginningand end of an ac tivity, (b) permits the observer to m ake his judg m entleisurely at his convenience rather than in the tense set of having tocatch a fleeting impression, and(c) permits quantification.

I I . PROBLEM AND ME THO D

The problem of this study was to investigate (1) how the phenomenon described at the outset would vary with speed, (2) whethethe observed shrinkage is a dropping out of pa rts of the stimulus or apsychological telescoping, and (3) how the shrinkage is affected by thshape of the stimulus.

(1) Apparatus.A modified McDougall wheel at Columbia University was used as constructed and described

by Kellogg (1 9). Its diameter was two feet. It permitted the insertion of various stencils intoa section and was illuminated from behind a milk glass by four 25-watt bulbs, so that the stencilpatterns were seen as light figures on black ground.

In front of the center of the wheel a fiat black box (presenting the variable) was mounted,illuminated separately, from within. On the side of the box facing the observer, a horizontalline was cut in a stencil, covered on the back by white paper. Th e illuminations of this lin ea ndof the stencil pattern of the main apparatus were subjectively equ ated. Th e length of the linecould be varied through rolling shutters, ma nipulated b y the experimenter. T he center of theline was in front of the axis of the wheel and was marked as the fixation point. Th e box did n otobstruct the main pattern on its circular path. Th e apparatus is shown in Fig. 1.

(2) Experimental situation.Th e experiments took place in ordinary room illumination. Th e observers sat two to four

feet away from the apparatus. Th e instructions were: "Y ou w ill see two illuminated figures: onestationary line here in the center, the length of which can be varied, and some other designrotating around the periphery of the disc in front of you. Look at the mark in the center of thestationary line. Th e line will at one time be very short at the beginning and expand slowly, thenext time it will be long at the beginning and contract slowly. When th e stationar y line which isslowly being lengthened or shortened seems to you to be equal in length to the line or figureyou see moving around it, say 'No w .' M ake judgm ents only when your eyes are fixated on thecenter point of the line. M ake quick judg m ents; do not deliberate in any way . W e are notinterested in how correctly you judge the actual length of the figure; we only want to know howlong the figure seems to yo u. Eve rything w ill be explained to you at the end of the experime nt."

Th en the wheel was set to r otate, the lights in both the standard and variable were turned o n which served as cue to get readyand t he variable was slowly lengthened or shortened. W hen


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the observer said 'N ow ,'the apparatus lights were turnedoff, and the length of the variable (thejudgment) was measured.

The method of limits was used. An average of four judgments was obtainedfor each situa-tion, except in Experiment I where 10 judgmentswere used.

(3) Observers.The observers were undergraduateand graduate students and older college graduates,in

part unfamiliar with psychological experimentation,40 in all. Since the problem was explainedat the end of the session, no observer was used twice; some sessions consistedof several experi-ments. There were8 to 12 observers per experiment.

FIG. 1. The apparatusDiameter of the disk: 50 cm.A: Moving arc stimulus illuminated from behind,the standard, 13 cm. long. B: Adjustable

line illuminated from behind, operatedby the experimenter, the variable, with the eye fixationpoint. C: Chin rest for the observer, about3 feet away from B, used only in the beginning of thestudy and abandoned when found unnecessary. D: Constant-speed motor with variousad-justments.

Actually the entire apparatus was uniformly black. Different shades appearin the photo-graph due to overexposure.

III. EXPERIMENTS I-IV AND RESULTS

Experiment I: An arc as the moving stimulus (basic experiment).Purpose: To determine the relationship of the extent of shrinkage

of an arc to variation in speed.


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H . L. A N S B A C H E R

Procedure:The standard was an arc at a radial distance of 20.7 cm. It was 13 cm. long (1/10 of the

circumference of its circle) and .4 cm. wide. Speeds of o, .5, .7, I, and 1.3 revolutions per secwere used. Judg men ts were obtained from 12 observers.

