Vkon
Rrr. Vol. 16. pp.
187-191.
Pcrgamon
Press
1976. Printed
tn Greor
Bnrain.
MOTION AFTEREFFECT AS A FUNCTION OF THE CONTRAST OF SINUSOIDAL GRATINGS MAX J. KECK and THOMAS D. PALELY, Department of Physics, John Carroll University. Cleveland. OH 44118. U.S.A. and ALL~V PANTLE Department
of Psychology, Miami University. Oxford. OH 45056. U.S.A.
(Receiced 18 September 1974; in recised_rbrm 6 rlpril 1975) Abstract-The motion aftereffect resulting from adaptation to moving vertical gratings has been measured as a function of the grating contrast from threshold up to 10.5:,. The gratings consisted of spatial sinusoidal intensity modulations which were generaTed on the face of an oscilloscope. The contrasts of the adapting grating and of the test grating could be varied independently. Both the duration and the initial apparent speed of the aftereffect wert measured. When the test grating contrast was held constant, the motion aftereffect magnitude increased rapidly with the contrast of the adapting
grating up to about 3%; for higher contrasts incremental incr?ases in contrast resulted in much smaller aftereffect increases than at the lower contrasts. When the adapting grating contrast was held constant, the aftereffect was found to be strongest for the lowest tesl contrasts and became weaker as the test contrast was increased. These psychophysical results support the hypothesis that fovea1 directionspecific motion detecting mechanisms show only a limited or compressed response to stimulus contrast.
the contrasts of the adapting and test gratings to be varied independently.
IYTRODUCFIOS
Adaptation to a moving visual stimulus results in a motion aftereffect (MAE); that is, following inspection of a moving stimulus, a stationary stimulus appears to move backward. The purpose of the present study is to make a systematic psychophysical examination of the dependence of the MAE on the contrasts of the moving (adapting) and the stationary (test) stimuli. In a previous study Pantle and Sekuler (1969) examined the contrast response of human visual mechanisms sensitive to motion using a method of selective adaptation. They measured the contrast threshold elevation for square wave gratings moving in the opposite direction and in the same direction as an adapting grating. The difference between these elevated contrast thresholds for various adapting contrasts provided a measure of the contrast response of directionally sensitive mechanisms. The motion aftereffect, which was measured in the present study, provides another way to study the response of direction sensitive mechanisms. We used one dimensional sine wave gratings as stimuli so that the spatial perodicity of the gratings would consist of only one spatial frequency. The parameters characterizing the sinusoidal gratings, with the exception of contrast, were held constant throughout the experiments, i.e. the spatial frequency, the mean luminance, the orientation, and the speed of the gratings. In this paper we attempt to answer the following two questions: (1) to what extent does the contrast of the adapting grating influence the adaptation to the moving grating, as measured by the MAE; and (2) does the contrast of the test grating influence the measurement of that adaptation? In order to answer these questions the experiments were set up so as to allow
EXPERIMENTAL METHOD
Vertically oriented gratings were generated on the face of a Tektronix 5 15.-Ioscilloscope with a green (P-3 1) phosphor. Tte gratings consisted of a sinusoidal luminance modulation along the horizontal axis. The mean luminance was held constant throughout the experiments at 0.6 R-L. The conuast. i.e. the ratio of the modulation amplitude to the mean luminance, could be varied between 0 and 10-5x. This intensity modulation was achieved by voltage modulating the :-a.ws of the oscilloscope with a sinusoidal voltage. The contrast was directly proportional to this a.c. voltage; 1.2OV corresponded to a contrast of 1@5%. Subjects viewed the grating display binocularly from a distance of 2 m. At this distance, the rectangular gratings subtended a visual angle of 2.7’ horizontally and 1.4’ vertically. The surround of the display was black. The spatial frequency of both the adapting and test gratings was 4c/ deg. The adapting gating moved across the oscilloscope face in a horizontal direction at a speed of I.ljdeg&c. At this JFed the temporal frequency at any position on the oscilloscope face. due to the traversal of the grating, was 5 Hz. The test _erating was stationary. Subjects were directed to fixate a small black dot in the center of the oscilloscope face. At the beginning of each session subjects were dark adapted for IOmin. Each session consisted of approx 35 trials. On each trial the moving adapting Dating was presented for 30~. followed immediately by the stationary test grating. The intertrial interval was 9Osec. During this interval the subject looked away from the oscilloscope display. The experimental room was dark except for a small amount of scattered light produced by the apparatus. The two dependent variables which were measured in these experiments are the initial speed of the MAE and its duration. In some trials the initial speed was measured
187
