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Rhythmic registration of movement, apparent movement and apparent rest
Acta Psychologica 29 (1969) 134-149; 0 North-Holland Publishing Co., Amsterrlam Not to be reproduced by photoprint or microfilm without written permission from the publishcrr
RHYTHMIC RJ5GISTRATIC3N OF
V&/f&+,
APPARENT MOVEMENT ANI3 APPARENT REST
ABRAM R. Vk ‘MUISEN Psycholo&&
Laboratoriup, ,Vniversitfit vain Amsterdam, Keizersgracht 611-613, Amsterdam ,
In principle, visual movment is perceived when: (a) t:he pro&t.ion of an object moves across the retina, or (b) a movi’I.-g object is followed by ,the eyes, so that the projection of the object remains at a fixed spot of the retina. Movement must then be inferred from secondary cues such as the relative movement of background or foreground, kinestheitic stimuli produced by rotations of eyes, neck or trunk-, or the movement pa.ral1ax.l The first type of movement is probably the most primitive. We shall eal! ;It primary movement and consider it first. Like roeived c&our, sound, scent or shape, perceived movement is sin exclusive!y ‘phenomenal’ affair. Phenomenal movement is formed out of spe&ic 2-Xes~,we-&h =e present in the input of the eyes and brain. In cease of a perception of actual movement these cues can be supposed to be functionally congruent with, i.e. representative of, movement of the outer object on the one side, and of perceived enomen@ movement on the other. Functional congruity, however, that movement itself must be among the properties ,s not im of the input of the brain, which determine the perception of the movement. The specific churacteristics which produce phenomee,rzaI need not be movement themselves; they only have to be ~~~r~e~t~v~ of ;f. Representativeness implies a relationship, as for em.mple that between the stationary position of an accelerator, and ion or speed of the car. The position and not the movement z’s d&&ion of mm&ar parallax (195 1) is: ‘When a human is moving through space fixates an object that is not moving, a angular ve:locity exists between the lines of sight to the fixated zct and some otl~er stationary object. If the objects move while the observer ~!;ra~o%,ionless, tk same situation of differential angular velocity exists’. 134
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OF MOVEMENT
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of the accelerator is representative of the speed which the car ~21 assume In principle, acceleration and speed could be regulated by numerous devices, but in order to function adequately, any such device must show the desired functional correspondence between input and output. Accordingly, phenomenal movement, which is the correlate of physical movement, can in principle be produced by any combination of stimuli, which is representative of the movement in question. Our hypothesis is that phenomenal movement is inferred from stimuli, which we obtained from an in,term.ittent succession of quasi-stationary positions of the moving object. The
idea is that a11visible properties, including movement, are registered by meam of a rapid succession of sensory ‘snapshots’ of short expopdrc, An object wfiich moves too fast for the time of exposure will be seen as blurred, transparedtt and elongated, as it would appear in a photograph taken with an exposure of l/l 0 instead of the required 111 iB second. III most cases, however, ti stretch which can be covered by the object d.uring the time of registration is so small that it approaches zero, so that, in each single registration, the moving object is registered as if it were stationary. The movement of the object must then be inferred from at least two successive registrations. The assumption that visual registration occurs by means of a rapid succession of short exposures implies that positive p ases of registration alternate with negative phases of non-registration. Since in primary movement perception the eyes do not follow the object, the alternation of positive and negative phases will be determined by inner mechanisms of the sensury apparatus, probably in combination with the brightness of the registered area. We may assume that the alternation of positive and negative phases OCCUIS rhythmically, that is in accordance with the general rhythmic activity of the nervous system. If this assumption is correct, the duration of the positive and negative phases will he a constant, if further conditions such as the brightness of the field are the same. There is no reason, however, to suppose that the duration of a positive phase is the same as the duration of a negative ,phase. me idea that phenomenal movement is derived from series of still retinal images of the moving object was originally derived from four considerations: (a) &J has been discussed already, the cues for movement need not move themselves, they only have to be representative of movement. (b) ‘The &stcnce of apparent movement. From a phenomenal point
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of view, apparent movement and real movem.:n’t are identical. In either case one sees real movement. Apparent movement originates when two or more stationary, spatially separated stimuli are registered mporal succession. Why then could not all phenomenal movement ate from a succession of spatially separated still retinal pro(c) The quantity of information which can be grasped by the organism is limited. Consequentl;r, the total quantity of information which is present in the input of the eyes is reduced to a quantity which can managed by tke nervous system. In principle, a. moving object through an inkinite number of positions. The organism cannot register all thesle positions, but only a limited1 number. Assuming that the ‘channel capacity’ is a constant, the degree of reduction will be ional to the retinal velocity of the stimulus. The faster the vement, the smaller will be tke number of registered positions of a tch covered in a certain time-unit, and the @eater will be the between successive retinal images. From this it also follows t only a few of the structural properties OF a moving object - witk exclusion of a very slow motion - can be noted if the object is not follow~ed by the eyes. Succ/:ssive projections on the retina will ly overlap and prevent the development of a single clear image. ) The retina consists, of a great quantity of photcL-electric cells. Each 11cars regicter changes in brightness or colour, but ai single cell cannot ovement. One may assume that the cells and the nerve fibres h convey tke registered information towards the brain function in dence with the general rhythmic activity of the nervous system. Even if the movement of a stimulus could be registered com1~1~ by the retina, one may wonder in which way this transversal ement skou!!d be transmitted to the brain.. If the retinal projection vement would Imove from a point A to a point B during the registration, it would either continue to move in the same during the transport to the brain, or it would have to be back a point whick is the correlate of A, then move towards B, problem is that transversal movement cannot be conveyed a ckannel if it is nti fixed in separate sub-channels. Tke ution is ttat the fixation occurs in the retint cells in a code is deciphered in the brain. In the retina, such a fixation can an alternation of short phases of registration and
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Although a number of arguments in favfxn- of the basis hypothesis have already been brought forward it is still possible that: (a) the arguments are wrong, and (b) short stretches of movement and sot quasi-stationary positions of the moving stimulus are registered by the nervous system. Fortunately, the basis hypothesis could be tested by means of a simple experiment. If the basis hypothesis is correct, it must be possibIe to synchronize reiterative elements of a moving stimulus with the supposed periods of registration of the nervous system. III. actual fact the author wondered what would happen if a moving, reiterative stimulus like the one of fig. 1, exactly covers a distance S or a multiple of it, in exactly one cycIe of registration and non-registration.
