Adaptation to second-order motion results in a motion aftereffect for directionally-ambiguous test stimuli

The magnitude of the motion aftereffect (MAE) obtained following adaptation to first- or second-order motion was measured in two experiments using a nulling method. The second-order motion adaptation stimulus was composed of contrast-modulated noise produced by multiplying two-dimensional random noise by a drifting, 1 c/deg, vertical sine grating. The first-order motion adaptation stimulus was composed of luminance-modulated noise produced by adding, rather than multiplying, the sine grating and noise field. The test stimuli were directionally-ambiguous first- or second-order motion patterns composed of either two oppositely drifting sine gratings added to static noise or its contrast-modulated equivalent. The amplitudes of the two drifting components were manipulated such that as one increased in amplitude the other decreased in amplitude by the same degree. This technique was employed to estimate the null point at which the test no longer appeared to drift in the direction opposite the adaptation direction. In the first experiment all stimuli were equated for visibility by presenting them at the same multiple of threshold and all possible combinations of first- and second-order motion adaptation and test stimuli were examined. The results were similar for all conditions: following adaptation the amplitude of the test component drifting in the same direction as adaptation needed to be approximately twice that of the oppositely drifting component in order to null the perception of unidirectional motion of the test. In a second experiment, the effects of manipulating the amplitude (visibility) of the first- and second-order motion adaptation stimuli on MAE magnitude were investigated. This revealed an approximately linear relationship between MAE magnitude and the amplitudes of the adaptation stimuli. The results demonstrate that, contrary to the findings of several previous studies, adaptation to second-order motion does produce a substantial movement aftereffect. Cross-adaptation between first- and second-order motion stimuli also occurs under appropriate conditions and produces aftereffects that are comparable in magnitude when the stimuli are equated for visibility.