Results: The results are presented in Table I and Fig. 2 and arebriefly: The greater the speed, up to the maximum investigated,theshorter the apparent lengthof the stimulus. No exception from thisgeneral trend was found,and there were reversalsin only three cases.

TABLE ISHOWING APPARENT LENGTH IN CM. OF 13 CM.

VARIOUS SPEEDS (EXPERIMENT

Group 1 starting with 0 r.p.s..

Obs. N o .

I

2

3456

Average 1-6

Group 2 starting w ith 1.3 r.p.s. 789

1 011

1 2

Average 7-12

Average for both groups combined

Rotation Speefor setting 2, 9.95 cm. Individual values are shown in Tab le I I I andthe averages plotted in Fig. 2. In comparing the triangles with the

. 5 .7 I. 1.3R E V O L U T I O N S P E R S E C O N D

F i e . 2. Resu lts of experiments I-VT-

corresponding angle sides, one may speak of a tendency of the tri-angles to shrink more, although their surface is%\ times larger. Oneof these differences (.07) is no t significant, bu t the other (.90) is nearlyso ( CR = 2.20) . Again, as in Experiment II , greater resistance toshrinkage is no t related to greater illuminated surface. The b lunttriangle shrinks nearly significantly more ( CR = 2.56) th an th e


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pointed. The blunt curve was found to shrink less than the pointed.This apparent contradiction will find its resolution in the Discussion.Introspectively, the same figures were reported from the triangles asfrom the angle sides, only that the new figures seemed brighter.

Summary: (i) Triangles, although larger, tend to shrink more thancorresponding angle sides. (2) A blunt triangle tends to shrink morethan a pointed one.

IV. DISCUSSION

(1) Hypothesis of visual physiological pulsations.

The first four experiments have shown that: (1) the greater thespeed, up to the maximum used, the greater the amount of shrinkage;(2) the perceived shrinkage is due to telescoping; (3) the shape of thestimulus affects the extent of shrinkage (for details see summaries ofExperiments II-IV); (4) Gestalt principles are effective in FMP.

Further consideration of point 3 leads to the following exposition.At 1.3 r.p.s. (rate representing judgments on all stimuli) a completecircle is formed in 770 ms. Since in each case used, the length of thestimulus was 1/10 of a circle, the time required to pass a point, or tomove its own length, is 1/10 of 770 or 77. ms., or in round numbers80 ms.

Now let us assume that during this travel period of 80 ms. thestimulus was 'stopped' by stroboscopic photography at 6 equal inter-vals. This would yield series of pictures as shown in Fig. 3, whichseries would in actual photography form arcs of 72 0 .

The 'stroboscopic' series of five stimuli in Fig. 3 are arranged frombottom to top according to their proneness to shrinkage. Inspectionindicates that with the plain arc of Experiment I the individual phasesoverlap completely. With the angle side of Experiment III, indi-vidual phases do not overlap at all. Between these two extremes wefind degrees of overlapping, occurring roughly in the same order asshrinkage.

In Experiment II the pointed sine curve shrank more than theblunt curve because the former apparently produced the greater over-lap; in Experiment IV the blunt triangle shrank more than the

pointed, for the same reason. Overlap thus emerges as the commondeterminant of shrinkage.

By what mechanism can overlap be related to shrinkage? Let usconsider the plain arc. In stroboscopic photography the developedfilm would show 6 degrees of light exposure, with the center receiving6 times as much light as the two extremes, and 5 stepwise gradationsin between. What would take place on the photographic plate canproperly be assumed to hold for the retina, with the difference that its


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12 H . L. A N S B A C H E R

exposure is distribu ted in the shapeof an isosceles triangle instead ofa histogram. The assumption that light stimulationof the retinaadds up in this fashion is justified, since the Bunson-Roscoe law holdsin the hum an eye up to 200 ms. ( Troland, 25, p. 671).In such tri-angular distribution of light exposure of the retina, apparently onlythat extension is seen which receives maximum stimulation, i.e., con-tinuous stimulation during the time interval in question. Th e otherpa rts become eclipsed, possibly through contras t effect, accounting forthe shrinkage.