and in other trials the duration >\a3 measured. The mIttal speed and the dtiration were always measured in scpararr sessions. fn those sessions where rhz duration of the aftereffect was measured. the subject started a timer when the test pattern appeared and stopped it when he judged the MAE to have stopped. The time was recorded to the nearest O-5sec. In those sessions where the initial speed of the hIAE was measured. the method of magnitude estimation was used. Subjects were shown a standard trial in which the adapting and test patterns were at a contrast of 1039,. The speed of the MAE produced by the standard. Zsec after the onset of the test pattern. was defined as 100. The standard was presented twice at the beg,inning of each session. On subsequent test trials, the subject was instructed to scale the apparent speed of the test pattern relative to this standard. In ail cases, the subject gave his estimate within Zsec after the onset of the test pattern. This is necessary as the apparent speed of the M.G decreases exponentially from its initial value (Taylor, 1963; Sekuler and Pantle. 1967). ESPERIMEXT I: MAE DEPENDEM-E OS THE COSTRAST OF THE hDAF’TfSG GRATING
In the first experiment the dependence of the MAE on the contrast of the adapting grating was measured for two values of the test grating contrast: 1.7 and lO$;. Each of three subjects viewed adapting gratings with 11 different contrasts and judged the durations of the MAE. Each subject completed three replications of the experiment. For each replication the conditions were presented in random order. For each condition. the mean duration and the standard error of the mean were found for a given subject. The pattern of rest&s was the same for al1 subjects. Excluding the standard errors for those conditions in which the subjects saw no aftereffects,’ the standard errors ranged from 0.2 to 5.7 set and had a mean of l-5. For each condition, the overall mean of the aftereffect durations of the three subjects was computed and is plotted in Fig. 1. The upper curve shows the results when the test grating contrast was 1.7%; the Iower curve shows the results when the test grating contrast was lO*S%.The main features of both curves are: f 1) for low contrasts the MAE duration increases rapidly with adapting contrast; and (2) for contrasts greater than about 3% there is a marked decrease in the rate of change of MAE duration with adapting contrast. There is no evidence of complete saturation for adapting contrasts up to lO.Sll/,. For low adapting contrasts the MAE duration measured with the higher test contrast (lOSo/,) was only a small fraction of that measured with the lower contrast (1.70/;;)test grating. Also note I None of the subjects saw aftereffects at the lowest adapting contrast (@7P/,fwhen the test contrast was l&5%. Two of the three subjects saw no aftereffects with adapting contrasts of 0% 0.9 or I% and a test contrast of 10-S%. The individual means and standard errors in the remaining conditions were not correlated. Ninety-seven per cent of the standard errors (a11 but two) were between @2 and 3-3 set and were approximateiy normaily distributed. 2 The distribution of standard errors was positively skewed. The mean estimates and the standard errors were not correfated.
a-
Fig. I. Duration of the MAE as a function of the contrast of the adaptins grating for two different test grating contrasts: 0 I.;“,, test grating contrast, $2 IO+?; test srating contrast.
that for ail values of the adapting contrast the MAE duration was shorter for the higher contrast test condition. The above conditions were repeated and the same three subjects were asked to judge the initial speed of the MAE. Each subject completed three replications. For each condition the mean initial speed estimate and the standard error of the mean were found for a given subject. The pattern of results was the same for all subjects. Again, excluding the conditions in which the subjects saw no aftereffects, the range of standard errors was 00-18~9 with a mean of 62.” For each condition, the overall mean of the speed estimates of the three subjects was obtained and is given in Fig. 2. The curves of Fig. 2 show that the higher contrast test grating resulted in a lower initial MAE speed for all adapting Contra& For both test contrasts, (1) initial MAE speed increased as a function of adapting contrast and (2) the rate of change of MAE speed with adapting contrast, was less at higher than at lower adpating contrasts. Unlike the duration curves of Fig. 1, both MAE speed curves in Fig. 2 are upprosimare~~ logarithmic functions for the range of adapting contrasts displayed. It is apparent from Figs. 1 and 2 that the initial speed and the duration of the MAE are not in direct proportion to each other over the range of adapting contrasts investigated. The reason for tl& is not entirely clear, but may be related to an adaptation of motion analyzers by the test grating during the course. of the MAE’s time decay Keck. 1974). The duration measurements required continuous viewing of the test grating, whereas the initial MAE speed estimates were completed in a couple of seconds. Despite the observed discrmancy bet~eer~ the two dependent variables plotted in Figs. 1 and 2. the data warrant the general conclusion that incrementai increases of the adapting contrast above 3% resulted in much smaller MAE increases than at lower adapting contrasts.
