MOTION A~EREFFE~S: EVIDENCE FOR PARALLEL PROCESSING IN MOTION PERCEPTION’ OLGA EUXERFAVREAU Department of Psychology, University of Montreal, Case Postale 6128, Montreal, 101, Quebec, Canada (l&riced

6 January 1975; in recisedform 31 .Clnrch 1975)

Abstract-Interocularly transferred motion aftereffects occur immediately after exposure stimuli but are no longer demonstrable 7 min later. By contrast. opposite dichoptic motion occur both immediately after exposure and 7 min later, with no evidence of a decrease The results imply that the processing of motion aftereffects and hence, by extension, motion

to moving aftereffects over time. perception

occurs in parallel systems.

After an observer has looked at a moving display for a while the stationary display appears to move briefly in the opposite direction. This phenomenon has been referred to as the waterfall effect-or as the motion aftereffect (MAE). The main purpose of the present paper is to investigate how MAEs are mediated within the visual system. It will be seen that the evidence requires postulating that at least two partially independent pathways are involved in processing MAEs. Before considering the evidence for parallel processing of MAEs, it is relevant to examine briefly some aspects of theories about MAEs and other negative aftereffects. Contemporary theoretical explanations of the MAE are generally in terms of the hmctioning of neurons which respond selectively to the direction of moving contours. The level of activity in the set of neurons that have just been stimulated by the adaptation procedure for producing MAEs is said to be depressed relative to the activity in the set of neurons which detect motion in the opposite direction. The nervous system consequently interprets the different relative activity levels as signalling motion in the non-adapted direction. Fatigue in the adapted cells has been the major mechanism used to explain the different activity levels, with some theorists also invoking release from ~hibition in opponent-

’ The work reported in this paper was made possible by grants from the following sources: National Research Council of Canada grant No. APAto D. C. Donderi; Defense Research Board of Canada grant No. 9425-10 to M. C. Corballis; a McGill Univeristy Graduate Faculty grant to the author; and National Research Council of Canada grant No. A9788 to the author. The author is grateful for the invaluable advice and criticism of D. C. Donderi and M. C. Corballis. * The use of terms denoting length of time for MAEs can be confusing-for, while the MAE only appears rather briefly on any one test, it can reappear if the stationary stimulus is refixated. In this paper ~~ur~o~ is used to denote the duration of the MAE on any single test; persistence denotes the period of time during which it can be re-induced without additional exposure to motion.

process units (Barlow and Hill, 1963; Coltheart, 1971; Over, 1971; Sekuler and Pantle, 1967; Sutherland 1961). Although this paper is not intended to be a critique of the fatigue theory of aftereffects. it should be noted that some recent work makes it seem necessary to re-examine the question of whether fatigue alone can cause negative aftereffects. A major objection to fatigue theory arises from the observation that some negative aftereffects can be quite persistent’ (e.g. Masland, 1969; Favreau, Emerson and Corballis, 1972; Stromeyer and Mansfield 1970). Neural fatigue, on the other hand. has not been shown to be detectable for longer than a few min after stimulation (e.g. Horn and Hill, 1966; Segundo and Bell, 1970). Failure to demonstrate relatively persistent neural fatigue does not, of course, constitute proof that fatigue does not cause negative aftereffects. i%e very few studies in which neural fatigue was observed directly do not seem to have used lengthy stimulation periods such as those that generally produce persistent aftereffects. Nevertheless, it seems prudent, for the present, to reserve judgment on the issues of whether MAEs and other negative aftereffects are caused by fatigue or by other processes. For the purpose of the present paper, we retain the assumption that MAEs reflect activity of neurons that respond selectively to the direction of moving contours. Whatever neural mechanism or mechanisms may underly MAEs, it is evident that the activity of the stimulated cells returns gradually to preadaptation levels, as it has been shown that the magnitude and duration of MAEs decreases with the time elapsed following exposure to the adapting stimulus (Masland, 1969; Anstis and Harris, in press; Wohlgemuth, 1911). This point is important in the present context as the decay of MAEs is the major dependent variable measured in the experiments reported below. As stated above, the results reported below indicate that MAEs are mediated in at least two partially separate pathways. Evidence that different kinds of neurons are involved in causing MAEs already exists. One indication of this is that MAEs can be mediated either by monocularly or by binocularly driven