Fig.1
On the basis of the possible ways of registration mentioned above, the following results can be predicted if the squares of fig,. 1 are put in to motion: (a) If registration does not occur periodically but continuously, one will see a continuous movement as long as the speed of the squares is not too high. If the speed is increased, the individual squares will ;ro longer be recognized, and only a vague, grey surface of the height of the squares will be perceived. (b) If registration occurs periodically, but not rhythmically, the same phenomenon as described under a) will be perh.:eived, since the squares are registered at arbitrary positions of the trkck. (c) If short stretches of movemerct are registered periodically and rhythmically, one cannot predict exactl,y what will be perceived, since not only the black squares, but also the white intervals move. It is certain, however, that the squares will not be seen as stakionary squares of the same width as drawn in fig. 31if the velocity of the squares is synchronized to the rhythm of registraticn. (d) If (quasi-) staticmny positims arc registered rhythmically, one will, like in the other alternatives, see a slowly proceeding series of squares as long as the actual movement is slow. At a certain moment only a. vague, greyish surface will be perceived, 9 the l/elocity is
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gradually increased Then, if the speed of the squares is synchronized to the rhythm of the registrations, one must see a stationary series of squares which looks like the original stimulus. The black squares and the white intervals will add up, since they will always appear at the places. The periods of non-registration will be filled up because the after-lag of the stimulated retina cells. If now the velocity of stimulus is slightly decreased, the squares will not be registered at the same places. As in the case of actual slow movement, each next ession, although partly overlapping the previous one, will stay a i little behind. Consequently, the squares must be seen as moving slowly into a retrograde direction. The reverse must happen if the d is slightly increased. In either case the perception of apparent movement is produced by a succession of stationary, spatially separ&d stimuli, in ,;he same way as real movement is produced. If the velocity is furthe,!r increased &yond the synchronized velocity., the eyes ll alternatively register black and white surfaces at specific places. It also happen that the speed becomes too fast for the period of re. In these cases only a greyish, vague surface of the height squares will be seen. le thiuking of an i.nstrum.ent by means -of which ,the hypothesis Id be tested, the author happened to remember an instrument which is used to regulate the r&&on sspeed of gramophones. This ‘stro’ consists of a disk on -which white radial stripes alternate black ones. Experimentation with this disk soon showed that all rceived phenomena surprisingly well co&rmed the hypothesis vrsual perception occurs on the basis of a rhythmic registration asi-) stationary positions of the moving object. se of the great conformity between predictions and obserwe shall not describe the perceived phenomena once again. are perceived as stationary at a certain velocity, they slowly e backwards or forwards if the velocity decreases or increases, etc. additional remarks, holvever, are necessary. e rotation speed is low, the movement of the radial stripes rotation of the whole disk can be easily perceived and the eyes. According to our theory, the impression of iginates here in the same way as the retrogade or proapparent movement yvhich originates if the disk is synchronized to the rhythm of the registrations. The impression one the slow apparent movement is not exactly the same as that
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of the slow real movement, however, The black and white of the apparently moving and also of the apparently stationary stripes is somewhat less intense than the black and white of ihe actually stationary or slowly moving stripes. oreove:, the apparently stationary or apparently moving stripes look less real than the actual. stripes. There may be more than one cause for the difference in appearance. The gramophone stroboscope is not an ideal instrument for experiments. l%tly, the black and white stripes are not exactly equal in size. In successive registrations, the stripes therefore do not cover each other completely. The black .and ;Ivhie will partly blend into grey, which may affect the whole appearance of the disk. In the second place, the disk contains a great deal of small, non-reiter.ltive irregularities. Due to these, the disk is perceived as rotating while the reiterative stripes are seen as stationary. This effect disappears only partly when a small part of the disk is observed through a slit in a piece of paper. The paradoxical effect, is that the statiimary stripes seem to float .in a somewhat unreal way on a rotating ground. This effect is further .enhanced<- since all small irregularites , which make the black and white more real and tangible when the disk is at rest, have disappeared because of the rotation speed which causes the stripes to appar as stationary. Consequently, the objectbound ‘surface’ colours of the stationary or slowly moving stripes seem to assume the appearance of less intense ‘free’ or ‘expanse’ c&ours (WOODWORTH, 1938). (2)
The
progressive or retrograde apparent move ent is necessarily slow if the stroboscope for gramophones is used. It originates from small displacements of the image of the stripes in successive registrations, No movement will ,be seen when the displacements become a. little larger, since the eyes then receive an alternation of. black and white sti.muli in successive registrations. Apparent movement can only originate - as long as .the, displacement ‘in successive registrations is smaller than half the width of one stripe. Beyond ‘that, the observer will only .perc.eive a more or less uniform grey. If the white sections are enlarged, it should be possible, however, to produce a somewhat faster progressive or retrograde movement, One ‘of the consequences of a process of perception, which occurs by means,of a fairly rapid rhythmic alternation ,of phases of registration of stationary stimuli, and phases of non-registration is that real and u~p&rent movement origi.nates in exactly the same way. In either case,
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the appearance of movement is created by a series of temporally and spatially separated stationary stimuli. Mostly these cues happen to coincide with an actually m.oving object. Sometimes, however, the senses are deceived as the object which produces movement cues does not really move. In reverse, the eyes are deceived when a reiterative, moving series of stimuli is st=en as stationary, or in relatively s:\ow progressive or retrograde motion.
In a study of the literature of the phenomena described above, thle author happened to jiind an article by Neuhaus, published in 1934. From a number of observations and experiments NeuhFus conclutdrd that perception occurs in short periodic phases of registration and non-registration. Because of the apparent importance of his article and also because Neuhaus’ findings supplement our own in several respects, a detailed survey of his article seems appropriate. Neuhaus used a rotating white disk of 30 to 40 cm. in diameter, with four perpendicular, black radii of approximately 1 lcm. in width. The speed of the disk could be varied. When the disk rotates slowly, one perceives a rotating cross. When the velocity of the disk is increased, one sees at a certain moment more than four &ii. ‘The number of radii which are perceived, increases zs the rotation speed of the disk increases. PURKINJE (1823) was the tist to describe this phenomenon. GEHRCKE and LAU (1923) examined it more in detail. Neuhaus accidentally discovered that the number of radii is also dependent on the illumination of the disk. Through observation obtained from 20 subljects he verified that: (a) the nurn&r of radii perceived increases if the intensity of the illumination decreases. In fig. 2 the relationships as noted by Neuhaus are recorded. In fig 2a, two diierent magnitudes of light reflected by the disk are taken as u~&ants. In fig. 2b the contants are $VWvelocities of roation. “The results of all subjects corresponded with the general tendencies, aa-&ough there were slight interindividual variations, In a following experiment Neuhaus used two disks. The left Gsk mid only be seen by the left eye, the right one by the right eye. The illummation of one disk was kept constant, that of the other was varied. Front the results it appeared that under these conditions the eyes fuactii independently from each other. The number of radii the variable illumination changed in accordance with the relation-
RHYTHMIC:
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radii
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OF MOVEMENT
radii I
14-
12 -
10.
8;
6-
I
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e
0.4
0.6 Fig.
0.8
?
1 set
1
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I
10 30
*
’
1
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50 70 90 110 130 lux Fig.
2a
1 set
2b
ships found in experiments for both eyes, while on the other disk the number remained constant. Besides, what was noted by the one eye could be compared immediately with what was noted by the other eye. Neuhaus also did a few tentative experiments with colours. Here again the number of perceived radii varied with average brightness of the disk. A bright red cross showed fewer rad an a dark red onz. Quite interesting are the varying impressions obtained at different velocities of the disk. If the disk revolves slowly, the radii move across the surface and can be followed by the eyes. If the rotation speed increases, the radii can no longer be followed by the eye:;. If a certain point on the disk is fixated, one sees that the radii do not move across the disk at all. Qn the contrary, they are seen to emerge rapidly at scme spot ,md diszippear again. If the speed ,is further increased, the number of perceived radii increases, and now they remain at their own places. They are not rigidly fixed, but very restless3 They do not, however, continue to move. Due to contrast, the fields between the radii seem considerably brighter than before. Under certain cxmditkms, t.& radii can move either in the direction of 2
Probably the construction
of the disk was not very accurate, or the al&r-
nation of succtxive phases lis not completely regular.