ESPERlitfENT If: M.IE DEPFSDE?XE 0% THE CONTRAST OF THE TEST GRATTISG
In the second experiment
the contrast of the test grating was varied, and the contrast of the adapting
Motion aftereffect function
Fig. 2. Relative initial speed of the MAE as a function of the contrast of the adapting grating for two different test grating contrasts: 0 1.7% test grating contra% 0 103% test grating contrast. grating was held constant at 5.27;. Each of four subjects viewed the adapting grating followed by a test grating with one of eight different contrasts between O-9 and 10.5% and judged the durations of the MAE. Each subject completed three replications of the experiment. For each replication the conditions were presented in random order. For each condition. the mean duration and the standard error of the mean were found for a given subject. The standard errors were positively skewed, ranged from’ 0.3 to 68 set, and had a mean of 2.6. For each condition, the overall mean of the aftereffect durations of the four subjects was computed and is plotted in Fig. 3. The curve shows that as the contrast of the test grating was increased. the MAE duration became shorter. The maximum value of the MAE occurred for the lowest test contrasts. At the highest test contrast used, 105%, the MAE was only about one half of its maximum value. The above conditions were repeated, and the same four subjects were asked to judge the initial speed of the MAE. Again each subject completed three replications. For each condition the mean initial speed estimate and the standard error of the mean were found for a given subject. The range of the standard errors was 00-18~9 and their mean was 8.4. For each condition, the overall mean of the initial speed estimates of the four subjects was obtained and is given in Fig. 4. Note that this curve is similar to the curve of Fig. 3. Low test contrasts result in fast MAEs, and high test contrasts result in slow MAEs.
189
It is interesting to note that as a consequence of the above results. one can actually get stronger aftereffects with lower contrast gratings. This agrees with qualitative observations made by Szilly (1905. as summarized by Holland, 1965, on p. 6): ‘weak contours produce the best after-effects and strong contours weaken them.” In Szilly’s experiments, as is the usual case for MAE experiments, the adapting and test targets were identical. Our experiments show that, other things being equal, an increase of adapting contrast results in a greater MAE. However, an equivalent increase of test contrast may more than offset the greater effectiveness of the higher adapting contrast. The net effect, when high contrast targets are used for both adapting and testing, may be a reduced MAE.
DISCUSSION Relationship
of the present results to a model of MAE
Barlow and Hill (1963b) recorded the responses of a direction-sensitive cell in the retina of the rabbit. When an irregular stimulus pattern was moved in the preferred direction through the receptive field of the cell, its response rate increased immediately above its spontaneous level of discharge. As the stimulus motion continued, the response of the cell declined, but even after 57 set the response was still well above the spontaneous level present before stimulation. When the moving stimulus was stopped, the response rate dropped abruptly to zero and only slowly recovered to the normal spontaneous level. When the stimulus was moved in the null (opposite) direction through the receptive field of the same cell, no changes in the activity of the cell were observed. The results of Barlow and Hill give strong support to the hypothesis (Sutherland, 1961; Barlow and Hill, 1963b; Sekuler and Pantle, 1967) that the MAE results from a temporary imbalance in the discharge of cells which have different preferred directions of movement. Presumably, the adapting stimulus which is used to generate an MAE desensitizes cells whose preferred direction of movement coincides with that of the adapting stimulus and leaves cells sensitive to the opposite direction of movement unaffected. If the extent of the desensitization of the cells which respond to the adapting stimulus depends upon how much they are excited by it, then the MAE ought
2,-
,
,-
Fig. 3. Duration of the MAE as a function of the contrast of the test grating. The adapting grating contrast was 52%.
Fig. 4. Relative initial speed of the MAE as a function of the contrast of the test grating. The adapting grating contrast was 5.2%.
190
hf. J. f;ECL T. D. I'ALELLA
to be an indirect measure of the degree of excitation of direction-sensitive cells by the adapting stimulus. The results in Figs. 1 and 2 show that the MAE increases rapidly with adapting contrast for contrasts near threshold. However, for adapting contrasts greater than about 3?b. incremental increases in contrast result in much smaller MAE increases than at the lower contrasts. This in turn implies that for our experimental conditions the excitation of directionsensitive analyzers increase only slightly with changes of adapting contrast above 3% (about six times threshold). The dependence of the MAE on the contrast of the tesr grating can also be explained in terms of the imbalance between the discharges of cells tuned to motion in opposite directions if it is assumed that the dir~tion-sensitive celIs respond to a stationary grating. Ordinarily the stationary pattern would be expected to produce equal responses in cells with different preferred directions of movement, with the result that the test grating would appear stationary. However, if one class of direction-sensitive cells had been previously adapted (desensitized), the response of each class to the stationary test pattern would not be equal; the adapted (desensitized) cells would respond less. The imbalance of activity would result in apparent backward motion. i.e. an MAE. If this analysis of the MAE is correct. the data in Figs. 3 and 4 suggest that the imbalance in the responses of adapted and unadapted analyzers decreased as the test grating contrast was increased. The result is surprising since it seems more logical that a high contrast test grating would magnify the response imbalance. One possible reason for the weaker MAE at high test contrasts is the limited or compressed contrast response of direction-sensitive cells suggested by the results of experiment I. An increase of test contrast above 3”/, might lead to small increases in the response of unadapted cells, but to large increases in the response of adapted cells. Consequently, the net effect of an increase of test contrast above 3% would be a reduction of the imbalance of the outputs of adapted and unadapted cells, and hence a smaller MAE. Relationship of the present results to other physioiogical and psychophysical datn
