Abstract
Adaptation to a moving stimulus changes the perception of a stationary grating and also reduces contrast sensitivity to the adaptor. We determined whether the first effect could be predicted from the second. The contrast discrimination ( T vs C) function for a drifting 7.5 Hz grating test stimulus was determined when observers were adapted to a low contrast (0.075) grating of the same spatial and temporal frequency, moving in either the same or the opposite direction as the test. The effect of an adaptor moving in the same direction was to move the T vs C function upwards and to the right, in a manner consistent with an increase in divisive inhibition. We also measured the effect of adaptation on the motion-null point for a counterphasing grating containing two components, one moving in the same direction as the adaptor and the other in the opposite direction. Adaptation increased the amount of contrast of the adapted component required to achieve the motion-null point. However, this shift could not be predicted from the effects of adaptation on contrast sensitivity. In particular, the balance point was shifted in gratings of high contrast where there was no effect of adaptation on contrast discrimination. We suggest that adaptation has a subtractive (recalibration) effect in addition to its effects on the contrast transduction function, and that this subtractive effect may explain the movement after-effect seen with stationary tests.
Highlights
After adaptation to an upwards-moving grating, a stationary grating will appear to move downwards
We wanted to use a relatively low contrast adaptor in order to avoid saturating the effect on contrast detection (Blake, Tadin, Sobel, Raissian, & Chong, 2006), so we first measured the function relating adaptor contrast to test detection threshold, separately for tests moving in the same and opposite directions as the adaptor, and for a stationary grating of the same spatial frequency
The weak adaptor allowed us to use counterphasing gratings of higher contrast, in the masking region of the T vs C function where there was no effect of adaptation on sensitivity
Summary
After adaptation to an upwards-moving grating, a stationary grating will appear to move downwards. A flickering grating will appear to move downwards, even though it is composed of physically equal upwards- and downwards-moving components. The seminal study supporting the contrast adaptation theory was carried out by Sekuler and Ganz (1963) who found a reduction in contrast sensitivity for gratings moving in the adapted direction, but not in the opposite direction. Their psychophysical experiment echoed the finding from physiology that directionally-tuned detectors in rabbit retina lose sensitivity when subjected to prolonged stimulation (Barlow & Hill, 1963). Neuroimaging studies have supported the two stage model by showing that the BOLD response in V5/MT to a moving stimulus is reduced by an oppositely-moving stimulus; while there is little evidence for this opponency effect in V1 (Heeger, Boynton, Demb, Seidemann, & Newsome, 1999)
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