Motion aftereffect magnitude as a measure of the spatio-temporal response properties of direction-sensitive analyzers

I rs#ntr Rrs. Vol. 14. pp. 1219-i 236. Pergamon

Press 1974. Printed m Great Brrtain

MOTION AFTEREFFECT MAGNITUDE AS A MEASURE OF THE SPATIO-TEMPORAL RESPONSE PROPERTIES OF DIRECIION-SENSITIVE ANALYZERS’ ALLANPANTLE Department of Psychology, Miami University, Oxford OH 45056, U.S.A. (Receirled 16 August 1973; in revised form 5 March 1974) Abstract-After an observer views an adapting pattern moving uniformly in one direction for a prolonged period of time, a stationary pattern will.appear to move in the opposite direction. In the present experiments observers inspected spatially periodic, adapting patterns which were moved at different speeds in different experimental conditions. The magnitude of the motion aftereffect which was generated in each condition was measured. There was an interaction between pattern characteristics and adapting speed. For a variety of patterns the temporal frequency, rather than the velocity, of the adapting patterns was the critical determinant of aftereffect magnitude. The psychophysical results suggest (1) that the responses of direction-sensitive analyzers in humans are controlled by the temporal frequency of drifting patterns rather than their velocitv. and (2) that the ueak resoonse frequency of direction-sensitive analyzers is about 5 Hz under low photopic levels of illumination. -

Ifan observer adapts to a pattern of uniformly moving for a prolonged period of time, and then the motion is stopped, the contours will appear to move in the opposite direction. Apparent movement which is produced in this manner has been called motion aftereffect (MAE) and it can be generated by adaptation to linear, rotational or radial (spiral) motion. A number of researchers have measured the strength or magnitude of MAE’s elicited by adapting patterns moving at different speeds. In these studies the strength of the MAE was defined by its duration (e.g. Sekuler and Ganz, 1963) or its apparent speed (e.g. Scott and Noland, 1965). In each study a curvilinear relationship between the magnitude of MAE and the speed of the adapting stimulus was found. The magnitude of MAE first increased with an increase of adapting speed, reached a maximum for adapting speeds of 2-6 “/set, and then declined with further increases of adapting speed. Although the function which relates MAE magnitude and adapting speed appears to be invariant under a variety of conditions (Scott and Noland. 1965X studies of contrast sensitivity to moving gratings (e.g. contours

Watanabe. Mori. Nagata and Hiwatashi, 1968) suggest that the function might not be independent of the spatial characteristics of the stimuli which are used to elicit the aftereffect. Contrast sensitivity to a grating with a spatial frequency less than 3 c/deg is an inverted W-shaped function of the velocity of the grating, and the location of the sensitivity function along the velocity axis depends upon the grating’s spatial frequency. Table 1 (first and second columns) shows the optimal speed, i.e. the speed which results in maximum sensitivity, for gratings of different spatial frequency (Watanabe et al., 1968).’ The range of optimal speeds is reiatively large, and there is an approximate inverse relationship between the spatial frequency of a grating and its optimal speed. By comparison, the temporal frequency of stimulation that is produced at a given retinal point by the various gratings when they are maximally visible is approximately constant. The optimal temporal rate of stimulation is about 5 Hz.

Table 1. Optimal speed of movement and temporal frequency for gratings of different spatial frequency

’ The first experiment reported in this article was conducted in the Department of Psychology at the Uni~rsity of California, Los Angeles. The second experiment was carried out in the Department of Psychology at Wright State University. ’ Watanabe et al. (1968) obtained two types of contrast threshold-one at which flicker was just apparent. and another at which the grating stripes became just visible. The optimal speeds and optimal flicker rates in Table 1 were derived from the flicker threshold data. 1229

Spatial frequency (ckdeg)

Optima1 speed of movement V&c)

1-23 0.90 0.63 0.45 0.25 0.18 O-09

4.3 6.5 10.0 19.0 32.0 75.0

3.0

Optimal frequency of temporal luminance modulation (Hz) 3.69 3.87 4.10 4.50 4.75 5.76 6.75

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ALLANPANTLE

An extension of Watanabe er al.‘s results to motion adaptation suggests that the relationship between MAE magnitude and the temporal frequency of an adapting stimulus. not its speed ought to be invariant with changes of its spatial characteristics. In order to test this hypothesis we measured the magnitude of MAE’s generated by patterns with different spatial characteristics. We were especially interested in determining what combinations of adapting velocity and pattern characteristics produce equivalent aftereffects. EXPERIMENT