In a physiological study, Barlow and Hill (1963a) investigated the contrast response of single cells seiectively sensitive to direction of motion. Their stimulus was a 1’ spot of variable luminance superimposed on a uniform background with a luminance of 0.2 mL. The minimum ratio of spot luminance to background luminance necessary to obtain a response was reported at 1.2/l. For spot to background Iuminance ratios greater than 1.5/l (about two times greater than the threshold contrast) the response was found to be ~epe~e~r of contrast. It should be noted that the spatial frequency content of the non-periodic spot stimulus is significantly different from our single frequency gratings. Maffei and Fiorentini (1973) obtained a similar result when recording the response of complex cells of the striate cortex of the cat. They employed moving sinusoidal gratings as stimuli and measured the response of the cortical cells as a function of the contrast of the gratings. They found that
and ,A. P-\\:rt
the rate of discharge saturated at about 5’,, I.! tlrnps threshold) for cells u-ith a receptive field of 2 5’ (thw Fig. 9). However. the same figure also shokvs responses recorded from other complex cells which did not exhibit the saturation. Ganz and Lange (1973) have reported that the responses of direction-sensitive cells in the cat cortex saturate at a low contrast. The contrast response of motion detectors has been investigated ps~choph~sically by Pantle and Sekuler (1969). Subjects adapted to moving square-wave gmtings (038 cjdeg, temporal frequency = 2.3 Hz) of various contrasts. Following adaptation to a grating with a particular contrast. the contrast threshold for a moving test grating was found to bz elevated. The threshold elevation was greater for test gratings moving in the same direction as the adapting grating than for test gratings moving in the opposite direction. The difference between these two threshofd elevations was reasoned to be a measure of the desensitization of direction-sensitive mechanisms. Their results show that the response of direction-sensitive mechanisms is a power function of the contrast of the stimulus up to 16*/;,(i-6 times threshold), and the response saturates for higher contrasts. In comparing their data with our results. it must be kept in mind that the contrast threshold for the square-wave gratings used by Pantie and Sekuler was much higher (2.4”~ than that of the gratings used in the present study (O.jg,,j. Moreover the gratings employed by Pantle and Sekuler contained harmonic frequencies at louver contrasts in addition to the fundamen~l frequency of 038 ddeg. Given the differences in stimuli. it is interesting that the response saturation observed by Pantle and Sekuler occurred at the same subjective contrast (Z-6 times threshold) as the fairly pronounced decrease in the rate of change of MAE strength with adapting contrast (3”;. 6 times threshold) in our experiment I. While the results of the present studies can be expfained in terms of a limit or compression of the contrast response of motion analyzers, they might also reflect the operation of inhibitory processes. For example the weaker MAEs obtained at high test contrast (experiment II) may be due to the inhibition of transient, motion-det~ting systems by a pattern system at high contrast (Tolhurst, 1973; Breitmeyer, Love and Wepman. 1974) or to inhibition between direction-sensitive mechanisms (Sekuler and Levinson, 1974). Acknowledgements-We wish to thank Robert Sekuler for critical discussions. and Harry Nash for numerous suggestions throughout the course of this work.
REFERENCES
Barlow H. B. and Hill R. M. (1963a) SeIective sensitivity to direction of movement in ganglion cells of the rabbit retina. Science 139. 412-414. Barlow H. B. and Hill R. M. (1963b) Evidence for a physiological explanation of the waterfall phenomenon and figural after-effects. Xarure, Land. 200, 13451347. Breitmeyer 3.. Love R. and Wepman B. (1974) Contour suppression during stroboscopic motion and metacontrast. Vision Res. 14. 1351-1456.
Motion aftereffect function Ganz L. and Lange A. (1973) Changes in motion sensitivity of cat visual cortex neurons durmg the course of dark adaptation. Presented at 1973 meeting of the Association for Research in Vision and Ophthalmology. Sarasota. Fla. Keck M. J. (197-t) Motion aftereffect time decay as a function of the contrast of sinusoidal test gratings. Presented at the 1974 meeting of the Assoc. for Research in Vision and Ophthalmology, Sarasota. Fla. Maffei L. and Fiorentini A. (1973) The visual cortex as a spatial frequency analyser. t’ision Res. 13, 1255-1267. Pantle A. and Sekuler R. (1969) Contrast response of human visual mechanisms sensitive to orientation and direction of motion. C’isiorl Rrs. 9. 397-tO6.
IYI
Sekuler R. and Pantle A. (1967) A model for after-effects of seen movement. t’ision Res. 7. X7--139. Sekuler R. and Levinson E. (197-L)Mechanisms of motion perception. Ps~cho~oyia 17. 3U9. Sutherland N. S. (1961) Fipural aftereffects and apparent size. Q. JI Ps~chol. 13, X2-225. Szilly A. von (1905, summarized by Holland H. C., 1965) The Spiral .4jf‘rer-E&r. p. 6. Pergamon Press. London. Taylor M. M. (1963) Tracking the decay of the after-effect of seen rotary movement. Percept. :Voc. SkiUs 16, 119129. Tolhurst D. J. (1973) Separate channels for the analysis of the shape and the movement of a moving visual stimulus. J. Physiol., Lond. 231. 385-402.