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the roiatioa of the disk or in the opposite sense. This stroboscopic phenomenljn is even better observed on the white sections of the disk. The width of the radii remains approximately the same at various rates. Only towards the periphery of the disk the radii are completely dissolved,s and at high speed they cannot be clearly seen at :all. At high sped, the colour of the radii is less saturated and paler than at lower speed. The facts described here can easily be explained by assuming that our sense-*ryapparatus works in a periodic way’ says Neuhaus. That is, tlhere is an intermittent succession of phases of registration 4 (positive phases) p.nd of non-registration (negative phases). This mechanism is believed to be connected with the rhythmic activity of our nervous system. If the velocity of rotation is low, Neuhaus continues, the images of the cross partly overlap. If the velocity increases, the radii are not registered in uninterrupted continuous succession., but once here’ once there. The intervals are ‘completed’. If the velocity is further increased, one perceives more than the objective number of radii. At times there a{e 6, at times 8. One cannot *.ways easily tell the exact number, but ;tt a certain speed there arc definitely 8 radii. This happens when the retin lag of one registration is still active when a new registration occurs. At a certain velocity, the radii are always in the same place, when t4fe eyes are registering. Then the stimuli add up, and one sees the radii quite clearly. If at this moment the velocity is increased’ the radii start .moving forward, since the new registraticns are always a e ahead of the old ones, and a stroboscopic effect results. A reverse: movement occurs if the velocity decreases. Neuhaus has tried to calculate the duration of the phases of registration and non-registration,, but he did not exactly state on which data his calculations were IWXL He fbund ‘10 ms for. the pitive phase, and 40 ms for the negative phase. These values would only apply to spccif~ conditions of illumination. The number of periods per unit ud time increases if the illumination decreases’ as appears -. 3 At the: periphery of the disk, the forward velocity is highest. ,1 l’+khws
used the term perception (‘Wahmehmung). We prefer to distinau&h between the rhythmic registration of the retina and $enom.enat percep-
‘J%elatter is not r3ytLm.k. Neuhaus- made this distinction too, but his terminolcqy is different from ours. .
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from the increasing number of perceived ra&i. Neuhaus supposec; that the duration of the positive phases does nr.:t change under varying conditions, so that all changes in periods would be due to variations in the negative phases. This relationship would correspond with the smaller lag of a weak stimulation, as compared v:iCh that of a 2trong stimulation. Apparent movement,
apparent re,rt and rtfal movement
A.hbu& the problem of periodic registration was approached from different angles, the correspondence between Neuhaus’ and our findings is striking. Neuhaus had observed the pl :;nomenon of ‘apparent rest’, but it took him several years 2nd another accidental observation before he found an explanation of the perceived data. The present author, on the other hand, started from the phenomena of perceptual reduction and apparent movement. From these phenomena he concluded that perception of movement might be based on a registration of a limited number of stationary positions of the moving stimulus. It took him some years, however, before the deduction was made that perception might occur by means of short, rhythmic phases of registration and non-registration. As a matter of fact. the latter conclusion resulted from the distinction which must be made between ‘pursuit movement’, that is movement which is perceived when a stimulus is followed by the eyes, and movement which results from displacements of the retinal image. The original assumption was based on an analysis of pursuit movement and an - incorrect - identification of fixation with registration. A fixation of all the positions which are covered by the moving object seemed to be impossible. Only a number of ‘critical positions’ ‘could be registered, while the unregistered intervais were believed to be completed, like small gaps in a circle, or a distance between two stimuli in apparent movement are completed. In pursuit movement the observer can more or less deliberately define the positions which are fixated. Eut this is impossible in primary movement perception, when the eyes are usua.Uq kept stilL One is then completely at the mercy of the inner activity of the eyes or die: nervous system. After having discovered the distinction between fixation hnd registration it seemed to be unlogical that the alternation of registrations and non-registrations would occur by ch.mce. From this conclusion it was only one step to the hypothesis that registration occurs by means of short, rhythmic phases of registration and non-
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registration. Moreover, such rhythmic alternation is in accordance with the rhyt.hmic actiGty of the nervous system.5 The difference in approach may have been the cause that Neuhaus just missed an important finding, which was implicit in his observations. He realized that perception occurs by means of a periodic alternation of positive and negative phases of registration. He must also have not&d that stationary positions are registered, but he failed to discern the implication of this discovery with respect to the perception of movement. In correspondence with traditional conceptions of his time, he continued to distinguish between the perception of red movement and the perception of apparent movement, which is effected by a succession c:f stationary stimuli separated by certain spatial and tempora.! intervals,. In case of slow mcvement, Neuhaus therefore did not even notice a problem. The radii are seen in a continuous forward movement, since ‘the periods of registration succeed each other so rapidly that the obtained images, although somewhat dislocated, still overlap’. At 2::&i;;r velocities, the registrations do not overlap anymore, and nctw the perception of movement is explained as a strobcopic etfect. The intervals are completed, says Neuhaus, since: “In what we pzrceive, we are strongly dlependent on what we know or expect. We know that the crcoss is in rotation, and for that reason we see it in rotation”. It is evident that Neuhaus’ explanation of apparent movement by means of relevant expectancy or knowledge is wrong here. Apparent movement is also perceived if one knows that the stimuli are stationary. And em objec:t of which one knows that it moves fast in a forward direction is sometimes seen as stationary or moving slowly in a progressive or rezrograde direction, as Neuhaus himself has shown. As a matter of fact, Neuhaus failed to discern the inescapable necessity to explain uZ2phenomenal movement as apparent movement if actual movement is registered by means of registrations of stationary positions of the stimulus. If the velocity is low, the distance which is covered between two registrations, and which must be filled up, is small. The iact that the successive images partly overlap is only incidental in this respect, since it does not explain the origination of movement. The retinal lag may prolong an experience, and cause nomena as the apparent extension of moving objects or the mixing 3
tic&
~.XTXCrelationship is ccrmplex,however, as appears from different on the subject. Cf. JASPER, 1958; HUBER and WIESEL, 1959.
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of colours, it dms not bridge spa&j displacements by means of mst’emenl. The size of these displacements is of no importance in this respect. pursuit
movtement
rceptisn, the pcjrception of movement seems to be a rapid alternation of short phases of registration and non-registration. A succession 0: short, quasi-stationary stimuli separated by spatial an temporal intervals may be supposed to be the basis of visual movement and velocity. The relation between experienced velocity and spatial and temporal intervals between the stimuli reszd by the formula Vph = C.&./&. ‘;r th,iS fmm.da, vph menal velocity, c varies with the distancz of vph and s, and tr indicate distance and time between the s,timuli as regist,ered - and ncit as consciously ctxperienced - by the organism. The magmtude of the faotors which determine perceived velocity are not objective measures, but must be defined by the organism, so that errors in the perceived velocity ay originate from different sources. Distance and time between successive stimuli can be defined by different methods. Distance between successive stimuli can for example be measured by means of dislocations of retinal projections, but also by means of proprioceptive stimuli caused by the turning of head, neck or trunk. Judged distance between observer and stimulus, which mus,t also be taken into account, will not always be accurate, and cannot at all be defined in the first moments of observation. If different methods of measuring do not produce equal results, the same actual velocity will result in different phenomenal velocities. An example of variations in phenomenal velocity is shown in the so-called Aubert-Fleischl paradox, which implies that a velocity is perceived faster if a stationary point is fixated than with parrsuit eye movement (see COHEN, 1962). Experienced velocity is also faster in the first moments of observation than. afterwards (see ASHOUR, 1964). According to IARBUS (1962), the eyes cannot immediately pick up the movement, but on y after ;L delay of 100 to 200 milliseconds. In the first short period of adaptation, velocity would be perceived with stationary fixation, and aftl.:rwards with pursuit. MASHOUR (1964) recorded the movements of the eyes during the ptlrsuit of a spot of light moving along a horizontal motion-track. One of his conclusions was that the eyes move and stop intermittently when