With changes in the hypothetical duration of the stimulus processof 80 ms., changes in light distribution would be as follows: (1) W ithin

E p . o .

Jttrtribotlon or (t lula tl on or tb . retina during tlM tlaa I t Ukua 13 e. a tlwlaa to traral tba diatanea of 1U o n lancth. Conxaa

of atlfloloa eooaldarad 'atopptd* pbotograpnloallj 6 tlaaa duringtill, pariod.

D>tTMOforarlap fro*lntpaotlonof praoadiaf

Pareantanof ahrlnk- f aa t

1.3 rp.

n.i

u

IS

FIG. 3. Showing distribution of retinal stimulation in relation to degreeof overlap and percentage of shrinkage.

less than 80 ms., distribu tion would t ak e th e formof a plateau, whichwould be the longer and lower, the shorter th e du ration of the process.In the extreme case, where this would last only one instant, therewould be no plateau, and the brief maximum stimulation wouldcoincide with the length of the stimu lus. Th is is the situation in Ex-periment I I I with the angle side, where we find that the perceivedobject is practically of the same length as the stimulus. (2) If thestimulus process lasted beyond 80 ms., the plateau would become in-creasingly longer, with initial ascent and final descent becoming pro-portionately insignificant. Under these conditions shrinkage shouldnot be expected.


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Actually, the stimulation in our experiments was continuous andstill the existence of shrinkage has been demonstra ted . I t thereforebecomes necessary to assume that physiological intervals break up th ephysical con tinu ity, thereby creating th e same physiological situa tionas would the hypothetical 80 ms. stimulation period. T hus, weadvance the hypo thes i s :the visual mechanism is active for only a certainperiod, which is followed by another period of inactivity; in other words,the visual mechanism functions in pulsations.

Such a hypothesis of visual pulsations has, in recen t years, receivedstrong physiological sup port throu gh the electroretinogram studies ofB artley. He found several instances where deviations of the sub -jective experience from the objective stimulus situation could berelated to "a n intrnisic periodicity in the visual path way som ewhere"(4) or to some pulsations in the optic nerve discharge ( 6) . T he firstdeduction was made from the Briicke or Bartley effect, the second,from observations in recurrent vision and records of the optic nervedischarge. A similar deduction was made from a third experiment inwhich subjective flicker rate was compared with objective flashrate (3).

Previously, Frohlich (12) had made "the unqualified assumptionthat the sense organs . . . respond to stimuli with rhythmical exci-ta ti o n s" ( p. 45) . Th is he based on the general attribut e of all livingsubstance to respond t o th e most diverse kind of stimuli with a seriesof excitation waves (p. 37).

In Bartley's experiments rhythmicity was present in two of thestimulus conditions, while in the third, recurrent vision, the stimuluswas a brief flash; subjective rhyth micity was found in all three experi-

ments. In our experiments, rhyth m icity was present neither in thestimulus situation nor in the subjective situation, but both were con-tinuous. Nevertheless, a hypo thesis of pulsations in the optic mecha-nism offers the best exp lanation of our findings. In our experimen tswe would then have, using Frohlich's (12, p. 27) terminology, arhythm of the first order "w ith which any living substance may respondto a [non-rhythmic] stim ulus" (on the physiological level), in B artley 'sexperiments, a rhythm of thesecond order "with which a living sub-stance responds to a stimulus which is rhythmicalitself."

Since according to our theory pulsations transform real m ovem entinto discrete physiological'stills,* and since nevertheless we perceivemovement, it follows th a t th e 'st ill s' would enter perception re-trans-lated into movement, even as the stills in moving pictures (and allapp arent movement) are translated into perceptual movement. Per-ception of real movement would thus rest on the same principle asperception of apparent movement, with the exception that the 'stills'in the former would be of physiological rather than physical origin.