Rrr. Vol. 16. pp.
187-191.
Pcrgamon
Press
1976. Printed
tn Greor
Bnrain.
MOTION AFTEREFFECT AS A FUNCTION OF THE CONTRAST OF SINUSOIDAL GRATINGS MAX J. KECK and THOMAS D. PALELY, Department of Physics, John Carroll University. Cleveland. OH 44118. U.S.A. and ALL~V PANTLE Department
of Psychology, Miami University. Oxford. OH 45056. U.S.A.
(Receiced 18 September 1974; in recised_rbrm 6 rlpril 1975) Abstract-The motion aftereffect resulting from adaptation to moving vertical gratings has been measured as a function of the grating contrast from threshold up to 10.5:,. The gratings consisted of spatial sinusoidal intensity modulations which were generaTed on the face of an oscilloscope. The contrasts of the adapting grating and of the test grating could be varied independently. Both the duration and the initial apparent speed of the aftereffect wert measured. When the test grating contrast was held constant, the motion aftereffect magnitude increased rapidly with the contrast of the adapting
grating up to about 3%; for higher contrasts incremental incr?ases in contrast resulted in much smaller aftereffect increases than at the lower contrasts. When the adapting grating contrast was held constant, the aftereffect was found to be strongest for the lowest tesl contrasts and became weaker as the test contrast was increased. These psychophysical results support the hypothesis that fovea1 directionspecific motion detecting mechanisms show only a limited or compressed response to stimulus contrast.
the contrasts of the adapting and test gratings to be varied independently.
IYTRODUCFIOS
Adaptation to a moving visual stimulus results in a motion aftereffect (MAE); that is, following inspection of a moving stimulus, a stationary stimulus appears to move backward. The purpose of the present study is to make a systematic psychophysical examination of the dependence of the MAE on the contrasts of the moving (adapting) and the stationary (test) stimuli. In a previous study Pantle and Sekuler (1969) examined the contrast response of human visual mechanisms sensitive to motion using a method of selective adaptation. They measured the contrast threshold elevation for square wave gratings moving in the opposite direction and in the same direction as an adapting grating. The difference between these elevated contrast thresholds for various adapting contrasts provided a measure of the contrast response of directionally sensitive mechanisms. The motion aftereffect, which was measured in the present study, provides another way to study the response of direction sensitive mechanisms. We used one dimensional sine wave gratings as stimuli so that the spatial perodicity of the gratings would consist of only one spatial frequency. The parameters characterizing the sinusoidal gratings, with the exception of contrast, were held constant throughout the experiments, i.e. the spatial frequency, the mean luminance, the orientation, and the speed of the gratings. In this paper we attempt to answer the following two questions: (1) to what extent does the contrast of the adapting grating influence the adaptation to the moving grating, as measured by the MAE; and (2) does the contrast of the test grating influence the measurement of that adaptation? In order to answer these questions the experiments were set up so as to allow
EXPERIMENTAL METHOD
Vertically oriented gratings were generated on the face of a Tektronix 5 15.-Ioscilloscope with a green (P-3 1) phosphor. Tte gratings consisted of a sinusoidal luminance modulation along the horizontal axis. The mean luminance was held constant throughout the experiments at 0.6 R-L. The conuast. i.e. the ratio of the modulation amplitude to the mean luminance, could be varied between 0 and 10-5x. This intensity modulation was achieved by voltage modulating the :-a.ws of the oscilloscope with a sinusoidal voltage. The contrast was directly proportional to this a.c. voltage; 1.2OV corresponded to a contrast of 1@5%. Subjects viewed the grating display binocularly from a distance of 2 m. At this distance, the rectangular gratings subtended a visual angle of 2.7’ horizontally and 1.4’ vertically. The surround of the display was black. The spatial frequency of both the adapting and test gratings was 4c/ deg. The adapting gating moved across the oscilloscope face in a horizontal direction at a speed of I.ljdeg&c. At this JFed the temporal frequency at any position on the oscilloscope face. due to the traversal of the grating, was 5 Hz. The test _erating was stationary. Subjects were directed to fixate a small black dot in the center of the oscilloscope face. At the beginning of each session subjects were dark adapted for IOmin. Each session consisted of approx 35 trials. On each trial the moving adapting Dating was presented for 30~. followed immediately by the stationary test grating. The intertrial interval was 9Osec. During this interval the subject looked away from the oscilloscope display. The experimental room was dark except for a small amount of scattered light produced by the apparatus. The two dependent variables which were measured in these experiments are the initial speed of the MAE and its duration. In some trials the initial speed was measured
187