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following an object, When following an object,; information would be gained about the structure of the object. During the stops, the image mov8esacross the retina and information would be obtained about the speed of the object. The latter c,onclusion seems to be somewhat premsture, however. The fact that pursuit movements are not continuous does not necessarily imply that the vekocity is measured during stops or other irreguiarities. In the area of apparent movement, ~~ptinudmovement which looks like real movement is produced if the temporal interval between two stationary, spatially separated stimuli is about 60 milliseconds (WERTBEIMER,1912). Up to about 30 milliseconds two stimuli are reported as cccurring simultaneously, and at about 200 milliseconds stimulus succz-ssion is experienced. ‘Part movements’ (i.e. movement over a part ale the distance between the stimuli] are reported at intervals lying between those for optimal movement and for ’ succession. The so-called phi movement (the less real ‘movement that dormsnot involve object movement’) is said to occur at ,temporal intervals lying between those for optimal movement and simultaneity. Experiments with optimal movement f-nave shown that the spatial separation between the stimuli increases with an increase in the temcmral interval &ORTE, 19 15 ; NEUHAUS,1930). Since vDh= sr/tr he implicazion may be that the so-called optimal movement is a standard velocity. if we pre to believe the above relationships it could also be a rhreshold velocity, i.e. the lowest possible velocity at which two spatially separated stimul! can be ‘completed’. Because of the relative slowness of optimal movement, it may look more real than the quick, jumping appearance of the faster phi phenomenon. One may assume that, as long as, the eyes are not adapted to certain conditions of the moving object, such as its distance from the eyes, movement is produced in the most primitive way, i.e. by means of displacements of the object projection across the stationary retina. y under specific conditions the movement will then be experienced as the actual movement of a real object. Often, however, it will, like the phi phenomenon, be experienced as a kind of abstract mcrtion and not hs really mtegra&d with an object. But even under gdmal conditions, Colestructure of a moving olbject cannot be clearly seen ans of primary movement perceptim. fn primary mLwement perception, the eyes dc not follow the &ject. ~otrsequence is :hat the moving object cannot be fixated for more
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than a bury short period of time. If the polint of, fixation is not exactly at the C-omxA dis”ance, or if the retinal projection of the object does not move across the fovea, the object is not clearly perceived at all. aim a more or less vague and distorted In ali these cases one
image of the o en if the point of fixation happens to be position, the object cannot be clearly at the correct perceived during the period that its retinal projection moves across the fovea. e Teason is that each of the stationary retinal projections obtained in successive registrations is somewhat displaced with respect to the previous projection. e displacements produce the cues for the perception of movement, ut at the same time prevent the development of a clear impression of the structural propertics. Approximately the same effect is obtained here as in the case of a stih camera taking a number of photographs of a moving object, ia rapid succession, without transportation of the film. The structural characteristics of successive exposures blend and obscure ‘each other, and the resulting image only tells something :ibout the height, the approximate distribution of the brightness of the object, and the distance which is coverer1 between the first and the last exposure. Only when the velocity c-f the retinal projections of the moving object is slow, the deviations between two or three successive reastrations are small and something of the structural characteristics may be noted, during short periods of observation. In most cases, however, o y some dim outlines ence primary and an unreal brightness or wiour is perceived. movement tends to be experienced as a rathtir abstract phenomenon if it cannot be inserted into a clear structural impression; the object itself is hardly seen at all during the movement. ‘Something’ moving is seen, and not the object as one knows it. This unclear appearance is very unsatisfactory to the perceiver, if he wants to see more than movement only. A clear image can only be obtained by fixating the object and following it with the eyes. It is theref-rre not surprising that an irresistible urge towards pursuit eye movement originates as soon as the existence of movement is registered somewhere in the field of vision. If the moving object is followed by the eyes, its movement cannot be seen in a direct way, however. From now on, movement and velocity must be defined by secondary cues. Secmciary cues are the relative, inverse movements of backgrosmd or foreground, propioceptive stimuli resulting ‘from the turning of the eyes, neck or trunk, or the movement parallax.
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in velocity may in the first place be not4 on the basis
of partly overlapping retinal images caused by the change,d velocity. The pursuit movement, adapted to the former velocity, thez:kno longer provi&s a clear image of the stimulus. Thus, all noticeable changes in velocity may in first instarze be re,gistered by dislocations of successive retinal projections of the fixated object. For that reason, data obtained by primary movement proeption can function as warningsignals, to indicate changes in the environment. Besides that, however, all movement which cannot be derived from seco dary cues, will be perceived with primary movemlent perception. me interpretation of lihe secondary cues must, at least: partly, be learned. First of all the young individual must learn the cues which are :mdicative of movement snd velocity, and under which circumstances. He must also learn to define velocity by means of different methods and the reliability of these methods. One complication is, among other things, the fact th.at eye movements can ‘be us(?d to define the velocity of an object, but are mostiiy used to change the point of fixalion and inspect figural qualities of an object, in which case they must be neglected. Among the basic percept.ual skills that must be learned is the adaptation of The velocity - which is directly dIetermined by retinal displaceme& or propriticeptive stimuli - to the estimated distance of the moving s,timulus. If two objects are moving at the same speed, the velocity of the nearer object would be cs,timated or perceived as being faster, if no adaptation to the distance were to take place. Therefore the velocity of an object can only be defined if its distance is correctly defined. The direct consequence is a Cference in perceived velocity before and during the fixation of an object. In the first moments of per~~pticw, the distance of the object cannot be defined at once, and the velocity originated from the retinal dislocations may be projected on a standard distance, which iu many case.s may be greater lthan the actual distance. Whether and to which extent this occurs also in prolonged primary movement perception can only be established experimentally. It s#eems doubtful, however, whether the Aubert-Fleisch paradox is relevant to all aspects of prolonged primary perception.
TWOEnds of visual movement perception can be distinguished. Primary movement perception results from displacements of the retinal image. Pursuit
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movement is perceived when the object is followed by the eye=, and the retinal image remains at a fixed position of the reti;ta. The hypothesis is made that primary movement is based on a rhythmtc registration of quasi-stationary positions of the moving stimulus. If the hypethecis in correct, ‘apparent movement’ is produced by the same stimuli as ‘real moverr?ent’. Moreover, the existence of apparent rest. that is the perception of a moving stimulus as stationary, can be predicted on the basis of the hypothesis. Arguments in favour of the hypothesis are brought forward, whereas the results of a simple experimtnt appeaed to be in conformance with the predictions. In 1934, Neuhaus published an article in which the same hypothesis was suggested. On the basis of experiments, Neuhaus .:ould draw a number of conclusions, but he did not discern the implications with respect to the perceptual identity of real and apparent movement. Primary movement perception cannct produce ;I clear perception of the structural properties of a moving o):jer!. I’or that reason, !he perceives will try to follow a moving object with the eyes as soon as it is discerned. Some implications are discussed.
CottEN, R. L., 1962. A further study of velocity synthesis. Stand. 1. Psychol. 3, 137-142. CEHRCKE, E. and E. LAU, 1923. Versuche iiber das Sehen von Bewegungen. Psychol. Forsch. See: Neuhaus, 1934. GRAHAM, C. H., 1951. Visual perception. In: Stevens, S.S. (ed.), Handbuak of exper. psychol. New York: Wiley. HUBEL, D. H. and T. N. WIESEL, 1959. Receptive fields of single neurones in the cat’s striate cortex. J. Physiol. 148, 574,591. IARBUS, A. L., 1962. Duizheniia glaz pri vospriiati dvizhushchikhsia ob’ektov (Eye-movements in perception of moving objects). Bioflzika, 7, 64-69. (Init. Biophysics, Moscow). JASPER, H., 1958. Reticular-cortical systems and theories of the integrative action of the train, In: W. F. Harlow and C. N. Woolsey (eds.), Biological and bqiochemical bases of behavior. Madison Univ. of Wisconsin Press. KOKT~, A., 19 15. Kinematoskopische Untersuchungen. Z. Psychol. 72, 143-296. relations in the perception of velocity. MASHOUR, M., 1964. Psychophysical Stockholm: Almquist and Wiksell. NEUNAUS,W., 1930. Experimcntelle Untersuchunger: der Scheinbewegung. Arch. ges. Psychol. 75, 315-352. -, 1934. Ueber den periodischen Verlauf der Wahrnehmung. Z. Psychol. 132, 349-359. PURKINJE, 1823. See Neuhaus, 1?34. WERTHEIMER, M., 1912. I,. Psychol. See Woodworth, 1938. WOODWORTH,R. S., 1938. Exp. Psychol. New York: Halt.