I

In the first study MAE’s were generated by adaptation to rotating discs with alternating black and white sectors. Four discs. each with sectors of a different size, were used in order to determine whether the relationship between MAE magnitude and adapting velocity depends upon the spatial characteristics of the stimulus used to generate the aftereffect. Method The stimuli were stiff cardboard discs 305 cm dia. Each disc contained radial black and white sectors. The black and white sectors alternated and were of equal size on any one disc. Four different discs were constructed by varying the number (and consequently size) of the sectors on the disc -either 8. 16.32 or 64 black and white sector pairs. Each disc could be mounted individually on the shaft of a variable speed motor (Heller 2T60) and rotated at speeds ranging from 6 to 160 revimin. The motor and disc were placed at one end of a table. The subject sat at the opposite end with his chin on a rest and his eyes at the same height as the center of the disc. The viewing distance was 250 cm. The disc was iiluminated with a sharply focused beam from a slide projector. The resultant luminance of a black sector of the disc was 0% mL; the luminance of a white sector, 3.6 mL. The contrast of the sectors was 72 per cent. The experimental room was dark except for the disc and a small amount of scattered light reflected from it. A flat black poster board was placed immediately behind the disc to provide a homogeneous dark background. The subjects were nine undergraduate students from introductory psychology classes at the University of California. Los Angeles. The subjects served in seven experimental sessions-one practice session followed by six test sessions. Each session lasted about 50 min. In the first session the subject practiced making judgments of the velocity and duration of MAE’s These judgments were of the same type as those required in the later test sessions. A representative subset of the experimental conditions was used during the practice session. In a test session sixteen MAE’s were measured, each on a separate trial with a specific combination of the independent variables-the number of sectors on the stimulus disc and the adapting velocity. Each trial began with a signal from the experimenter whereupon the subject positioned his chin in the rest and fixated the center of the stimulus disc. The disc was then set in motion. After a 30-set

adapting period the disc was stopped and a timer activated. The subject immediately (within a second or two) gave an estimate of the initial speed of the MAE he observed. When the MAE ceased. the subject pushed a button stopping the timer. The timer provided a measure of the MAE duration. During the time between trials (9&120 set). the subject looked away from the stimulus disc. The method of magnitude estimation was used to measure the apparent speed of the MAE (e.g. Sekuler and Pantle. 1967). The subiect used the initial soeed of the MAE generated by a 30-s& exposure to the 16-sector disc rotating at 54 rev/min as the standard. The velocity of the standard was designated “100” and it was presented twice at the start of each test session, The subject judged the initial velocity of all aftereffects in relation to that of the standard. He assigned a number to the velocity of each aftereffect in proportion to its speed. For example, if the initial speed of the aftereffect were twice the standard speed. the subject assigned it a value “200”. if half as fast. the value “SK. Aftereffects were measured after adaptation to each of the four stimulus discs rotating at each of eight different speeds between 6.5 and 154 revjmin. In any one test session. only two stimuius discs were used. Therefore two sessions were required for one replication of the experiment. Each subject completed three replications. The order of presentation of the discs was different for each subject. No disc was used a second time until all the discs were used once. The order in which different adapting velocities were presented was counterbalanced across subjects for each disc (sector size). RcsrIts

The two response measures. initial speed and duration of MAE, were highly correlated. In Fig. 1 aftereffect duration is plotted as a function of adapting velocity. The separate curves give the results obtained with different adapting discs. i.e. sector sizes. Each data point is the mean of the median MAE durations of the individual subjects. The average standard error, excluding those conditions where no aftereffects were obtained was 1.75. As Fig. 1 clearly shows. MAE duration decreased as adapting velocity increased.3

ADAPTING

’ Once the duration declined to zero (i.e. there was no aftcrcffect) it remained at zero for all faster speeds. For the a:rkc of clarit>. the 7Cro aftereffects in those conditions arc not represented in Figs. 1-4.

VELOCITY

(I? PM)

Fig. 1. Motion aftereffect duration as a function of the adapting velocity of discs having 64 (triangles). 31 (open circles). 16 (filled circles} or 8 (squares) pairs of alternating black-white sectors.