and in other trials the duration >\a3 measured. The mIttal speed and the dtiration were always measured in scpararr sessions. fn those sessions where rhz duration of the aftereffect was measured. the subject started a timer when the test pattern appeared and stopped it when he judged the MAE to have stopped. The time was recorded to the nearest O-5sec. In those sessions where the initial speed of the hIAE was measured. the method of magnitude estimation was used. Subjects were shown a standard trial in which the adapting and test patterns were at a contrast of 1039,. The speed of the MAE produced by the standard. Zsec after the onset of the test pattern. was defined as 100. The standard was presented twice at the beg,inning of each session. On subsequent test trials, the subject was instructed to scale the apparent speed of the test pattern relative to this standard. In ail cases, the subject gave his estimate within Zsec after the onset of the test pattern. This is necessary as the apparent speed of the M.G decreases exponentially from its initial value (Taylor, 1963; Sekuler and Pantle. 1967). ESPERIMEXT I: MAE DEPENDEM-E OS THE COSTRAST OF THE hDAF’TfSG GRATING
In the first experiment the dependence of the MAE on the contrast of the adapting grating was measured for two values of the test grating contrast: 1.7 and lO$;. Each of three subjects viewed adapting gratings with 11 different contrasts and judged the durations of the MAE. Each subject completed three replications of the experiment. For each replication the conditions were presented in random order. For each condition. the mean duration and the standard error of the mean were found for a given subject. The pattern of rest&s was the same for al1 subjects. Excluding the standard errors for those conditions in which the subjects saw no aftereffects,’ the standard errors ranged from 0.2 to 5.7 set and had a mean of l-5. For each condition, the overall mean of the aftereffect durations of the three subjects was computed and is plotted in Fig. 1. The upper curve shows the results when the test grating contrast was 1.7%; the Iower curve shows the results when the test grating contrast was lO*S%.The main features of both curves are: f 1) for low contrasts the MAE duration increases rapidly with adapting contrast; and (2) for contrasts greater than about 3% there is a marked decrease in the rate of change of MAE duration with adapting contrast. There is no evidence of complete saturation for adapting contrasts up to lO.Sll/,. For low adapting contrasts the MAE duration measured with the higher test contrast (lOSo/,) was only a small fraction of that measured with the lower contrast (1.70/;;)test grating. Also note I None of the subjects saw aftereffects at the lowest adapting contrast (@7P/,fwhen the test contrast was l&5%. Two of the three subjects saw no aftereffects with adapting contrasts of 0% 0.9 or I% and a test contrast of 10-S%. The individual means and standard errors in the remaining conditions were not correlated. Ninety-seven per cent of the standard errors (a11 but two) were between @2 and 3-3 set and were approximateiy normaily distributed. 2 The distribution of standard errors was positively skewed. The mean estimates and the standard errors were not correfated.
a-
Fig. I. Duration of the MAE as a function of the contrast of the adaptins grating for two different test grating contrasts: 0 I.;“,, test grating contrast, $2 IO+?; test srating contrast.
that for ail values of the adapting contrast the MAE duration was shorter for the higher contrast test condition. The above conditions were repeated and the same three subjects were asked to judge the initial speed of the MAE. Each subject completed three replications. For each condition the mean initial speed estimate and the standard error of the mean were found for a given subject. The pattern of results was the same for all subjects. Again, excluding the conditions in which the subjects saw no aftereffects, the range of standard errors was 00-18~9 with a mean of 62.” For each condition, the overall mean of the speed estimates of the three subjects was obtained and is given in Fig. 2. The curves of Fig. 2 show that the higher contrast test grating resulted in a lower initial MAE speed for all adapting Contra& For both test contrasts, (1) initial MAE speed increased as a function of adapting contrast and (2) the rate of change of MAE speed with adapting contrast, was less at higher than at lower adpating contrasts. Unlike the duration curves of Fig. 1, both MAE speed curves in Fig. 2 are upprosimare~~ logarithmic functions for the range of adapting contrasts displayed. It is apparent from Figs. 1 and 2 that the initial speed and the duration of the MAE are not in direct proportion to each other over the range of adapting contrasts investigated. The reason for tl& is not entirely clear, but may be related to an adaptation of motion analyzers by the test grating during the course. of the MAE’s time decay Keck. 1974). The duration measurements required continuous viewing of the test grating, whereas the initial MAE speed estimates were completed in a couple of seconds. Despite the observed discrmancy bet~eer~ the two dependent variables plotted in Figs. 1 and 2. the data warrant the general conclusion that incrementai increases of the adapting contrast above 3% resulted in much smaller MAE increases than at lower adapting contrasts.