neurons. tVhen each eye is exposed to opposite dirsctions of motion. either simultaneously or alternately. the direction of the M.4E is contingent on which eye views the stationary display (Gates, 19%; Wohigrmuth. 1911). XAEs obtained under these conditions are referred to as dichoptic X.AEs, and must clearly be caused by monocularly driven neurons. On the other hand, when one eye alone is adapted to a moving pattern, an appropriate MAE is esperienced when the unadapted eye is tested with the stationary pattern (Barlow and Brindley, 1963; Freud. 1964; Walls, 1953; Wohlgemuth, 1911). Neurons that mediate interocular transfer must be binocularlv driven. The fact that MAEs can be mediated either by neurons that receive input from both eyes or from one eye only has been interpreted by previous investigators as indicating that these neurons are located serially at different levels of processing within a single retino-cortical pathway (e.g. Anstis and Moulden. 1970). Dichoptic &lAEs have been attributed to cells located at any level peripheral to the point where the inputs from both eyes are combined. Interocular transfer was believed to occur at or central to the level where information from both eyes converges onto the same neurons. The research reported in this paper interprets the differential persistence of dichoptic and interocularly transferred MAEs as evidence that they occur in parallel systems. Before reporting the experiments. we shall briefly examine some previous work which also implies that neurons that mediate MAEs can be distinguished by different rates of return to preadaptation activity levels. Masland (1968. 1969) exposed observers to rotating spirals for 15 min. When the apparent magnitude of the MAE was sub~quen~ly sampled every 2 min for 30 min, there was a rapid drop within 5-10 min following adaptation, and then a very slow decay during the remainder of the test period. Masland attributed the two phases of decay to two different processes, fatigue and conditioned inhibition. Wowever, an alternate explanation is that the two decay rates index return to preadaptation levels of firing in two different populations of neurons. In our own laboratory we have obtained some indirect evidence that units which mediate MAEs may have different rates of recovery from adaptation. We have found that, under certain conditions, color-contingent MAEs increase in duration with time following adaptation (FakTeau, 1974, in preparation). In the original experiments in which color-contingent MAEs were reported (Favreau er al.. 1972; Mayhew and Anstis. 1972) the inducing stimuli were black and white spirals onto which red and green lights were projected atternateiy. The direct of rotation of the spiral was reversed when the color of the light changed so that one color was always associated with one direction of rotation. Under these conditions color-contingent MAEs show the above-mentioned increase in duration. Suppose that MAEs can be mediated by motion-detecting units that are sensitive to color as well as by units that are sensitive only to brightness contrast but not to color (units of these types have been observed by Hubel and Wiesel, 1968, and by Gouras, 1972, 1974). It can then be seen that, although the above procedure imposes a differential directional bias on the coior-sensitive units. thz con-

trast s,‘nsitiii’ lint13 JR equali> adapted for ,tppos~ts directions of motion. LVhsn color-contingent $&Es are instead induced with tno different spiraIs th:it arc painted in diffirmt colors of equal brightness, the more usual deLTease in aftereffect duration over time is observed (Favreau, 1971). Thus it could be chat the contrast sensitive units may haye interfered uith the early appearence of color-contingent MAEs in rhe original procedure. If there are contrast-sensitive units that recover more rapidly than color-sensitive units do. then. when they have recovered it ma)- become possible for color-contingent MAEs to be perceived. 1~ is thus sten that there is evidence that. first, ;It.%& are mediated by either of two sets of neurons: those that are monocularly driven and those that are binocularly drivsn. Second. and somewhat more tentatively, there may be fwo sets of neurons that mediate MAEs and these may be characterized by different rates of recovery from adaptation. Let us see. now, whether the monocularlv and binocularly driven units can be distinguished with respect to rate of recovery from adaptation. it \vas hvpothesized above that at least some contrast-sensitive neurons recover more rapidly from adaptation than do color-sensitive neurons. Coltheart (1973f. in a review of the literature on color-related aftereffects. has pointed out that these are most likely mediated by monocularly driven units. He reached this conclusion for two reasons. First. because interocular transfer cannot normally be obtained with color-reiated aftereffects. Secondly. he pointed out &hat of the few cortical color-detectors that were reported by Wubel and Wiesel (1965). all ivere exclusively monocularly driven. Coltheart’s speculation has been strengthened by more recent work in which it has also been found that cortical color-sensitive neurons are predominantly monocular (Dow and Gouras, 1973: Gouras, 1972, 1974). The literature on contrastsensitive neurons. on the other hand. indicates that, of those located in the cortex. some are monocularly driven and others are bin~ularIy driven (e.g. Hubel and Wiesel. 1961. 1965. 1968: Blakemore and Pettigrew. 1970). We have hypothesized that the color sensitive units. which. we now see are predominantly monocular, have a relatively slow rate of recovery from adaptation. Let us extend this hypothesis to include all monocularly driven units. that is. those that are contrast-sensitive as well as those that are colorsensitive. Let us. furthermore. assume that binocularly driven units. which. it appears. are sensitive only to contrast and not to color. have a faster rate of recovery from adaptation. These assumptions lead us to make the following prediction: that dichoptic MAEs persist longer than interocularly transferred iMAEs. The esperiments reported below test this prediction.