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14 H. L. ANSBACHER

Koffka ( 21, p . 281) s ta te s : " I t has been proved beyond a do ub t th a tas far as psychophysical dynamics are concerned there is no differencebe tween s t roboscopic a n d ' rea l ' m ot ion ." T he present cons idera tionswould carry this theory one s tep fur ther, namely, that s t roboscopicmot ion is physiological ly more e lementary.

These views are in accord with an inference made by Smith andKappauf (24) from a study in which they succeeded in el ici t ing opticnystagmus in cats in response to apparent movement s t imulat ion.They bel ieve " that the physiological representat ion of real movement. . . is a lso a ser ies of discrete neural p at te rn s . "

(2) Calculation of a constant.If the hypothesis of visual physiological pulsations is correct, we

should be able to calculate from the data in Experim ent I a time con-stant which would correspond to the duration of one pulsation, thepulsation time. Th is is attem pted in the following.

If at .5 r.p.s. it takes the apparatus disk 2000 ms. to pass a singleretinal po int ( RP) , and it takes th e 13 cm. stimulus arc (which is 1/10of th e circumference of the disk) 200 ms. Like calculations for each

speed are found in Table IV, Column 3.From the rate (Column 1) and the apparent length of the arc(Column 4), one can calculate how long it would take the apparentlength to pass one R P ( Column 5) . Column 6 gives the differencebetw een Columns 3 and 5. Th is difference increases with speed from46 ms. to 61 ms. We shall take the average value of 55 ms. as basic,representing a con stant time factor present at all four speeds. Thegradual increase to61 ms. will be briefly discussed under the resu lts ofExperiment V.

The time constant, being the difference between the duration ofreal movement over a single RP and the duration ofperceived move-ment over a single RP, expresses the duration during whichno per-ceived' movement occurs at a single RP, although real movement doesoccur.

During the actual phenomenon, however, we are dealing not witha single R P b ut w ith a series of RP 's . The occurrence over a seriesof RP's is described in Fig. 4, which is based on the measures ofTable IV. In Fig . 4 the Y-axes indicate time in ms., while the X -axesindicate two measures of distance: (1) the distance in cms. travelledby th e stimulus, and (2) the extension of the stimulation of the re tinalsurface, expressed in hypo thetical R P 's . Speeds are plo tted fromTable IV, Column 3. The distances between the speed lines parallelto the X -axes represen t the length of the stimulus arc . T he speedlines at the left represent the function ofwithdrawal of stimulation


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.5 rps2OO

O 4 8 12 16 20 24

I rpsIOO

O 4 8 12 16 2O 24

.7 rps2OO

100

O 4 8 12 16 2 0 24

1.3 rpsIO O

O 4 8 12 16 2O 24

1

i

DISTANCE IN CM. TRAVELLED BY THE STIMULUSFIG. 4. Showing distribution of retinal stimulation during the various speeds of Experiment I, based on the measures of Table IV-


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16 H. L. ANSBACHER

from th e various RP 's. (A t .5 r.p.s. the left speed line indicates th a tstimulation was withdrawn from RP o immediately; from RP 3, after46 m s.; and from R P 13, after 200 ms.) The speed lines at the right

represent the function of onset of stimulation of RP's beyond theR P 1 3 .

We have found above that the time constant of 55 ms. expressesthe duration during which no perceived movement occurs at a singleR P . In Fig. 4 we see the stimulation conditions of a series of RP's(o to 13) during the time constan t. By plo tting at .5 r.p.s., for ex-ample, the 46 ms. (time constant) line, we see that it represents theduration during which no withdrawal of stimulation has occurredbeyond RP 3, and during which all new stimulation has occurred

TABLE IVSHOWING TIME REQUIRED FOR AN ARC OF THE REAL LENGTH AND AN ARC OF THE APPARENT

LENGTH TO PA S S A SINGLE RETINAL POINT (RP) AT THE VARIOUS RATES OPS P EED , AND THE D IF F EREN CE BETWEEN TH ES E T W O DURATIONS

{

R.pj.