Motion aftereffect magnitude Table 2. Summary of results of Friedman Analyses of the effect of adapting velocity on MAE speea and auration

Experiment I-MAE

duration

I-MAE

velocity

II-MAE

duration

II-MAE

velocity

df

2

64 32 16 8 64 32 16 8

7 7 7 7 7 7 7 7

38.0 51.0 48.9 21.5 38.0 53.8 40.8 19.5

Spatial frequency (c/deg)

Experiment

Significance level

Number of sectors

6 3 6 3

That the decrease is statistically significant for all sector sizes is shown by Friedman analyses of variance (Siegel, 1956) (see Table 2). Furthermore, the relationship between MAE duration and adapting velocity was not invariant with changes of sector size. The smaller the sector size, the more quickly MAE duration declined as adapting velocity increased. For each rev/min velocity in Fig. 1, there is a corresponding temporal frequency of modulation produced

(Hz)

Fig. 2. Motion aftereffect duration as a function of the temporal frequency of discs during adaptation. Discs contained 64 (triangles), 32 (open circles). 16 (filled circles). or eight (squares) pairs of alternating black-white sectors.


(P)

df 6 7 6 7

36.5 29.3 39.9 38.9


by one of the adapting discs. The temporal frequency of modulation is given by the formula .f; = N x v/60 whereA = temporal frequency in Hz, N = the number of black-white sector pairs (cycles) on the disc. and v = adapting velocity in rev/min. In Fig. 2 the duration data have been replotted with temporal modulation frequency as the abscissa. The duration curves obtained with the adapting discs containing 64.32 and 16 black-white sector pairs approximately superimpose. For a range of sector sixes. then. MAE duration was largely dependent upon temporal frequency alone. MAE durations were less at all temporal frequencies for the adapting disc with eight black-white sector pairs. Figure 3 shows the initial MAE velocity as a function of the speed of rotation of the adapting disc. The

5

FREQUENCY

m

IO

ADAPTING

20

50

VELOCITY

100

200

(R PMI

Fig. 3. Initial apparent velocity of motion aftereffects as a function of the adapting velocity of discs having 64 (triangles), 32 (open circles), 16 (filled circles) or 8 (squares) pairs of alternating black-white sectors.

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ALLAN PANTLE

sinusoidal gratings in order to avoid the possible influence of harmonic components on the measured frequency response. A wider range of temporal drift rates was studied than in the first experiment. The drift rates extended from 0.3 through 40 Hz. Method

0

I

“‘I I

2

5

IO

FREQUENCY

20

SO

100

(Hz)

Fig. 4. Initial apparent velocity of motion aftereffects as a function of the temporal frequency of discs during adaptation. Discs contained 64 (triangles). 32 (open circles). 16 (filled circles) or 8 (squares) pairs of alternating blackwhite sectors. separate functions represent data obtained with different discs. Each data point is the mean of the median velocity estimates of the individual subjects. For those conditions in which aftereffects were obtained the average standard error of the mean was 8.65. As was found with MAE duration. initial MAE velocity (1) decreased significantly (see Table 2 for Friedman analyses) as adapting velocity increased and (2) fell off more quickly when the aftereffects were produced by discs with smaller sectors. When the data are plotted on a temporal frequency abscissa (Fig. 4) the velocity functions for the adapting discs having 64, 32 and 16 black-white sector pairs superimpose in the 10-40 Hz region. This result parallels the duration data and further supports the conclusion that temporal frequency. rather than velocity per se. was the critical determinant of the changes of MAE magnitude. Like MAE durations. MAE velocities were less at all temporal frequencies for the adapting disc with eight black-white sector pairs. EXPERIMENT

II

The first experiment demonstrates the importance of the temporal frequency of adapting stimuli on MAE’s, A second experiment was designed to determine more precisely the temporal frequency response of the mechanisms which underlie the MAE. The temporal modulation which is produced at a given retinal point by rotating discs like those used in the first experiment has a rectangular wave-form which contains a number of harmonic frequency components in addition to the fundamental (nominal frequency). Moreover, only a restricted range of intermediate-to-high temporal frequencies was investigated in the first experiment. In the second experiment MAE’s were generated by drifting

