Perception, 1977, volume 6, pages 719-725

Recovery from adaptation to moving gratings

Max J Keck, Benjamin Pentz Department of Physics, John Carroll University, Cleveland, Ohio 44118, USA Received 15 October 1976

Abstract. Short-term adaptation to moving sinusoidal gratings results in a motion aftereffect which decays in time. The time decay of the motion aftereffect has been measured psycho physically, and it is found to depend on (i) the spontaneous recovery from the adapted state, and (ii) the contrast of the test grating. We have measured the decays for various test conditions. An extrapolation of the measurements allows us to obtain a decay which represents the time course of the spontaneous recovery of the direction-sensitive mechanisms. 1 Introduction

Adaptation to a moving visual stimulus results in a motion aftereffect (MAE); that is, after a subject views a moving stimulus, a stationary stimulus appears to move backward. This paper reports measurements of the time decay of the MAE speed resulting from short-term adaptation to moving 4 cycles deg -1 sinusoidal gratings. Recently Keck et al (1976) found that the observed MAE speed depends on the contrast of the stationary test grating. The present study examines in detail the effect of the test-grating contrast on the time course of the MAE decay. A study of the time decay of MAE speed was reported by Taylor (1963), who measured the speed of the observed apparent backward rotation of a stationary test disc immediately after the inspection of a rotating disc. The same two discs, which were marked with an irregular pattern, were used throughout. The MAE speed was found to decay exponentially as long as the inspection time was less than 80 s. (For longer inspection times a second long-lasting exponential decay was observed by Taylor to be superimposed on the shorter decay. This second decay with its much longer time constant and consequent long-lasting aftereffects, is not of concern here as our inspection times were kept short.) The test conditions were not varied in these experiments. Ross and Taylor (1964) measured the motion-aftereffect decay with a test pattern that was illuminated with either a bright or a dim light. The decay was found to last longer with the dimmer test pattern. The contrast of the test pattern was not varied, only its luminance was varied. Spigel (1962) studied the duration to the MAE for various test conditions, consisting of interpolated periods of stimulation between the offset of the adapting motion and the onset of the test period. The interpolated conditions were total darkness, unpatterned illumination, unpatterned illumination of reduced intensity, and a combination of patterned illumination followed by darkness. For each subject the interpolated conditions were presented for a time that was equal to that subject's normal MAE duration. After the interpolated interval, a residual MAE duration was obtained. The residual MAE durations were found to be unexpectedly long for all interpolated conditions, and Spigel suggested that this means that the interpolated stimulation had brought about some inhibition of the decay process. The present experiments measure the dependence of the MAE time decay on the duration of an interpolated period, and on the contrast of the test grating used to measure the time decay.

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2 Experimental Vertically oriented gratings were generated on the face of a Tektronix 515A oscilloscope with a green (P-31) phosphor. The gratings consisted of a sinusoidal luminance modulation along the horizontal axis. The mean luminance was constant throughout the experiments at 0-6 millilamberts. The contrast, i.e. the ratio of the modulation amplitude to the mean luminance, could be varied between 0% and 10-5%. This intensity modulation was achieved by voltage modulating the z axis of the oscilloscope with a sinusoidal voltage. The contrast was directly proportional to the AC voltage; 1 -20 V corresponded to a contrast of 10-5%. 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 deg horizontally and 1 -4 deg vertically. The immediate surround of the display was black. The spatial frequency of both the adapting and the test gratings was 4 cycles deg -1 . The adapting grating moved across the oscilloscope face in a horizontal direction with a speed of 1 -25 deg s"1. At this speed the temporal frequency at any position on the oscilloscope face, due to the traversal of the grating, was 5 Hz. The test grating 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 10 min. For all trials in these experiments the adapting conditions were identical, i.e., the contrast of the moving grating was 10-5%, the frequency was 5 Hz, and the inspection time was 30 s. At the end of the 30 s adapting period the motion was stopped and simultaneously the contrast was changed to a desired value to form the test grating. Subjects viewed the test grating continuously and were asked to give MAE speed estimates at preselected times by using the method of magnitude estimation. A standard trial was presented and the resulting initial MAE speed was designated as 10. The standard was presented twice at the beginning of each session along with two other trials for familiarization. When subjects judged the MAE to have stopped, they turned off a timer. Upon completion of the MAE there was a 90 s rest interval, during which the subjects looked away from the oscilloscope face. The intertrial interval was chosen so that the MAE from the previous trial would be completely decayed, in order to avoid buildup from trial to trial. The adapting motion was always in the same direction. The experimental room was dark except for a small amount of scattered light produced by the apparatus. 3 Experiment 1. MAE time decay as a function of test contrast In the first experiment the time decay of the MAE speed was measured for two values of the test grating contrast, a low value of 1 -7% which is about three times greater than threshold, and a higher value of 10-5%. One of the following four test conditions was presented immediately after adaptation to the moving grating: (1) a test contrast of 1 -7%, (2) a test contrast of 10-5%, (3) a test contrast of 10-5% which was switched to 1 -7% 8 s after the cessation of the adapting motion, and (4) a test contrast of 1-7% which was switched to 10-5% 8 s after the cessation of the adapting motion. The above four test conditions were presented in a random sequence. Each subject observed three sequences. Test condition 1 was chosen for the standard trial. The three subjects observed the test gratings continuously and were asked to estimate the apparent MAE speeds initially, after 5 s, after 10 s, after 15 s, and immediately after the contrast was changed at 8 s for conditions 3 and 4. Subjects also turned off a timer when they judged the MAE to have stopped. The mean values of the responses from the three subjects are graphed in figure 1. The open circles show the MAE decay for the 1 -7% test contrast (condition 1).