57

1

a

Length in cm.of real arc

13131313

3

Duration in ma.of real move-

ment overone RP

2O 0143100

77

4

Length in cm.of apparentar c

108.45-22 .7

5

Duration in ms.of perceivedmovement

over one RP

15492416

6

Difference betweencolumns 3 and 5.Duration in nu.

during which noperceived movementoccurs over one RP.

although actualmovement does occur

46

l o61

beyond R P 13. This distance from R P 3 to R P 13 over which nochange in stimulus conditions did occur during 46 ms. corresponds tothe length of the perceived moving stimulus, which is 10 cm. Inother words, that much of the moving stimulus is seen which stimu-lates the retina uniformly du ring 46 ms. A t .7, 1, and 1.3 r.p.s.,corresponding relationships are found in Fig . 4. In each instance, theretinal extension for which no change in stimulus conditions occurredduring the time constant and which received uniform maximum stim-ulation during that time, corresponds to the length of the perceivedobjec t. The res t of the stimulation is eclipsed, possibly through con-trast effect.

We have shown, then, th at the time constan t of 55 ms. is directlyrelated to the perceived length. In terms of the hypothesis of visualpulsations, the constant would correspond to the pulsation time, or.rathe r to the 'o n ' response of the pulsation. Regarding the durationof the 'off' response, no indica tion is available.


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Where could a constant of 55 ms. fit in existing knowledge?We have found six instances where a similar value was derived:(1) Frohlich (13, p. 58) found a visual sensation-timeof 40-150 ms.,depending on the light intensity used. (In the present experimentsthe effects of change in light intensity were not investigated.)(2) Vierordt (26, pp. 123 ff.) thought that perceptionwas composedof single impressions which had an average duration of 41 ms.(3) Modern moving picturesare shown at the rate of 20 frames persec , which is 50 m s. per frame including the pause. (4) For the phiphenomenon the best appearance of movement occursat an intervalof about 60 ms. (Woodworth, 29, p. 682). (5) In recurrent vision,the initial short dark interval lastson the average about 40 ms. andis followed by the first positive after image,or Hering's image, whichlasts about 50 ms. (Bartley, 6). (6) Regarding flicker frequency,Bartley (5, p. 124) found that with increasing illuminationthe rateof last seen or marginal flicker remains constantat a frequency ofabout 20 per sec, while objective frequency increases from below20to nearly 60 per sec. Here again a period of approximately 55 ms.

is involved.I t may well be tha t the above values and our own constant are allrelated to the same mechanism, and it is hoped that the presentfindings will be an aid in the ultimate synthesis.

V. ADDITIONAL EXPERIMENTS (V-VII) AND RESULTS

In the Discussion we have assumed that the length of the per-ceived object is determined by the length of maximal constant lightdistribution on the retina (complete overlapof stimulation) duringalimited interval of physiological pulsation. The three additionalex-periments were designed to test the first part of this assumption.

Experiment V: Replacement of real movement by apparentmovement.

Purpose: In real movement, we have seen that maximal constantlight distribution on the retina during the pulsation time of 55 ms.is reduced in accordance with the speed of movement, and tha t theperceived length shrinksat the same rate . If light distribution reallycauses the shrinkage phenomen on, thena change in the former shouldresult in a corresponding changein the latter. A change in light dis-tribution can be produced through replacing real movementby ap-parent movement. In apparent movement, the stimulus does notmove, and, consequently, the length of maximal constant light distri-bution on the retina (duringthe physiological pulsation) always corre-sponds to th e stimulus leng th, irrespectiveof the speed of th e ap pa ren tmovement. Thus, in apparent movement, the seen length should


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18 H. L. ANSBACHER

always be equal to the length of the stimulus. It was the purpose ofthe present experiment to test this prediction.