The stimuli were vertical sinusoidal gratings displayed on the face of a Fairchild 708A oscilloscope with a P31 (bluegreen) phosphor. The technique described by Enroth-Cugell and Robson (1966) was used to generate the patterns. Two different gratings were used in the experiment. one with a spatial frequency of 3 cdeg and the other with a spatial frequency of 6 c(deg. The space-average luminance of the gratings was 2.2 mL. Their contrast was 22 per cent. The subject viewed the patterns binocularly from a distance of I12 cm. At this distance the pattern subtended a visual angle of 4‘ 36’ horizontally and 3. 34’ vertically. The experimental room was dark except for a small amount of scattered light produced by apparatus lights. A small fixation spot was located in the center of the pattern. The subjects were eight undergraduate students from introductory psychology classes at Wright State University. The velocity and duration of MAE’s were measured in the same way as in the first experiment with a few exceptions. Each subject served in four experimental sessions-one practice session and three test sessions. The adapting period on each trial was 45 sec. Eight different adapting velocities were used with the 3 c/deg grating; seven. with the 6 c/deg grating. The adapting velocities were between 0.1 and 13.3 “/sec. The corresponding frequencies of temporal modulation were between 0.3 and 40 Hz. The initial speed of the MAE generated by a 4%~ exposure to the 6 c/deg grating drifting at I.67 “/set (IO Hz) was used as the standard for the magnitude estimates of MAE speeds in different conditions. The standard speed was designated “100”. Fifteen aftereffects (one for each condition of the experiment) were measured in a single test session. Each subject completed three replications. The orders of presentation of the gratings and of the adapting velocities for each grating were counterbalanced across subjects.

Results The result of Experiment II are shown in Figs. 5 and 6. The initial apparent velocity of MAE in the various experimental conditions is given in Fig. 5; MAE duration, in Fig. 6. The abscissa is the temporal drift rate of the adapting grating; i.e. the number of cycles of the adapting grating passing a given retinal point per sec. In each figure, each data point is the mean of the median responses of the individual subjects. The average standard error of the mean was 11.43 for the velocity estimates and 2.71 for the durations. The solid curve in each figure depicts data obtained with the 3 c/deg grating; the dashed curve. data obtained with the 6 cJdeg grating. The variations of MAE magnitude shown by each of the four functions in Figs. 5 and 6 are statistically significant (see Friedman analyses in Table 2). With both the 3 and 6 c/deg gratings MAE magnitude, defined either in terms of its apparent velocity or duration. was an inverted U-shaped function of the temporal frequency of the adapting grating. The peaks of the func-

Motion aftereffect magnitude

L a

25

t DRIFT

RATE

(Hz)

Fig. 5. Initial apparent velocity of motion aftereffects as a function of the temporal drift rate of gratings during adaptation. Solid curve: data for 3 c/deg sinusoidal grating; dashed curve: data for 6 c/&g sinusoidal grating.

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adapting velocity (see Figs. 1 and 3). The relationship depended upon the spatial characteristics of the stimulus used to generate the aftereffect. In view of these results the exact relationship found in past studies (e.g. Scott and Noland, 1965) between MAE magnitude and adapting velocity cannot be considered unique and by itself it has no special theoretical significance. On the other hand the relationship between MAE magnitude and the temporal frequency of the adapting stimulus was invariant for a range of sector sizes. In Figs. 2 and 4 where MAE magnitude is plotted as a function of temporal frequency, the results for three of the sector sizes superimpose, especially in the region above 10 Hz. In other words, as long as the temporal frequency of the adapting stimulus was constant, neither a change of sector size (within limits) nor the reciprocal change of adapting velocity required to keep temporal frequency constant had any effect on MAE magnitude. For a range of sector sizes the falloff of MAE magnitude was governed by the temporal frequency of the adapting stimulus alone. Similarly, in experiment II it was the temporal frequency of the adapting stimulus. not its velocity, which determined whether the aftereffect was maximal or not. For both the 3 and 6 c/deg gratings the maximum aftereffect was obtained at the Sante temporal drift rat-5 Hz (see Figs. 5 and 6). However, the optimal velocity for generating an aftereffect was d@rent for each grating-l.67 ‘/set for the 3 c/deg grating and 0.83 ‘/set for the 6 c/deg grating. In defining the conditions which are optimal for generating an MAE, it is the product of spatial frequency and adapting velocity (i.e. temporal frequency) which matters, not velocity per se.

O 4/0:3

0’6 .

1’2 .

DRIFT

2’1 .