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The filled circles show the decay when the test grating contrast is 10-5% (condition 2, the test grating has the same contrast as the adapting grating). Note that the MAE ends earlier for the higher test contrast. This result is in agreement with the findings of Keck et al (1976), namely, that both the initial speed and the duration of the MAE are reduced as the contrast of the test grating is increased. For test condition 3 the decay for the first 8 s is that of the lower 10-5% curve, and when the contrast is reduced at 8 s the MAE speed is observed to increase suddenly, as shown by the triangles and the dashed curve in figure 1. Note that the remaining decay is prolonged considerably after the contrast reduction at 8 s. For test condition 4 the decay for the first 8 s is that of the upper 1 -7% curve, and when the contrast is increased to 10-5% at 8 s the MAE speed decreases more rapidly, as shown by the squares and the dashed curve; the decay being completed at the end of 18 s rather than at 25 s.

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15 Time (s)

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Figure 1. Relative MAE speed as a function of time after cessation of adapting motion. The mean values of the responses from three subjects are plotted for the following test conditions:

0 condition (1), 1 -7% contrast; • condition (2), 10-5% contrast; •* condition (3), 10-5% for 8 s then changed to 1 -7% contrast; on condition (4), 1 -7% for 8 s then changed to 10-5% contrast. The bars indicate the mean value of the standard errors; they range 0-42 to 0-53. 4 Experiment 2. MAE time decay for extrapolated zero test contrast From the results of experiment 1 we can conclude that a low-contrast test grating results in a longer-lasting MAE. Would an even longer-lasting MAE decay be obtained if the test grating contrast were reduced below the 1 • 7% value used in experiment 1 ? Unfortunately the question cannot be answered experimentally by further reducing the test grating contrast, since it becomes difficult to see the test grating near its contrast threshold, and therefore reliable MAE speed estimates cannot be obtained. To get around this difficulty we delayed the presentation of the test grating by presenting a uniform field (0 -6 millilambert) for an interval immediately following the end of the adapting period. The uniform-field interval was varied in length from trial to trial according to a random schedule. In this way the effects of zero test contrast on the MAE could be measured. In this experiment the time decay of the MAE speed was obtained for test gratings which were presented after uniform-field intervals of 2, 5, 10, 15, 20, 25, 30, 40, or 50 s. Subjects were asked to estimate the speed when the test grating was first presented (initial speed), and again 5 s later. They were also asked to turn off a timer when they judged the MAE to have stopped. In all cases the test grating contrast was 1 • 7%. The condition where the test grating was presented after 2 s uniform field interval, was chosen for the standard trial.

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Figure 2 shows a sample decay curve for MAE speed when subject CG observed the MAE with a test grating whose presentation was delayed by a 1 0 s uniform-field interval. We are primarily interested in the initial point at 11 s, for it represents a measurement of MAE speed when the test stimulus is at zero contrast for 10 s. 10

Recovery from adaptation to moving gratings.

Perception, 1977, volume 6, pages 719-725 Recovery from adaptation to moving gratings Max J Keck, Benjamin Pentz Department of Physics, John Carroll...
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