Procedure:An apparatus corresponding withthe original was constructed on the principle of a moving

electric sign. It showed a translucent white circle (completingthe arc of Experiment I) on ablack cardboard. In front of the center of the circle the original black box withthe adjustablewhite line ( see box B, F ig.l) was mounted, permitting measurementof the seen length. Behindthe circle 10 groups of three small bulbs were mountedso that each group, whenlit, illuminatedI /IO of the circle, i.e.,an arc of 13 cm. length. To create apparent movementa wind-up phono-graph motor was used,in which the turntable was replaced by a stationary plate of insulatingboard. On this board10 contact plates were arrangedat intervals of their own width. Attachedto the drive of the motor was a wooden arm with a brush contact which contactedthe plates asit rotate d. Through proper wiring the10 parts of the arc could thus be illuminatedin succession.

The motor was calibrated for 1, 1.3, and 2 r.p.s., which in addition to zero r.p.s., werethespeeds used in this experiment. Judgm ents were obtained from eight observers.

Results: The results are presented in Table V and Fig. 2. At zerospeed the apparent length was 10.4 .95 cm. which compares with

TABLE VSHOWING AP PAR ENT LENGTH IN CM. OF A 13 CM. ARC IN A P PA R E N T

M O V E M E N T AT VAR IOUS S P EEDS (EXP ER IM ENT V)

Obs. No.

2324252627282930

Average

Rotation Speeds in Revolutions per Second

0

1012.311.310.38.9

10

9-910.5

10.4i.95

1

9-39.18.78.16.77-78 48.4

8.3-77

1- 3

8.9nr7-58.78.4

8..87

2

9.47-58.4

S8.58

Arrow indicatessequence of

stimuli

Average o r.p.s. vs. average I r.p.s.: D /Sigma D = 8.

an apparent length of 11.3 1.75 cm. for zero speed in Experiment I.The difference between the two measures, while not reliable(CR = i-5), is interesting to account for: in the present experimentthere was no contrast effect in the fast-slow sequence due to thegreatly limited shrinkage.

At 1, 1.3, and 2 r.p.s., averages of 8.3, 8, and 8 cm. respectivelywere obtained. Thus, in apparent movement, increase in speedbeyond one r.p.s. does not produce further shrinkage.

The difference of 2.1 cm. in subjective length, between o r.p.s. and1 r.p.s. , is, however, highly reliable (CR = 8). Since the stimulusdistribution is the same for these two speeds, as for all other speeds,the shrinkage must be attributed to a psychological factor in the per-


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ception of movem ent. While this factor distinguishes between stand -ing still and fairly fast movement, it does, apparently, not distinguishbetween further increases in speed.

If movement itself, irrespective of light distribution, accounts fora shrinkage of2.1 cm., tota l shrinkage in all the preceding experimentsmust be regarded as the product of physiological shrinkage andpsychological shrinkage. Inspec tion of Fig. 4 shows th a t, with every-thing else remaining constant, the counterpart of an increase inshrinkage would be increased pulsation tim e. Thus , if we couldpartial out in Experiment I the increased shrinkage due to the psy-

chological factor, we should arrive a t lower pulsa tion times. Infuture studies this might be attempted; it may be that the gradualincrease in pulsation times in Column 6 of Table IV can be attr ibu tedto the psychological factor.

Summary: (1) In apparent movement of the arc, where the lengthof maximal constant light distribution always corresponds to thestimulus length, little shrinkage is found, and there is no further in-crease in shrinkage with increase in speed. (2) Since the littleshrinkage found cannot be attributed to stimulus distribution, a psy-chological component in the general shrinkage phenomenon isindicated.

Experiment VI: Replacement of the moving arc by the beginning-and end-points of the arc.