;

RATE

lb

2;

(Hz)

Fig. 6. Motion aftereffect duration as a function of the temporal drift rate of gratings during adaptation. Solid curve: data for 3 c/deg sinusoidal grating; dashed curve: data for 6 c/deg sinusoidal grating. tions are located at 5 Hz with a decline of MAE magnitude at both lower and higher temporal frequencies. In

other words the optimal temporal frequency for generating an aftereffect was 5 Hz. Weaker aftereffects (i.e. slower and shorter MAE’s) were obtained following adaptation at lower and higher temporal frequencies. DISCUSSION Temporal jequency tude

as a determinant of MAE magni-

In experiment I no single function was found to describe the relationship between MAE magnitude and

To summarize, experiments I and II both show that within limits changes in the spatial properties of the stimuli used to generate MAE’s and changes of adapting velocity do not alter MAE characteristics as long as the temporal frequency of the adapting stimulus is held constant. The invariances have important implications for the mechanisms underlying MAE and for models of MAE. The implications are discussed in a later section. In the next section the temporal frequency response lunctions shown in Figs. 5 and 6 are compared with temporal contrast sensitivity functions. A comparison of temporal frequency characteristics dejned by MAE and by contrast sensitivity

A number of investigators (Sutherland. 1961; Barlow and Hill, 1963; Sekuler and Pantle, 1967) have proposed that motion adaptation produces a change in the responsiveness of direction-sensitive analyzers which in turn gives rise to MAE. Assuming that this hypothesis is correct, the frequency response functions of Figs. 5 and 6 reflect the response properties of direction-sensitive analyzers alone, independent of other information processing mechanisms in the visual system. It is not surprising, then. that the functions in Figs. 5 and 6 differ in some respects from temporal contrast sensitivity functions whose shapes probably depend at

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ALLAN PANTLE

least partly upon other mechanisms.J For example, the frequency response functions in Figs. 5 and 6 obtained with both the 3 and 6 cjdeg gratings exhibit a low-frequency cutoff. Under comparable stimulus conditions. contrast sensitivity to a temporally modulated sinusoidal grating falls off at low temporal frequencies if the spatial frequency is less than 4 cideg. hut not if the spatial frequency is greater than 4 c/deg (van Nes. Koenderink. Nas and Bouman. 1967; Watanabe rr al., 1968; Robson. 1966). It is interesting that the bars of a temporally modulated 6 c/deg grating have been reported to be visible at a lower contrast than their movement or flicker (van Nes er al.. 1967; Keesey and Jones. 1972; Kulikowski and Tolhurst. 1973). The inability of an observer to see the movement or flicker at the contrast threshold” presumably means that the threshold is determined by some system which is more sensitive to the grating than direction-sensitive analyzers are. Under such circumstances contrast sensitivity measurements cannot be used to specify the response properties of directionsensitive analyzers. The response properties of the direction-sensitive analyzers would be concealed by the action of the more sensitive system. As a consequence, one would not necessarily expect contrast sensitivity measurements using a 6 c/deg grating to yield a temporal frequency response function like those shown in Figs. 5 and 6. On the other hand, the movement or flicker of a 3 c./deg grating is visible at the contrast threshold (van Nes et al.. 1967; Keesey and Jones, 1972; Kulikowski and Tolhurst. 1973). Apparently at the lower spatial frequency, direction-sensitive analyzers are relatively more sensitive and contribute to the observer’s ability to detect the grating. As one might expect, temporal contrast sensitivity functions obtained with a 3 c/deg grating do resemble the frequency response functions obtained in experiment II.

4 A temporal contrast sensitivity function gives the reciprocal of the amount of contrast required for an observer to drtect a temporally modulated stimulus at different frequencies. In some studies temporally modulated gratings of different spatial frequencies have been used as stimuli. Temporal modulation is produced by flickering the gratings in place on the retina (Rohson. 1966) or by allowing the gratings to drift across the retina (vsan Nes. Koenderink. Nas and Bouman. 1967: Watanabe 1’1ul.. 1968). ’ Hcrc. and in the remainder of this section. the term “contrast threshold” is used to refer to the minimal contrast required for an ohscrver to detect the presence of the gratis the inverse of the contrast ing. “Contrast sensitivity” threshold for detecting the presence of the grating. ’ The discussion does not explain the results obtained with the disc which had eight black and white sector pairs. This pattern may have been near the lower limit of the spatial frequency range for affecting direction-xnsitive analyzcrs. At the edge of the disc. each sector of the pattern subtended I I6 visual angle. which corresponds to a spatial frequency of 04 c:deg. MAE’s obtained with this pattern were weaker at all temporal frequencies.