ESPERIMEXT

1: PERSISTEXE TRASSFERRED

OF I~-i’EROCUXARLY M1AF.s

.a single-throw Sf turn arithmetic spiral was Cut OUt Of black matte paper and pasted on an S-cm tvhite cardboard disk. The disk had previously been covered with an irregular jtippled black and white pattern ILerratone. NO. Lfi.;.Q. The btack matte paper and stippled background each covered about half of the total area of the disk.

Motion aftereffects The spiral was mounted on the shaft of a variable speed motor which rotated at 5 rev/mm during adaptation The shaft of the motor projected through a hole in a white cardboard screen so that the observers saw the verticallyoriented spiral with the screen as background. The spiral and screen were illuminated by light from a projector. The luminance on the white area of the spiral was 430 mL. Thirty-six observers participated in this experiment They wore comfortable rubber-padded welding goggles from which the lenses had been removed. The observers held a piece of cardboard over one eyepiece of the goggles. A lever-operated hand-held microswitch connected to’ a timer provided a means of indicating the duration of the MAE. The observers sat 150 cm from the spiral, which thus subtended 3” of visual angle. They were told that the spiral would rotate for 75 min and were asked to keep their gaze fixated at the center. They were instructed that at the end of the adaptation period the spiral would be stopped and they would then be asked to say whether it appeared to be turning clockwise or counterclockwise (the apparent direction of motion of a spiral is usually described as expanding or contracting, but at slow rotation speeds it is easier to identify direction of rotation). The response wasa forced choice--observers were asked to guess the direction even if they saw no motion. They were further instructed to depress the lever if they did see any motion, and to keep it depressed until the motion ceased. Direction of rotation and exposure of dominant and nondom~ant eyes were counterbalan~d among observers. There were two tests immediately following the 7.5.min adaptation period. For the first test the observers occluded the adapted eye and looked at the stationary spiral wjth the unadapted eye (interocular transfer). The second test, given directly after the first, tested the apparent direction of motion with the adapted eye. Both tests were repeated in the same order 7 min later. During the interval between tests the spiral was hidden from view by a screen. The 7-min delay period was chosen because it coincides with the end of the initial phase of rapid decay of MAEs that had been reported by Masland (1968, 1969). Repeated measuring of MAEs appears to diminish their duration; bemuse of this only the two points in time that were of interest in the present context were sampled. All the experiments were conducted by a research assistant who was unaware of the hypotheses being tested. Results Scoring. On each test each observer gave two responses: (1) the direction in which the stationary spiral appeared to rotate, and (2), the duration, if any, of the apparent motion. These two responses were combined to provide a single score in the following way. When the reported direction was an appropriate aftereffect (i.e. opposite to the adaptation direction) the duration was recorded as a positive value. Similarly, when the reported direction was inappropriate, the duration was recorded as a negative value. Thus. for example. an observer who reported an