RHYTHMIC RJ5GISTRATIC3N OF
V&/f&+,
APPARENT MOVEMENT ANI3 APPARENT REST
ABRAM R. Vk ‘MUISEN Psycholo&&
Laboratoriup, ,Vniversitfit vain Amsterdam, Keizersgracht 611-613, Amsterdam ,
In principle, visual movment is perceived when: (a) t:he pro&t.ion of an object moves across the retina, or (b) a movi’I.-g object is followed by ,the eyes, so that the projection of the object remains at a fixed spot of the retina. Movement must then be inferred from secondary cues such as the relative movement of background or foreground, kinestheitic stimuli produced by rotations of eyes, neck or trunk-, or the movement pa.ral1ax.l The first type of movement is probably the most primitive. We shall eal! ;It primary movement and consider it first. Like roeived c&our, sound, scent or shape, perceived movement is sin exclusive!y ‘phenomenal’ affair. Phenomenal movement is formed out of spe&ic 2-Xes~,we-&h =e present in the input of the eyes and brain. In cease of a perception of actual movement these cues can be supposed to be functionally congruent with, i.e. representative of, movement of the outer object on the one side, and of perceived enomen@ movement on the other. Functional congruity, however, that movement itself must be among the properties ,s not im of the input of the brain, which determine the perception of the movement. The specific churacteristics which produce phenomee,rzaI need not be movement themselves; they only have to be ~~~r~e~t~v~ of ;f. Representativeness implies a relationship, as for em.mple that between the stationary position of an accelerator, and ion or speed of the car. The position and not the movement z’s d&&ion of mm&ar parallax (195 1) is: ‘When a human is moving through space fixates an object that is not moving, a angular ve:locity exists between the lines of sight to the fixated zct and some otl~er stationary object. If the objects move while the observer ~!;ra~o%,ionless, tk same situation of differential angular velocity exists’. 134
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of the accelerator is representative of the speed which the car ~21 assume In principle, acceleration and speed could be regulated by numerous devices, but in order to function adequately, any such device must show the desired functional correspondence between input and output. Accordingly, phenomenal movement, which is the correlate of physical movement, can in principle be produced by any combination of stimuli, which is representative of the movement in question. Our hypothesis is that phenomenal movement is inferred from stimuli, which we obtained from an in,term.ittent succession of quasi-stationary positions of the moving object. The
idea is that a11visible properties, including movement, are registered by meam of a rapid succession of sensory ‘snapshots’ of short expopdrc, An object wfiich moves too fast for the time of exposure will be seen as blurred, transparedtt and elongated, as it would appear in a photograph taken with an exposure of l/l 0 instead of the required 111 iB second. III most cases, however, ti stretch which can be covered by the object d.uring the time of registration is so small that it approaches zero, so that, in each single registration, the moving object is registered as if it were stationary. The movement of the object must then be inferred from at least two successive registrations. The assumption that visual registration occurs by means of a rapid succession of short exposures implies that positive p ases of registration alternate with negative phases of non-registration. Since in primary movement perception the eyes do not follow the object, the alternation of positive and negative phases will be determined by inner mechanisms of the sensury apparatus, probably in combination with the brightness of the registered area. We may assume that the alternation of positive and negative phases OCCUIS rhythmically, that is in accordance with the general rhythmic activity of the nervous system. If this assumption is correct, the duration of the positive and negative phases will he a constant, if further conditions such as the brightness of the field are the same. There is no reason, however, to suppose that the duration of a positive phase is the same as the duration of a negative ,phase. me idea that phenomenal movement is derived from series of still retinal images of the moving object was originally derived from four considerations: (a) &J has been discussed already, the cues for movement need not move themselves, they only have to be representative of movement. (b) ‘The &stcnce of apparent movement. From a phenomenal point
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of view, apparent movement and real movem.:n’t are identical. In either case one sees real movement. Apparent movement originates when two or more stationary, spatially separated stimuli are registered mporal succession. Why then could not all phenomenal movement ate from a succession of spatially separated still retinal pro(c) The quantity of information which can be grasped by the organism is limited. Consequentl;r, the total quantity of information which is present in the input of the eyes is reduced to a quantity which can managed by tke nervous system. In principle, a. moving object through an inkinite number of positions. The organism cannot register all thesle positions, but only a limited1 number. Assuming that the ‘channel capacity’ is a constant, the degree of reduction will be ional to the retinal velocity of the stimulus. The faster the vement, the smaller will be tke number of registered positions of a tch covered in a certain time-unit, and the @eater will be the between successive retinal images. From this it also follows t only a few of the structural properties OF a moving object - witk exclusion of a very slow motion - can be noted if the object is not follow~ed by the eyes. Succ/:ssive projections on the retina will ly overlap and prevent the development of a single clear image. ) The retina consists, of a great quantity of photcL-electric cells. Each 11cars regicter changes in brightness or colour, but ai single cell cannot ovement. One may assume that the cells and the nerve fibres h convey tke registered information towards the brain function in dence with the general rhythmic activity of the nervous system. Even if the movement of a stimulus could be registered com1~1~ by the retina, one may wonder in which way this transversal ement skou!!d be transmitted to the brain.. If the retinal projection vement would Imove from a point A to a point B during the registration, it would either continue to move in the same during the transport to the brain, or it would have to be back a point whick is the correlate of A, then move towards B, problem is that transversal movement cannot be conveyed a ckannel if it is nti fixed in separate sub-channels. Tke ution is ttat the fixation occurs in the retint cells in a code is deciphered in the brain. In the retina, such a fixation can an alternation of short phases of registration and
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Although a number of arguments in favfxn- of the basis hypothesis have already been brought forward it is still possible that: (a) the arguments are wrong, and (b) short stretches of movement and sot quasi-stationary positions of the moving stimulus are registered by the nervous system. Fortunately, the basis hypothesis could be tested by means of a simple experiment. If the basis hypothesis is correct, it must be possibIe to synchronize reiterative elements of a moving stimulus with the supposed periods of registration of the nervous system. III. actual fact the author wondered what would happen if a moving, reiterative stimulus like the one of fig. 1, exactly covers a distance S or a multiple of it, in exactly one cycIe of registration and non-registration.