Interpretations chmactrristics

of the irmractiott and adapting wlocit\,

hrtwrrn

pattu71

In their model for the aftereffects of seen movement. Sekuler and Pantle (1967) suggested that MAE velocity and duration are directly related to the number of direction-sensitive analyzers which are adapted by prolonged observation of real motion. They explained the apparently invariant. curvilinear relationship between MAE magnitude and adapting velocity in terms of the number of analyzers which were adapted by various velocities of real motion. Because few analyzers would be excited and adapted by very slow moving or very fast moving stimuli. such speeds would produce weak aftereffects. At some intermediate speed the largest number of analyzers would be adapted and MAE mag.nitude would be at a maximum. The difficulty with Sekuler and Pantle’s explanation of the relationship between MAE magnitude and adapting velocity is that it neglects spatial variables. The present experiments demonstrate that adapting velocity interacts with spatial pattern variables in determining MAE magnitude: (1) For a range of sector sizes in experiment I. the falloff of MAE magnitude was governed by the product of sector frequency and adapting speed (N x r/60): i.e. by the temporal frequency of the adapting stimulus. (2) In experiment II. combinations of spatial frequency and adapting speed (3 cideg x I.67 -sec. and 6 c deg x 0.83 “/set) which produced a temporal modulation frequency of 5 Hz were optimal for generating an MAE. Other combinations resulted in weaker MAE’s. The invariant relationship between MAE characteristics and the temporal frequency of adapting stimuli can be interpreted in more than one way. Alternative interpretations depend upon the assumptions one makes about the spatial response properties of direction-sensitive analyzers. Consider two different interpretations, each of which receives some support from physiological research: (1) It is possible that the MAE’s obtained with ruch pattern in experiment I or II result from adaptation of the sumr set of broadly tuned (spatial) direction-sensitive analyzers. This simple assumption accounts for the high degree of similarity of the temporal frequencv response functions obtained with different patterns; (in Fig. 2. 4. 5 or 6) provided only that the temporal frequency response of the individual analyzers is independent of the spatial patterns used to stimulate them. Ganz and Lange (1973) have studied the responses of single,directionallyselective neurons in the car cortex to movinggratings. They found that the response ofa single cell to a moving grating varied with its speed. The relationship between thecell response and the velocity of thegratingvdried with the spatial frequency of the grating. When the spatial frequency of the grating was lowered, the optimal velocity for stimulating the cell increased.Theoptimal temporal frequency for stimulating the cell, however. remained the same for gratings of different spatial frequencies. The temporal response properties of single directionally selective cells studied by

Motion aftereffectmagnitude physiological methods are like those inferred above from the MAE data. (2) While the above interpretation can account for the major findings of the present experiment. it may prove to be too simple. Campbell. Cooper. and Enroth-Cugeil(1969) and Maffei and Fiorentini (1973) have shown that the spatial response properties of movement-sensitive cells in the cat cortex vary from one cell to the next. The high spatial frequency cutoffs of the cells studied by Campbell et al. (1969) varied over a range of four octaves. The peak response frequencies of the complex cells investigated by Maffei and Fiorentini (1973) varied between 025 and 0.7 c/ deg. If direction-sensitive analyzers in humans have similar differences of spatial tuning, each of the different patterns employed in experiment I or in experiment II must have affected di$erent distributions of analyzers. If so. the present data indicate that the different subsets of analyzers had approx~ately the same temporal frequency response properties despite the differences in their spatial tuning. Otherwise, the functions which describe MAE magnitude as a function of temporal frequency (in Fig. 2, 4, 5 or 6) would not be similar. In summary, the present experiments demonstrate that adapting velocity interacts with pattern characteristics in determining MAE magnitude. The interaction indicates that the temporal frequency of an adapting stimulus is a critical variable controlling MAE magnitude. Although the present results can be used to delimit some of the spatio-tem~ral response properties of motion analyzers (e.g. their awruge temporal frequency response). they leave other properties open to further investigation (e.g. their spatial tuning characteristics).