183

appropriate MAE with a duration of 200 set was assigned a score of + 20.0. Observers who did not record a duration were assigned scores of +0 or -0 depending on the appropriateness of the forced-choice directional response. This scoring method makes maximal use of the data by including the information contained in the forced-choice guessing while also assigning greater values to longer durations. Since nonparametric statistics are used to analyze the data, the signing of zeros presents no problem. In order to determine whether the distribution of scores differed significantly from chance, each score was compared with a value of unsigned zero using the Wilcoxon matched-pairs signed-ranks test (Ferguson, 197I; Siegel, 1956). This meant, in effect, that all of the durations were ranked regardless of sign, with zeros assigned the lowest rank. The sum of ranks of the negative durations (FV-) was calculated. W- was referred to a table of critical values of W for the Wilcoxon test (Ferguson, 1971). On the assumption that the recorded MAEs occurred by chance, there should be no bias of magnitude in either direction (+ or -) across the entire set of independent scores at each delay interval. lnterocular transfer (unadaptedeye). The mean duration of the scores for the immediate test was 953 set (K’-- = 37.5, N = 36, P c 0005). Of the 36 observers, 31 reported appropriate MA& Thus it can be seen that the MAE did transfer interocularly. On the 7-min deiayed test the mean duration was 097 set (W- = 243, N = 36, NS). Only 19 of the 36 observers reported appropriate directions and only six of these had durations greater than zero. Clearly, the MAE which had transferred interocularly on the immediate test, had virtually disappeared 7 min later. The scores for each observer on the immediate test were compared with those on the delayed test. In most cases the difference could be arrived at by simple subtraction. Whenever possible, differences between signed zeros were also signed. Observer No. 10, for example, had a score of -0 on the immediate test and +0 on the delayed test. His difference score was calculated as -0 [immediatedelay = (-0) - (+0) = -01. That is, his score on the delayed test was relatively more appropriate than his score on the immediate test. SimilarIy, observer No. 20 had scores of +O and -0, respectively, to yield a difference score of i0. All the differences, including the signed, but not the unsigned zeros, were subjected to the Wilcoxon matched-pairs signed-ranks test. The decrease from the immediate to the delayed test was found to be significant (W= 10.5, iv = 28, P < 0005). The scores of eight of the observers were the same on both tests, reducing N to 28. Monocular MAE (adapted eye). In order to evaluate the data on interocular transfer, it is instructive to examine the MAEs obtained with the adapted eye. The mean duration score for the immediate test was 3740 set (W- = 0. N = 36, P c 0005). For the delayed test the mean duration was 6.67 set (W- = 44, N = 36, P c 000.5). Of the 36 observers. 32 reported appropriate M_AEs. Thus it can be seen that the failure of the interocularly transferred MAE to persist for 7 min was not due to the fact

Table I. Summary of experimental results: mean durations of immediate and delayed tests Adaptation Conditions Interocular Transfer > Dichoptic

Experiment

Immediate test Mean duration (set)

:

9.53 (p < 0005)* 12.50 @ < 005)

2, eye I 2. eye 2

5.19 @ < 0005) 16.42 (p < 0005)

Delayed test Mean duration (set)

Difference (Immediate-Delayed)

097 (NS)’ -5.21 (NS) 1097 (p < 0005) 1058 (p c 0005)

’ Probability that the distribution of signed duration scores differs from chance. ’ Probability that the difference between the immediate and delayed tests differs from chance.

p < 0005’ p < 005 NS NS

I si

OLGA EIZNERFAVREAC

that .MAE had simply dissipated in the whole visual system. it is possible that the failure to maintain the interocularly transferred !vfXE was due to a floor effect. since it was comparatively low on the immediate test. The results of the following experiments show that this is unlikely. (See Table 1 for a summary of the results of all the experiments.)

ferred MAE. but there ts no s:gniticant dscr~~ u:th:n the time period that was tested. In fact. in rhe c;1se oi eye 1, there was a nonsignificant incre;lss in duratmn on the delayed test. These results support the hypothssa that the monocularly driven units recover from adaptation less rapidly than do the binocularly driven units. Since the mean durations of both the interocularly transferred and the dichoptic MAEs were within the same range on the EXPERIMEZ;T 2: PERSfSTENCE OF DKHOPTIC YtMAEs immediate test. it seems unlikely that the disappearance of the former on the delayed test was due to a ffoor effect This experiment was perfotmed as a test of the pre- (in fact the mean for eye t was less than that for the inter. ocularly transferred MAEi. diction that dichoptic MAEs would decrease less on

a delayed test than do the interocularly transferred MAEs. Ordinarily the kinds of comparisons that are made between experiments 1 and 2 should be included in a single statistical analysis. However. the use of nonpar~etric statistics makes this kind of comparison impractical. Furthermore, it wili be seen that the results are so clear-cut that such an analysis is unnecessary.