Fig.1
On the basis of the possible ways of registration mentioned above, the following results can be predicted if the squares of fig,. 1 are put in to motion: (a) If registration does not occur periodically but continuously, one will see a continuous movement as long as the speed of the squares is not too high. If the speed is increased, the individual squares will ;ro longer be recognized, and only a vague, grey surface of the height of the squares will be perceived. (b) If registration occurs periodically, but not rhythmically, the same phenomenon as described under a) will be perh.:eived, since the squares are registered at arbitrary positions of the trkck. (c) If short stretches of movemerct are registered periodically and rhythmically, one cannot predict exactl,y what will be perceived, since not only the black squares, but also the white intervals move. It is certain, however, that the squares will not be seen as stakionary squares of the same width as drawn in fig. 31if the velocity of the squares is synchronized to the rhythm of registraticn. (d) If (quasi-) staticmny positims arc registered rhythmically, one will, like in the other alternatives, see a slowly proceeding series of squares as long as the actual movement is slow. At a certain moment only a. vague, greyish surface will be perceived, 9 the l/elocity is
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gradually increased Then, if the speed of the squares is synchronized to the rhythm of the registrations, one must see a stationary series of squares which looks like the original stimulus. The black squares and the white intervals will add up, since they will always appear at the places. The periods of non-registration will be filled up because the after-lag of the stimulated retina cells. If now the velocity of stimulus is slightly decreased, the squares will not be registered at the same places. As in the case of actual slow movement, each next ession, although partly overlapping the previous one, will stay a i little behind. Consequently, the squares must be seen as moving slowly into a retrograde direction. The reverse must happen if the d is slightly increased. In either case the perception of apparent movement is produced by a succession of stationary, spatially separ&d stimuli, in ,;he same way as real movement is produced. If the velocity is furthe,!r increased &yond the synchronized velocity., the eyes ll alternatively register black and white surfaces at specific places. It also happen that the speed becomes too fast for the period of re. In these cases only a greyish, vague surface of the height squares will be seen. le thiuking of an i.nstrum.ent by means -of which ,the hypothesis Id be tested, the author happened to remember an instrument which is used to regulate the r&&on sspeed of gramophones. This ‘stro’ consists of a disk on -which white radial stripes alternate black ones. Experimentation with this disk soon showed that all rceived phenomena surprisingly well co&rmed the hypothesis vrsual perception occurs on the basis of a rhythmic registration asi-) stationary positions of the moving object. se of the great conformity between predictions and obserwe shall not describe the perceived phenomena once again. are perceived as stationary at a certain velocity, they slowly e backwards or forwards if the velocity decreases or increases, etc. additional remarks, holvever, are necessary. e rotation speed is low, the movement of the radial stripes rotation of the whole disk can be easily perceived and the eyes. According to our theory, the impression of iginates here in the same way as the retrogade or proapparent movement yvhich originates if the disk is synchronized to the rhythm of the registrations. The impression one the slow apparent movement is not exactly the same as that
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of the slow real movement, however, The black and white of the apparently moving and also of the apparently stationary stripes is somewhat less intense than the black and white of ihe actually stationary or slowly moving stripes. oreove:, the apparently stationary or apparently moving stripes look less real than the actual. stripes. There may be more than one cause for the difference in appearance. The gramophone stroboscope is not an ideal instrument for experiments. l%tly, the black and white stripes are not exactly equal in size. In successive registrations, the stripes therefore do not cover each other completely. The black .and ;Ivhie will partly blend into grey, which may affect the whole appearance of the disk. In the second place, the disk contains a great deal of small, non-reiter.ltive irregularities. Due to these, the disk is perceived as rotating while the reiterative stripes are seen as stationary. This effect disappears only partly when a small part of the disk is observed through a slit in a piece of paper. The paradoxical effect, is that the statiimary stripes seem to float .in a somewhat unreal way on a rotating ground. This effect is further .enhanced<- since all small irregularites , which make the black and white more real and tangible when the disk is at rest, have disappeared because of the rotation speed which causes the stripes to appar as stationary. Consequently, the objectbound ‘surface’ colours of the stationary or slowly moving stripes seem to assume the appearance of less intense ‘free’ or ‘expanse’ c&ours (WOODWORTH, 1938). (2)
The
progressive or retrograde apparent move ent is necessarily slow if the stroboscope for gramophones is used. It originates from small displacements of the image of the stripes in successive registrations, No movement will ,be seen when the displacements become a. little larger, since the eyes then receive an alternation of. black and white sti.muli in successive registrations. Apparent movement can only originate - as long as .the, displacement ‘in successive registrations is smaller than half the width of one stripe. Beyond ‘that, the observer will only .perc.eive a more or less uniform grey. If the white sections are enlarged, it should be possible, however, to produce a somewhat faster progressive or retrograde movement, One ‘of the consequences of a process of perception, which occurs by means,of a fairly rapid rhythmic alternation ,of phases of registration of stationary stimuli, and phases of non-registration is that real and u~p&rent movement origi.nates in exactly the same way. In either case,
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the appearance of movement is created by a series of temporally and spatially separated stationary stimuli. Mostly these cues happen to coincide with an actually m.oving object. Sometimes, however, the senses are deceived as the object which produces movement cues does not really move. In reverse, the eyes are deceived when a reiterative, moving series of stimuli is st=en as stationary, or in relatively s:\ow progressive or retrograde motion.
In a study of the literature of the phenomena described above, thle author happened to jiind an article by Neuhaus, published in 1934. From a number of observations and experiments NeuhFus conclutdrd that perception occurs in short periodic phases of registration and non-registration. Because of the apparent importance of his article and also because Neuhaus’ findings supplement our own in several respects, a detailed survey of his article seems appropriate. Neuhaus used a rotating white disk of 30 to 40 cm. in diameter, with four perpendicular, black radii of approximately 1 lcm. in width. The speed of the disk could be varied. When the disk rotates slowly, one perceives a rotating cross. When the velocity of the disk is increased, one sees at a certain moment more than four &ii. ‘The number of radii which are perceived, increases zs the rotation speed of the disk increases. PURKINJE (1823) was the tist to describe this phenomenon. GEHRCKE and LAU (1923) examined it more in detail. Neuhaus accidentally discovered that the number of radii is also dependent on the illumination of the disk. Through observation obtained from 20 subljects he verified that: (a) the nurn&r of radii perceived increases if the intensity of the illumination decreases. In fig. 2 the relationships as noted by Neuhaus are recorded. In fig 2a, two diierent magnitudes of light reflected by the disk are taken as u~&ants. In fig. 2b the contants are $VWvelocities of roation. “The results of all subjects corresponded with the general tendencies, aa-&ough there were slight interindividual variations, In a following experiment Neuhaus used two disks. The left Gsk mid only be seen by the left eye, the right one by the right eye. The illummation of one disk was kept constant, that of the other was varied. Front the results it appeared that under these conditions the eyes fuactii independently from each other. The number of radii the variable illumination changed in accordance with the relation-
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radii
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radii I
14-
12 -
10.
8;
6-
I
:I\ \
4i
e
0.4
0.6 Fig.
0.8
?
1 set
1
1
I
10 30
*
’
1
1
’
50 70 90 110 130 lux Fig.
2a
1 set
2b
ships found in experiments for both eyes, while on the other disk the number remained constant. Besides, what was noted by the one eye could be compared immediately with what was noted by the other eye. Neuhaus also did a few tentative experiments with colours. Here again the number of perceived radii varied with average brightness of the disk. A bright red cross showed fewer rad an a dark red onz. Quite interesting are the varying impressions obtained at different velocities of the disk. If the disk revolves slowly, the radii move across the surface and can be followed by the eyes. If the rotation speed increases, the radii can no longer be followed by the eye:;. If a certain point on the disk is fixated, one sees that the radii do not move across the disk at all. Qn the contrary, they are seen to emerge rapidly at scme spot ,md diszippear again. If the speed ,is further increased, the number of perceived radii increases, and now they remain at their own places. They are not rigidly fixed, but very restless3 They do not, however, continue to move. Due to contrast, the fields between the radii seem considerably brighter than before. Under certain cxmditkms, t.& radii can move either in the direction of 2
Probably the construction
of the disk was not very accurate, or the al&r-
nation of succtxive phases lis not completely regular.
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A. R. W. MUIJEN
the roiatioa of the disk or in the opposite sense. This stroboscopic phenomenljn is even better observed on the white sections of the disk. The width of the radii remains approximately the same at various rates. Only towards the periphery of the disk the radii are completely dissolved,s and at high speed they cannot be clearly seen at :all. At high sped, the colour of the radii is less saturated and paler than at lower speed. The facts described here can easily be explained by assuming that our sense-*ryapparatus works in a periodic way’ says Neuhaus. That is, tlhere is an intermittent succession of phases of registration 4 (positive phases) p.nd of non-registration (negative phases). This mechanism is believed to be connected with the rhythmic activity of our nervous system. If the velocity of rotation is low, Neuhaus continues, the images of the cross partly overlap. If the velocity increases, the radii are not registered in uninterrupted continuous succession., but once here’ once there. The intervals are ‘completed’. If the velocity is further increased, one perceives more than the objective number of radii. At times there a{e 6, at times 8. One cannot *.ways easily tell the exact number, but ;tt a certain speed there arc definitely 8 radii. This happens when the retin lag of one registration is still active when a new registration occurs. At a certain velocity, the radii are always in the same place, when t4fe eyes are registering. Then the stimuli add up, and one sees the radii quite clearly. If at this moment the velocity is increased’ the radii start .moving forward, since the new registraticns are always a e ahead of the old ones, and a stroboscopic effect results. A reverse: movement occurs if the velocity decreases. Neuhaus has tried to calculate the duration of the phases of registration and non-registration,, but he did not exactly state on which data his calculations were IWXL He fbund ‘10 ms for. the pitive phase, and 40 ms for the negative phase. These values would only apply to spccif~ conditions of illumination. The number of periods per unit ud time increases if the illumination decreases’ as appears -. 3 At the: periphery of the disk, the forward velocity is highest. ,1 l’+khws
used the term perception (‘Wahmehmung). We prefer to distinau&h between the rhythmic registration of the retina and $enom.enat percep-
‘J%elatter is not r3ytLm.k. Neuhaus- made this distinction too, but his terminolcqy is different from ours. .