Purpose: Experiment III (the angle side) has already shown thatwithout stimu lation overlap there is practically no shrinkage. Thepurpose of the present experiment was to test this relationship oncemore, with different conditions. In Experim ent II I no retinal poin twas stimulated more than once, in the present experiment all retinalpoints are re-stimulated . The successive stimulations are not con-tinuous for a given time interval, however; rather they are separatedby 65 ms. at 1.3 r.p.s., and stimulation as well as re-stimulation lastonly 6 ms. each.

Procedure:The original apparatus was used for two series: (1) 11 cm. in the center of the original 13-cm.

stimulus arc were darkened, (2) the y were not darkened. Th e observers were the same as in

Experiment V, the speed was 1.3 r.p.s. Th e illumination came from a source of 500 wa tts becausewith the 100-watt illumination of the original experiment the two rotating points appeared toodim to permit convenient judgments.

Results: The results are presented in Ta ble VI and Fig. 2. Thevalue of 2.74 cm. for series 2 is virtua lly th e sam e as the correspondingvalue in Ex perimen t I. The value of 8.16 cm. for series1 is virtuallythe same as th e values found in app aren t movem ent ( Experime nt V) ,and the small shrinkage here, as in Expe riment V, m ay be attrib ute d

to a psychological com ponen t. The difference of 5.42 cm. between


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20 H. L. ANSBACHER

the values for the two seriesis highly signif icant (CR = 7.53) . T h eshr inkage of 8.16 cm. in t h e j u d g m e n t of series 1 is re l iably greatert h a n t h a t of 10.63 cm. in E x p e r i m e n t I I I ( C R = 3) an d m a y bea t t r i b u t e d to th e difference between th e figures. Th e two pointsdescribe but one circular l ine,as in apparent movement , whereas theangle side describes a t rack corresponding to that lef t by the blade ofa snow plow.

Summary: In the ro ta t ion of two p oints , where over lapof s t imula-t ion is pract ical ly ruled out , shr inkageis reduced to that which canbe a t t r i bu t ed to i ts psychological component.

Experiment VII: Changing the length of the moving a rc .Pur-pose: We found in Exper imen t I t h a t a 13 cm. arc movinga t

1.3 r.p.s . shrink s subject ive ly to 2.7 cm . T h e ques t ion to be answeredhere is : Would a 26 cm . arc under the same condi t ions shr ink pro-

TABLE VIS HOWING AP PAR ENT OVER -ALL DIS TANC E B ETWEEN TWO P OINTS (OR IGINAL 13-Cu. ARC WITH

M I D D L E I I C M . DAR KENED) , AND AP PAR ENT LENGTH OF 13-C11. ARC, BOTH ROTATINGAT 1.3 R.P.8. (EXPERIMENT V I). Judgm ent of two-point distance always

preceded judgm ent of arc length

Observer Number33

2425262728293

Average

Two Points

9-S11.97.66.57.88.36.86.98 . I 6 I . 6 8

13 Cm. i

42.9*-52.43-3i-91-43-S2.74

Average two points vs. average 13-cm. arc: D/Sigma D = 7.53.

portionately 20.6 cm., or would the shrinkage of10.3 remain constant?

According to our assumption that the seen length is determined pri-marily by the extent of maximal constant light distributionon theretina during one visual pulsation,the latter should be expected.Th is can easily be seen from Fig . 4, where an added 13 cm. would,atany speed, show itself directly ina corresponding lengtheningof theregion of maximal constant light stimulation. The purposeof thisexperiment was then to test this prediction.

Procedure:

A disk was cut for the original apparatus in which the size of the arc could be varied from6.5 to 26 cm. Jud gm ents were obtained from 10 observers with arc settings at 6.5, 13, 16.25,19.50, and 26 cm., at a speed of 1.3 r.p.s.

Results: The results are presented in Table VII . Th ey show tha tthe 13 cm. arc shrinks to 3.34 cm., the 26 cm. arc to 17.26 cm.Theshrinkage (difference between real and apparent size) is 9.66 cm. forthe former and 8.74 for the la tte r. The difference between these tw o


21/23


values is not significant, the CR being .85. Similar values were foundfor the intervening sizes. Shrinkage thenis not proportional to thelength of the arc, but rather a constant for a given speed.