The sameapparatus was used as in experiment I. The procedure was similar. with the following variations. The observers were instructed to occlude one eye as in experiment 2. and then I5 set later they exposed that eye and occluded the one that had been exposed first. When the observers changed eyes, the direction in which the spiral was rotating was reversed by the experimenter. The observer continued to change eyes and see opposite directions of rotation once every 15 set for a total of I5 min. For example, one observer saw clockwi& rotation with one eye alternating with counterclockwise rotation to the other eye. Thus each eye watched the same direction of rotation for a total of 75 mm. On the assumption that two sets of detectors for opposite directions were being adapted, in order to equate the amount of adaptation time with the first experiment, each set was exposed for 7-S min of adaptation. The order of presentation of direction of motion and the order of exposure of dominant eye were counterbalanced among observers. There were 24 observers in this experiment. In order to assure that any MAEs which were obtained would not be due to the last seen direction of rotation, the eye which had not been last exposed was tested first on both the immediate and deIayed tests. This was also the eye which had been exposed first and wilf be referred to as eye 1. The eye that was tested second wilt be referred to as eye 2. Results The results were scored as in experiment 1. Appropriate effects were those which depended on the eye being tested -thus an observer who had seen counterclockwise rotation with eye f and cbckwise rotation with eye 2 was expected to report clockwise and counterclockwise MAEs for eyes 1 and 2, respectively. On the immediate test the mean duration for eye i was 5.19 set (W- = 10, N = 24, P c 0.005) and for eye 2 it Thusit is seen was 16-43 set (W- = 6, N = 24, P c O-005). that significant dichoptic MAEs were obtained. On the delaved test the mean durations were 1097 set and [@58 set for each eye (applying the scoring method that was used in experiment 1 to determine whether the distribution of scores differs signit%antly from chance, it is found that W- = 31. N = 24, P c O-005 for eye 1; W- = 3.5, N = 24, P < OGO5for eye 2). The differences between the immediate and delayed tests were not statistically signiticant foreithereye(W= 92,N== 2i,foreye 1: W= 90, N = 22, NS, for eye 2). Thus it is seen that not only does the dichoptic MAE persist on the delayed test, unlike the interocularly trans-

ESPERIMEST 3: INTEROCLL.4R TRASSFER WITH I~TER.MITTE~T AD.APTATIOY’ There was a procedural difference between experiments 1 and :! which may have contributed spuriously to the persistence of MA& in the latter. Whereas in experiment 1 adaptation was continuous for 7.5 min, in experiment Z the adaptation for either eye was intermittent, although the total time was 7.5 min for each eye. The difference between the two conditions may be likened to the difference between massed and spaced trials, and it is possible that spaced trials cause more persistent %XEs. Experiment 3 was performed to learn whether the persistence of interocularly transferred MAEs would resemble dichoptic MAEs when intermittent adaptation is used. &ferhod The same apparatus was used as in the previous experiments. The procedure and testing were essentially the same as in experiment I except that during adaptation the adapted eye was alternately exposed and occluded every IS see for a total of I5 min. Thus the total amount of exposure to the rotating spiral was 75 min. Six observers were tested.

The mean duration on the immediate test for the unadapted eye was 12.50 set (W- = 25, N = 6, P < BO5). On the delayed test the mean duration was - 5-21 see (W= 7.5, N = 5, NS). One observer reported that the spiral appeared to be oscillating and was assigned a score of unsigned zero. There is, therefore. no evidence that rhe interocularly transferred MAE is still present on the delayed test. When the scores of each observer on the immediate and delayed tests are compared with each other. the decrease on the delayed test is statistically significant ( W = 0, N = 5, P c 0.05). DISCUSSION The relatively persistent dichoptic MAEs contrast markedly with the transient interocularly transferred MAEs, contirrning the original prediction that binocularly driven units recover more rapidly from adaptation than monocularly driven units do. It is worth noting that the present finding that the dichoptic MAE does not decrease (and even shows a tendency to increase in eye 1) does not necessarily imply that the monocular driven units do not begin to recover from adaptation within the 7 min following exposure to motion. The procedure for inducing dichoptic MAEs inevitably affects binocularly driven units, so that it is quite likely that on thz mediate test there is interference between the binocularly and monocularly driven units. The fact that it is in e>s