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from the increasing number of perceived ra&i. Neuhaus supposec; that the duration of the positive phases does nr.:t change under varying conditions, so that all changes in periods would be due to variations in the negative phases. This relationship would correspond with the smaller lag of a weak stimulation, as compared v:iCh that of a 2trong stimulation. Apparent movement,
apparent re,rt and rtfal movement
A.hbu& the problem of periodic registration was approached from different angles, the correspondence between Neuhaus’ and our findings is striking. Neuhaus had observed the pl :;nomenon of ‘apparent rest’, but it took him several years 2nd another accidental observation before he found an explanation of the perceived data. The present author, on the other hand, started from the phenomena of perceptual reduction and apparent movement. From these phenomena he concluded that perception of movement might be based on a registration of a limited number of stationary positions of the moving stimulus. It took him some years, however, before the deduction was made that perception might occur by means of short, rhythmic phases of registration and non-registration. As a matter of fact. the latter conclusion resulted from the distinction which must be made between ‘pursuit movement’, that is movement which is perceived when a stimulus is followed by the eyes, and movement which results from displacements of the retinal image. The original assumption was based on an analysis of pursuit movement and an - incorrect - identification of fixation with registration. A fixation of all the positions which are covered by the moving object seemed to be impossible. Only a number of ‘critical positions’ ‘could be registered, while the unregistered intervais were believed to be completed, like small gaps in a circle, or a distance between two stimuli in apparent movement are completed. In pursuit movement the observer can more or less deliberately define the positions which are fixated. Eut this is impossible in primary movement perception, when the eyes are usua.Uq kept stilL One is then completely at the mercy of the inner activity of the eyes or die: nervous system. After having discovered the distinction between fixation hnd registration it seemed to be unlogical that the alternation of registrations and non-registrations would occur by ch.mce. From this conclusion it was only one step to the hypothesis that registration occurs by means of short, rhythmic phases of registration and non-
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registration. Moreover, such rhythmic alternation is in accordance with the rhyt.hmic actiGty of the nervous system.5 The difference in approach may have been the cause that Neuhaus just missed an important finding, which was implicit in his observations. He realized that perception occurs by means of a periodic alternation of positive and negative phases of registration. He must also have not&d that stationary positions are registered, but he failed to discern the implication of this discovery with respect to the perception of movement. In correspondence with traditional conceptions of his time, he continued to distinguish between the perception of red movement and the perception of apparent movement, which is effected by a succession c:f stationary stimuli separated by certain spatial and tempora.! intervals,. In case of slow mcvement, Neuhaus therefore did not even notice a problem. The radii are seen in a continuous forward movement, since ‘the periods of registration succeed each other so rapidly that the obtained images, although somewhat dislocated, still overlap’. At 2::&i;;r velocities, the registrations do not overlap anymore, and nctw the perception of movement is explained as a strobcopic etfect. The intervals are completed, says Neuhaus, since: “In what we pzrceive, we are strongly dlependent on what we know or expect. We know that the crcoss is in rotation, and for that reason we see it in rotation”. It is evident that Neuhaus’ explanation of apparent movement by means of relevant expectancy or knowledge is wrong here. Apparent movement is also perceived if one knows that the stimuli are stationary. And em objec:t of which one knows that it moves fast in a forward direction is sometimes seen as stationary or moving slowly in a progressive or rezrograde direction, as Neuhaus himself has shown. As a matter of fact, Neuhaus failed to discern the inescapable necessity to explain uZ2phenomenal movement as apparent movement if actual movement is registered by means of registrations of stationary positions of the stimulus. If the velocity is low, the distance which is covered between two registrations, and which must be filled up, is small. The iact that the successive images partly overlap is only incidental in this respect, since it does not explain the origination of movement. The retinal lag may prolong an experience, and cause nomena as the apparent extension of moving objects or the mixing 3
tic&
~.XTXCrelationship is ccrmplex,however, as appears from different on the subject. Cf. JASPER, 1958; HUBER and WIESEL, 1959.
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of colours, it dms not bridge spa&j displacements by means of mst’emenl. The size of these displacements is of no importance in this respect. pursuit
movtement
rceptisn, the pcjrception of movement seems to be a rapid alternation of short phases of registration and non-registration. A succession 0: short, quasi-stationary stimuli separated by spatial an temporal intervals may be supposed to be the basis of visual movement and velocity. The relation between experienced velocity and spatial and temporal intervals between the stimuli reszd by the formula Vph = C.&./&. ‘;r th,iS fmm.da, vph menal velocity, c varies with the distancz of vph and s, and tr indicate distance and time between the s,timuli as regist,ered - and ncit as consciously ctxperienced - by the organism. The magmtude of the faotors which determine perceived velocity are not objective measures, but must be defined by the organism, so that errors in the perceived velocity ay originate from different sources. Distance and time between successive stimuli can be defined by different methods. Distance between successive stimuli can for example be measured by means of dislocations of retinal projections, but also by means of proprioceptive stimuli caused by the turning of head, neck or trunk. Judged distance between observer and stimulus, which mus,t also be taken into account, will not always be accurate, and cannot at all be defined in the first moments of observation. If different methods of measuring do not produce equal results, the same actual velocity will result in different phenomenal velocities. An example of variations in phenomenal velocity is shown in the so-called Aubert-Fleischl paradox, which implies that a velocity is perceived faster if a stationary point is fixated than with parrsuit eye movement (see COHEN, 1962). Experienced velocity is also faster in the first moments of observation than. afterwards (see ASHOUR, 1964). According to IARBUS (1962), the eyes cannot immediately pick up the movement, but on y after ;L delay of 100 to 200 milliseconds. In the first short period of adaptation, velocity would be perceived with stationary fixation, and aftl.:rwards with pursuit. MASHOUR (1964) recorded the movements of the eyes during the ptlrsuit of a spot of light moving along a horizontal motion-track. One of his conclusions was that the eyes move and stop intermittently when
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following an object, When following an object,; information would be gained about the structure of the object. During the stops, the image mov8esacross the retina and information would be obtained about the speed of the object. The latter c,onclusion seems to be somewhat premsture, however. The fact that pursuit movements are not continuous does not necessarily imply that the vekocity is measured during stops or other irreguiarities. In the area of apparent movement, ~~ptinudmovement which looks like real movement is produced if the temporal interval between two stationary, spatially separated stimuli is about 60 milliseconds (WERTBEIMER,1912). Up to about 30 milliseconds two stimuli are reported as cccurring simultaneously, and at about 200 milliseconds stimulus succz-ssion is experienced. ‘Part movements’ (i.e. movement over a part ale the distance between the stimuli] are reported at intervals lying between those for optimal movement and for ’ succession. The so-called phi movement (the less real ‘movement that dormsnot involve object movement’) is said to occur at ,temporal intervals lying between those for optimal movement and simultaneity. Experiments with optimal movement f-nave shown that the spatial separation between the stimuli increases with an increase in the temcmral interval &ORTE, 19 15 ; NEUHAUS,1930). Since vDh= sr/tr he implicazion may be that the so-called optimal movement is a standard velocity. if we pre to believe the above relationships it could also be a rhreshold velocity, i.e. the lowest possible velocity at which two spatially separated stimul! can be ‘completed’. Because of the relative slowness of optimal movement, it may look more real than the quick, jumping appearance of the faster phi phenomenon. One may assume that, as long as, the eyes are not adapted to certain conditions of the moving object, such as its distance from the eyes, movement is produced in the most primitive way, i.e. by means of displacements of the object projection across the stationary retina. y under specific conditions the movement will then be experienced as the actual movement of a real object. Often, however, it will, like the phi phenomenon, be experienced as a kind of abstract mcrtion and not hs really mtegra&d with an object. But even under gdmal conditions, Colestructure of a moving olbject cannot be clearly seen ans of primary movement perceptim. fn primary mLwement perception, the eyes dc not follow the &ject. ~otrsequence is :hat the moving object cannot be fixated for more