Acknowludyenr~ncs-I wish to express my sincere thanks to Miss Sheryl Perry for her help in conducting experiment I

1235

and to Mr. Stephen Lehmkuhle for his help in collecting the data for experiment II. REFERENCES Barlow H. B. and Hill R. M. (1963) Evidence for a physiological explanation of the waterfall phenomenon and figural aftereffects. Nature. Land. 200. 1345 1347. Campbell F. W.. Cooper G. S. and Enroth-Cugell C’.( 19691 The spatial selectivity of the visual cells of the cat. J. Physiol.. Lmd. 203, 223-235. Ganz L. and Lange A. (1973) Changes in motion sensitivity of cat visual cortex neurons during the course of dark adaptation, Paper presented at the Spring meeting of the Association for Research in Vision and Ophthalmology, Sarasota. Florida. Keesey U. T. and Jones R. M. (1972) Flicker and pattern detection: A comparison of thresholds-II. J. opt. Sot. Am. 62, 1395A. Kulikowski J. J. and Tolhurst D. J. (1973) Psychophysi~l evidence for sustained and transient detectors in human vision. J. Ph_rsiol.. Lo&. 232. 149-162. MatTei L. and Fiorentini A. (1973) The visual cortex as a spatial frequency analyzer. Vision Res. 13, 1255-1267. Robson J. G. (1966) Spatial and temporal contrast-sensitivity functions of the visual system. J. opt. Sot. Ain. 56, 1141-l 142. Scott T. R. and Noland J. H. (1965) Some stimulus dimensions of rotating spirals. Psjchol. Rev. 72, 344-357. Sekuler R. and Ganz L. (1963) Aftereffect of seen motion witha stabilized retinal image. ScWirc~‘.h’.Y. 139.419-420. Sekuler R. and Pantle A. (1967) A model for the aftereffects of seen movement, C’isionRes. 7.427-439. Siegel S. (1956) ~on~ara~etric Statistics for the Be~~~ior~~ &ieticps. McGraw-Hill, New York. ’ Sutherland N. S. (1961) Figural after-effects and apparent size. Q. fl P.\-p.Psrchof. 13, 222-228. van Nes F. L.. Koenderink J. J.. Nas H. and Bouman M. A. (1967) Spatio-temporal modulation transfer in the human eye. j. opt. Sof. Afn. 57, 1082-1088. Watanabe A., Mori T.. Nagata S. and Hiwatashi. K. (1968) Spatial sine wave responses of the human visual system. Vision Rrs. 8, 1245- 1263.

R&urn&-Quand un sujet regarde une structure d’adaptation qui se deplace uniformement dans une direction donnee pendant une duree prolongee, une structure immobile semble se deplacer en sens inverse. Dam les experiences actuelles, les sujets examinent des structures d’adaptation. spatialement periodiques qui se deplacent a differentes vitesses dans diverses conditions experimentales. On mesure la grandeur de l’effet cons&& de mouvement engendre darts chaque condition. I1 y a une interaction entre les caracteristiques de la structure et la vitesse d’adaptation. Pour diverses structures la frequence temporelle dadaptation est le paramttre critique de la grandeur de I’effet conlcutif, phttcit que Ia vitesse. Ces resultats psychophysiques suggerent que (1) les reponses des analyseurs sensibles a la direction chez l’homme sont controltes par la frequence des structures en mouvement plutot que par leur vitesse et (2) la frequence maximale de reponse des analyseurs sensibles a la direction est d’environ 5 Hz aux bas niveaux photopiclues d’tclairage.

ein Beobachter ILngere Zeit auf ein Sehding. das sich monoton in einer Richtung bewegt. adaptiert. so scheint ein station&es Sehding sich in gegenllufiger Richtung zu bewegen. Bei den vorliegenden Experimenten betrachteten Beobachter Adaptations-Gitter mit verschiedenen Geschwindigkeiten unter unterschiedlichen experimentellen Bedingungen. Die G&se des Nacheffekts der Bewegung, der sich stets einstellte. wurde gemessen. Es bestand eine Wechselwirkung zwischen der ~hdingcharakteristika und der Adaptationsge~hwindigkeit. Bei zahlreichen Anordnungen war die Zeitfrequenz des Sehdings-mehr als seine Ge~hwind~gkeitd~e kritische Zusammenfassung-Wenn

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ALLAN PANTLE

GrSsse fiir den Nacheffekt. Die psychophys~~hen Ergebnisse lassen vermuten: (I) dass die Reizantworten r;chrun~sabh~ngiger Analysatoren im Menschen starker durch die zeitliche Frequenz des bewegten Sehdings als durch seine Geschwindigkeit bestimmt sind; und (2) dam das Maximum der Reizantwort dieses richtungsabhlngigen Analysators bei ca. 5 Hz unter niedrigem photopischen Beleuchtungsniveau liegt.

Pe3mMe--Hocne

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