Motion aftereffects 1 that the MAE shows a tendencv to increase on the delayed test makes this explanaiion appear valid. This is because eye 2 had been exposed to the rotating stimulus just prior to the testing of eye 1 and therefore the interference from interocular transfer would be expected to be stronger for eye 1. Let us consider the implications of the experimental results. As noted in the introduction, the occurrence of both interocularly transferred as well as dichoptic MAEs has been previously taken as an indication that these aftereffects are caused by fatigue at different levels of processing within the retina-cortical pathway. The different decay rates of these two kinds of MAEs contradicts this view. If the dichoptic MAE is mediated distally to the level at which input from both eyes is combined, then it should disappear at least as soon as the interocularly transferred MAE, because central units depend on peripheral ones for their input. The level of activity in peripheral units would have to be reflected in the output of central units. The results, however. %ow that the binocularly driven units recover from adaptation more quickly than the monocularly driven ones. Consequently, the two sets of units must be processing information concerning motion perception in parallel rather than sequentially, and could thus be located in two partially distinct neural systems. It is interesting to speculate about the anatomical locus of these two systems. For simplicity we shall refer to the set of binocular neurons as System I and to the monocular set as System II. Taking into account the present limited state of knowledge about the visual system, at least two possibilities can be examined. One would be to assume that Systems I and II are located in the superior colliculus and visual cortex, respectively. It has already been suggested that the superior colliculus may be the site of the spiral aftereffect (Richards, 1971; Richards and Smith, 1969). The electrophysiologically determined properties of cells in the superior colliculus and visual cortex resemble, to some extent, the hypothetical characteristics attributed to Systems I and II. Cells in the superior colhculus are particularly sensitive to moving contours, a property which makes them likely candidates as mediators of MAEs. Moreover, they are almost overwhelmingly binocularly driven (e.g. Cynader and Berman, 1972; Sterling and Wickelgren, 1969). We could thus tentatively locate System I in the superior colliculus. The visual cortex, on the other hand, contains proportionately manv more monocularly driven cells and could thus be-identified as the site of System II. These hvo structures also differ on other dimensions that are relevant in the present context. While the neurons in the visual cortex tend to be relatively finely tuned for angle of orientation (e.g. Gouras, 1972; Hubel and Wiesel, 1965, 1968), the neurons in the superior colliculus are not particularly selective for the specific contours of a moving stimulus (Cynader and Berman, 1972; Goldberg and Wurtz, 1972; Sterling and Wickelgren, 1969). This distinction is relevant because Masland has shown that during the first phase of rapid decay of the MAE, apparent motion can be detected on a great variety of stimuli which can be quite different from the inducing stimulus (this has also been noted by Wohlgemuth. 19I I ). During its second phase. however, the

185

MAE is specific to the adapting stimulus. This distinction of psychophysical properties coincides with the neural characteristics of the visual cortex and superior colliculus in accordance with our hypothesis. since it is only in the early phase of the MAE that the binocularly driven units are involved. Color is another dimension on which the superior colliculus and visual cortex may be distinguished, albeit tentatively. Color-sensitive cells have been identified in the latter, but there is at present no reason to believe that the superior colliculus contains colorsensitive cells, although the evidence for this is meagre. The strongest comes from a study by Humphrey (1970) who found that decorticate monkeys can respond to brightness differences, but are unable to discriminate color changes when the colors vary over a wide range of brightness. In decorticate monkeys it is assumed that visual discrimination is effected by the colliculus. The distinction for color perception also fits our hypothesis as it was the system that contains monocularly driven units that was said to contain color detectors. Thus it can be seen that the properties of the superior colliculus fit the psychophysical data for the two hypothetical systems involved in motion perception. However, another way in which the two systems can be divided is within the retino-cortical tract. Recent research has indicated that visual information reaches the cortex via parallel pathways, as certain properties of simple, complex and hypercomplex cells imply that these cells are not hierarchically organized (Bishop and Henry, 1972; Hofhnann and Stone, 1971; Stone, 1972). It has, for example, been suggested that X and Y retinal ganglion cells (Enroth-Cugell and Robson, 1966), which have also been referred to as sustained and transient, or tonic and phasic feed differentially into simple, complex, and hypercomplex cells (e.g. Hoffman and Stone, 1971). The tonic and phasic cells can be distinguished on the basis of sensitivity to color, only the latter being sensitive to color (Gouras, 1968, 1969), as weU as spontaneous firing rate (Enroth-Cugell and Robson, 1966). Since we had suggested that the two motion-detecting systems respond differently to color, and since we have shown that they can be distinguished in terms of level of activity following stimulation, it is possible that the tonic and phasic cells represent the initial level at which Systems II and I, respectively, are separated. While the above speculations are interesting, it is important to note that we are nowhere near an adequate understanding of “where” motion perception occurs. We allow ourselves to entertain the notion that aftereffects are lodged in certain sets of neurons simply because we know of the existence of these neurons. It may well be that sets of neurons located in as-yet unexplored parts of the visual system are responsible for negative aftereffects, and that mechanisms that we cannot yet imagine are their cause. The work reported in this paper has simply suggested that motion aftereffects, and thus presumably, motion perception, occur in more than one place in the visual system. REFERENCES Anstis S. M. and Harris J. P. (in press) Movement aftereffects contingent on binocular disparity. Perception.

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