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than a bury short period of time. If the polint of, fixation is not exactly at the C-omxA dis”ance, or if the retinal projection of the object does not move across the fovea, the object is not clearly perceived at all. aim a more or less vague and distorted In ali these cases one
image of the o en if the point of fixation happens to be position, the object cannot be clearly at the correct perceived during the period that its retinal projection moves across the fovea. e Teason is that each of the stationary retinal projections obtained in successive registrations is somewhat displaced with respect to the previous projection. e displacements produce the cues for the perception of movement, ut at the same time prevent the development of a clear impression of the structural propertics. Approximately the same effect is obtained here as in the case of a stih camera taking a number of photographs of a moving object, ia rapid succession, without transportation of the film. The structural characteristics of successive exposures blend and obscure ‘each other, and the resulting image only tells something :ibout the height, the approximate distribution of the brightness of the object, and the distance which is coverer1 between the first and the last exposure. Only when the velocity c-f the retinal projections of the moving object is slow, the deviations between two or three successive reastrations are small and something of the structural characteristics may be noted, during short periods of observation. In most cases, however, o y some dim outlines ence primary and an unreal brightness or wiour is perceived. movement tends to be experienced as a rathtir abstract phenomenon if it cannot be inserted into a clear structural impression; the object itself is hardly seen at all during the movement. ‘Something’ moving is seen, and not the object as one knows it. This unclear appearance is very unsatisfactory to the perceiver, if he wants to see more than movement only. A clear image can only be obtained by fixating the object and following it with the eyes. It is theref-rre not surprising that an irresistible urge towards pursuit eye movement originates as soon as the existence of movement is registered somewhere in the field of vision. If the moving object is followed by the eyes, its movement cannot be seen in a direct way, however. From now on, movement and velocity must be defined by secondary cues. Secmciary cues are the relative, inverse movements of backgrosmd or foreground, propioceptive stimuli resulting ‘from the turning of the eyes, neck or trunk, or the movement parallax.
148 Changes
A. R. W. MUIJEN
in velocity may in the first place be not4 on the basis
of partly overlapping retinal images caused by the change,d velocity. The pursuit movement, adapted to the former velocity, thez:kno longer provi&s a clear image of the stimulus. Thus, all noticeable changes in velocity may in first instarze be re,gistered by dislocations of successive retinal projections of the fixated object. For that reason, data obtained by primary movement proeption can function as warningsignals, to indicate changes in the environment. Besides that, however, all movement which cannot be derived from seco dary cues, will be perceived with primary movemlent perception. me interpretation of lihe secondary cues must, at least: partly, be learned. First of all the young individual must learn the cues which are :mdicative of movement snd velocity, and under which circumstances. He must also learn to define velocity by means of different methods and the reliability of these methods. One complication is, among other things, the fact th.at eye movements can ‘be us(?d to define the velocity of an object, but are mostiiy used to change the point of fixalion and inspect figural qualities of an object, in which case they must be neglected. Among the basic percept.ual skills that must be learned is the adaptation of The velocity - which is directly dIetermined by retinal displaceme& or propriticeptive stimuli - to the estimated distance of the moving s,timulus. If two objects are moving at the same speed, the velocity of the nearer object would be cs,timated or perceived as being faster, if no adaptation to the distance were to take place. Therefore the velocity of an object can only be defined if its distance is correctly defined. The direct consequence is a Cference in perceived velocity before and during the fixation of an object. In the first moments of per~~pticw, the distance of the object cannot be defined at once, and the velocity originated from the retinal dislocations may be projected on a standard distance, which iu many case.s may be greater lthan the actual distance. Whether and to which extent this occurs also in prolonged primary movement perception can only be established experimentally. It s#eems doubtful, however, whether the Aubert-Fleisch paradox is relevant to all aspects of prolonged primary perception.
TWOEnds of visual movement perception can be distinguished. Primary movement perception results from displacements of the retinal image. Pursuit
RBYTHMIC REGISTRATION OF MOVE!,MENT
149
movement is perceived when the object is followed by the eye=, and the retinal image remains at a fixed position of the reti;ta. The hypothesis is made that primary movement is based on a rhythmtc registration of quasi-stationary positions of the moving stimulus. If the hypethecis in correct, ‘apparent movement’ is produced by the same stimuli as ‘real moverr?ent’. Moreover, the existence of apparent rest. that is the perception of a moving stimulus as stationary, can be predicted on the basis of the hypothesis. Arguments in favour of the hypothesis are brought forward, whereas the results of a simple experimtnt appeaed to be in conformance with the predictions. In 1934, Neuhaus published an article in which the same hypothesis was suggested. On the basis of experiments, Neuhaus .:ould draw a number of conclusions, but he did not discern the implications with respect to the perceptual identity of real and apparent movement. Primary movement perception cannct produce ;I clear perception of the structural properties of a moving o):jer!. I’or that reason, !he perceives will try to follow a moving object with the eyes as soon as it is discerned. Some implications are discussed.
CottEN, R. L., 1962. A further study of velocity synthesis. Stand. 1. Psychol. 3, 137-142. CEHRCKE, E. and E. LAU, 1923. Versuche iiber das Sehen von Bewegungen. Psychol. Forsch. See: Neuhaus, 1934. GRAHAM, C. H., 1951. Visual perception. In: Stevens, S.S. (ed.), Handbuak of exper. psychol. New York: Wiley. HUBEL, D. H. and T. N. WIESEL, 1959. Receptive fields of single neurones in the cat’s striate cortex. J. Physiol. 148, 574,591. IARBUS, A. L., 1962. Duizheniia glaz pri vospriiati dvizhushchikhsia ob’ektov (Eye-movements in perception of moving objects). Bioflzika, 7, 64-69. (Init. Biophysics, Moscow). JASPER, H., 1958. Reticular-cortical systems and theories of the integrative action of the train, In: W. F. Harlow and C. N. Woolsey (eds.), Biological and bqiochemical bases of behavior. Madison Univ. of Wisconsin Press. KOKT~, A., 19 15. Kinematoskopische Untersuchungen. Z. Psychol. 72, 143-296. relations in the perception of velocity. MASHOUR, M., 1964. Psychophysical Stockholm: Almquist and Wiksell. NEUNAUS,W., 1930. Experimcntelle Untersuchunger: der Scheinbewegung. Arch. ges. Psychol. 75, 315-352. -, 1934. Ueber den periodischen Verlauf der Wahrnehmung. Z. Psychol. 132, 349-359. PURKINJE, 1823. See Neuhaus, 1?34. WERTHEIMER, M., 1912. I,. Psychol. See Woodworth, 1938. WOODWORTH,R. S., 1938. Exp. Psychol. New York: Halt.