DEPENDENCE ON STIMULUS ONSET ASYNCHRONY IN APPARENT MOTION - EVIDENCEFOR 2 MECHANISMS

The detection of the direction of motion was measured as a function of the spatial and temporal offset for a kinematogram stimulus presented in two-frame apparent motion. The stimulus was made up of Gabor funct ion micro-patterns randomly distributed across the stimulus field. We show that for short stimulus onset asynchronies (SOA) performance can be predicted from the spatio-temporal Fourier power spectrum of the st imulus, whereas for long SOAs the pattern of performance is qualitivel y different from such a prediction. The dependence of motion perceptio n on SOA exhibits an abrupt change from one mode of behaviour to the o ther. These findings are suggestive of the operation of distinct mecha nisms, one ''quasi-linear'' and one ''nonlinear'', which can be separa ted by temporal parameters.