While shrinkage does not increase with the size of the arc, thevariability of the shrinkage does:SD of shrinkage for the 13 cm. arcis 1.86, for the 26 cm. arc, 3-67, almost exactly double.

Regarding the shrinkage value obtained fromthe 6.5 cm. arc, itmust necessarily be less than the other values obtained, sincethestimulus cannot shrink beyondits length.

TABLE VII

SHOWI NG APPARE NT L E NGT H IN CM. OF AR C S OF VARIOUS SIZES ROTATINGAT 1.3 R.P.S . ( E XPE RI M E NT VII)

Obs. No.

313233

3435363738394 0

Mean

Real Minus Ap-parent Size(Shrinkage)

Length of Arc in Cm.

6.50

1-5(3

[ . 1

1*65

i-5t. i

4.1

I.67

4.83*1.13

1 3

2

*1.6

3-72.96.92.43-1

V s6.83-34

9.66i.86

16.15

1 1

5-3

1:14-310.765-13-5

12.S

7-4

8.8s2.o6

19.50

13-99-38.9

13.69.6IS-S7.68.27.8

16.4

11.08

8.42^3.21

2 6

1912.75-4

'9192 0

10.91814.624

17.26

8.74*3.67

Arrowindicatessequenceof stimuli

** *

*

+

Shrinkage 13 cm. arc vs . 26 cm. arc: D/Sigma D = .85

Summary: As previously shown, that extensionof the retina whichdoes not receive maximal co nstant light stimulation during one visualpulsation determines the partof the stimulus which is not seen. Thisextension remains the same irrespective of the size of the stimulus.Thus we find that with increasein size, shrinkage remains constant,although the variability of the shrinkage increases roughlyin pro-portion with the increase in stimulus length.

VI. SUMMARY AND CONCLUSIONS

A phenomenon has been presented which consistsin the fact thatan illuminated arc of 360, rotating at less than fusion speed,and ob-served with fixed eyes, appearsto shrink to a fraction of its actuallength. Th is phenomenon is linked to related observations in the

literature.


22/23

2 2 H. L. ANSBACHER

Various aspects of the phenomenon were investigated, and it wasfound: ( i) The shrinkage is the greater, the greater the speed (w ithinthe limits of the present investigation) (Experime nt I) . (2) Th e

shrinkage is due to telescoping rather than to a dropping out of thebeginning or the end of the stimulus (Experimen t I I) . (3) A sinecurve shrinks less tha n an arc (Experim ent II ) . (4) An angle sideshrinks less tha n an arc (Experiment I I I ) . (5) Gestalt laws are opera-tive in the phenomenon ( Experiment II I ) . Greater surface of thestimulus does not necessarily mean less shrinkage, a triangle tendingto shrink more than one of its sides (Experiment IV).

In the Discussion, shrinkage was shown to be a function of the

degree of overlap of retinal stimulation, provided the existence ofvisual pulsations is assumed. The assum ption of pulsations is sup-ported by the work of B artley. A theo ry of visual pulsations wouldcarry the Gestalt theory of motion perception one step further,namely, that stroboscopic motion perception is physiologically moreelementary than 'real ' motion perception.

The duration of one pulsation was calculated from da ta of Experi-m ent I to be46-61 ms., and was compared to similar values previously

found significant in the field of visual perception.Three additional experiments (VVII) brought further evidenceth a t shrinkage is ac tually a function of the degree of overlap of retina lstimulation during the assumed period of visual pulsation.

Of the aspects not covered by this paper, the two most obviousones are: the effect of increased distance of the observer from theap pa ra tus , and the effect of change in illum ination. Regarding thesecond aspect data are at hand to indicate th a t shrinkage is facilitated

by increased illumination.(Manuscript received November 19, 1943)

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001-023 Distortion in the Perception of Real Movement.

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