ESPERlitfENT If: M.IE DEPFSDE?XE 0% THE CONTRAST OF THE TEST GRATTISG
In the second experiment
the contrast of the test grating was varied, and the contrast of the adapting
Motion aftereffect function
Fig. 2. Relative initial speed of the MAE as a function of the contrast of the adapting grating for two different test grating contrasts: 0 1.7% test grating contra% 0 103% test grating contrast. grating was held constant at 5.27;. Each of four subjects viewed the adapting grating followed by a test grating with one of eight different contrasts between O-9 and 10.5% and judged the durations of the MAE. Each subject completed three replications of the experiment. For each replication the conditions were presented in random order. For each condition. the mean duration and the standard error of the mean were found for a given subject. The standard errors were positively skewed, ranged from’ 0.3 to 68 set, and had a mean of 2.6. For each condition, the overall mean of the aftereffect durations of the four subjects was computed and is plotted in Fig. 3. The curve shows that as the contrast of the test grating was increased. the MAE duration became shorter. The maximum value of the MAE occurred for the lowest test contrasts. At the highest test contrast used, 105%, the MAE was only about one half of its maximum value. The above conditions were repeated, and the same four subjects were asked to judge the initial speed of the MAE. Again each subject completed three replications. For each condition the mean initial speed estimate and the standard error of the mean were found for a given subject. The range of the standard errors was 00-18~9 and their mean was 8.4. For each condition, the overall mean of the initial speed estimates of the four subjects was obtained and is given in Fig. 4. Note that this curve is similar to the curve of Fig. 3. Low test contrasts result in fast MAEs, and high test contrasts result in slow MAEs.
189
It is interesting to note that as a consequence of the above results. one can actually get stronger aftereffects with lower contrast gratings. This agrees with qualitative observations made by Szilly (1905. as summarized by Holland, 1965, on p. 6): ‘weak contours produce the best after-effects and strong contours weaken them.” In Szilly’s experiments, as is the usual case for MAE experiments, the adapting and test targets were identical. Our experiments show that, other things being equal, an increase of adapting contrast results in a greater MAE. However, an equivalent increase of test contrast may more than offset the greater effectiveness of the higher adapting contrast. The net effect, when high contrast targets are used for both adapting and testing, may be a reduced MAE.
DISCUSSION Relationship
of the present results to a model of MAE
Barlow and Hill (1963b) recorded the responses of a direction-sensitive cell in the retina of the rabbit. When an irregular stimulus pattern was moved in the preferred direction through the receptive field of the cell, its response rate increased immediately above its spontaneous level of discharge. As the stimulus motion continued, the response of the cell declined, but even after 57 set the response was still well above the spontaneous level present before stimulation. When the moving stimulus was stopped, the response rate dropped abruptly to zero and only slowly recovered to the normal spontaneous level. When the stimulus was moved in the null (opposite) direction through the receptive field of the same cell, no changes in the activity of the cell were observed. The results of Barlow and Hill give strong support to the hypothesis (Sutherland, 1961; Barlow and Hill, 1963b; Sekuler and Pantle, 1967) that the MAE results from a temporary imbalance in the discharge of cells which have different preferred directions of movement. Presumably, the adapting stimulus which is used to generate an MAE desensitizes cells whose preferred direction of movement coincides with that of the adapting stimulus and leaves cells sensitive to the opposite direction of movement unaffected. If the extent of the desensitization of the cells which respond to the adapting stimulus depends upon how much they are excited by it, then the MAE ought
2,-
,
,-
Fig. 3. Duration of the MAE as a function of the contrast of the test grating. The adapting grating contrast was 52%.
Fig. 4. Relative initial speed of the MAE as a function of the contrast of the test grating. The adapting grating contrast was 5.2%.
190
hf. J. f;ECL T. D. I'ALELLA
to be an indirect measure of the degree of excitation of direction-sensitive cells by the adapting stimulus. The results in Figs. 1 and 2 show that the MAE increases rapidly with adapting contrast for contrasts near threshold. However, for adapting contrasts greater than about 3?b. incremental increases in contrast result in much smaller MAE increases than at the lower contrasts. This in turn implies that for our experimental conditions the excitation of directionsensitive analyzers increase only slightly with changes of adapting contrast above 3% (about six times threshold). The dependence of the MAE on the contrast of the tesr grating can also be explained in terms of the imbalance between the discharges of cells tuned to motion in opposite directions if it is assumed that the dir~tion-sensitive celIs respond to a stationary grating. Ordinarily the stationary pattern would be expected to produce equal responses in cells with different preferred directions of movement, with the result that the test grating would appear stationary. However, if one class of direction-sensitive cells had been previously adapted (desensitized), the response of each class to the stationary test pattern would not be equal; the adapted (desensitized) cells would respond less. The imbalance of activity would result in apparent backward motion. i.e. an MAE. If this analysis of the MAE is correct. the data in Figs. 3 and 4 suggest that the imbalance in the responses of adapted and unadapted analyzers decreased as the test grating contrast was increased. The result is surprising since it seems more logical that a high contrast test grating would magnify the response imbalance. One possible reason for the weaker MAE at high test contrasts is the limited or compressed contrast response of direction-sensitive cells suggested by the results of experiment I. An increase of test contrast above 3”/, might lead to small increases in the response of unadapted cells, but to large increases in the response of adapted cells. Consequently, the net effect of an increase of test contrast above 3% would be a reduction of the imbalance of the outputs of adapted and unadapted cells, and hence a smaller MAE. Relationship of the present results to other physioiogical and psychophysical datn
In a physiological study, Barlow and Hill (1963a) investigated the contrast response of single cells seiectively sensitive to direction of motion. Their stimulus was a 1’ spot of variable luminance superimposed on a uniform background with a luminance of 0.2 mL. The minimum ratio of spot luminance to background luminance necessary to obtain a response was reported at 1.2/l. For spot to background Iuminance ratios greater than 1.5/l (about two times greater than the threshold contrast) the response was found to be ~epe~e~r of contrast. It should be noted that the spatial frequency content of the non-periodic spot stimulus is significantly different from our single frequency gratings. Maffei and Fiorentini (1973) obtained a similar result when recording the response of complex cells of the striate cortex of the cat. They employed moving sinusoidal gratings as stimuli and measured the response of the cortical cells as a function of the contrast of the gratings. They found that
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the rate of discharge saturated at about 5’,, I.! tlrnps threshold) for cells u-ith a receptive field of 2 5’ (thw Fig. 9). However. the same figure also shokvs responses recorded from other complex cells which did not exhibit the saturation. Ganz and Lange (1973) have reported that the responses of direction-sensitive cells in the cat cortex saturate at a low contrast. The contrast response of motion detectors has been investigated ps~choph~sically by Pantle and Sekuler (1969). Subjects adapted to moving square-wave gmtings (038 cjdeg, temporal frequency = 2.3 Hz) of various contrasts. Following adaptation to a grating with a particular contrast. the contrast threshold for a moving test grating was found to bz elevated. The threshold elevation was greater for test gratings moving in the same direction as the adapting grating than for test gratings moving in the opposite direction. The difference between these two threshofd elevations was reasoned to be a measure of the desensitization of direction-sensitive mechanisms. Their results show that the response of direction-sensitive mechanisms is a power function of the contrast of the stimulus up to 16*/;,(i-6 times threshold), and the response saturates for higher contrasts. In comparing their data with our results. it must be kept in mind that the contrast threshold for the square-wave gratings used by Pantie and Sekuler was much higher (2.4”~ than that of the gratings used in the present study (O.jg,,j. Moreover the gratings employed by Pantle and Sekuler contained harmonic frequencies at louver contrasts in addition to the fundamen~l frequency of 038 ddeg. Given the differences in stimuli. it is interesting that the response saturation observed by Pantle and Sekuler occurred at the same subjective contrast (Z-6 times threshold) as the fairly pronounced decrease in the rate of change of MAE strength with adapting contrast (3”;. 6 times threshold) in our experiment I. While the results of the present studies can be expfained in terms of a limit or compression of the contrast response of motion analyzers, they might also reflect the operation of inhibitory processes. For example the weaker MAEs obtained at high test contrast (experiment II) may be due to the inhibition of transient, motion-det~ting systems by a pattern system at high contrast (Tolhurst, 1973; Breitmeyer, Love and Wepman. 1974) or to inhibition between direction-sensitive mechanisms (Sekuler and Levinson, 1974). Acknowledgements-We wish to thank Robert Sekuler for critical discussions. and Harry Nash for numerous suggestions throughout the course of this work.
REFERENCES
Barlow H. B. and Hill R. M. (1963a) SeIective sensitivity to direction of movement in ganglion cells of the rabbit retina. Science 139. 412-414. Barlow H. B. and Hill R. M. (1963b) Evidence for a physiological explanation of the waterfall phenomenon and figural after-effects. Xarure, Land. 200, 13451347. Breitmeyer 3.. Love R. and Wepman B. (1974) Contour suppression during stroboscopic motion and metacontrast. Vision Res. 14. 1351-1456.
Motion aftereffect function Ganz L. and Lange A. (1973) Changes in motion sensitivity of cat visual cortex neurons durmg the course of dark adaptation. Presented at 1973 meeting of the Association for Research in Vision and Ophthalmology. Sarasota. Fla. Keck M. J. (197-t) Motion aftereffect time decay as a function of the contrast of sinusoidal test gratings. Presented at the 1974 meeting of the Assoc. for Research in Vision and Ophthalmology, Sarasota. Fla. Maffei L. and Fiorentini A. (1973) The visual cortex as a spatial frequency analyser. t’ision Res. 13, 1255-1267. Pantle A. and Sekuler R. (1969) Contrast response of human visual mechanisms sensitive to orientation and direction of motion. C’isiorl Rrs. 9. 397-tO6.
IYI
Sekuler R. and Pantle A. (1967) A model for after-effects of seen movement. t’ision Res. 7. X7--139. Sekuler R. and Levinson E. (197-L)Mechanisms of motion perception. Ps~cho~oyia 17. 3U9. Sutherland N. S. (1961) Fipural aftereffects and apparent size. Q. JI Ps~chol. 13, X2-225. Szilly A. von (1905, summarized by Holland H. C., 1965) The Spiral .4jf‘rer-E&r. p. 6. Pergamon Press. London. Taylor M. M. (1963) Tracking the decay of the after-effect of seen rotary movement. Percept. :Voc. SkiUs 16, 119129. Tolhurst D. J. (1973) Separate channels for the analysis of the shape and the movement of a moving visual stimulus. J. Physiol., Lond. 231. 385-402.
