Visual thresholds for shearing motion in monkey and man.

A reaction-time task was used to determine the visual motion thresholds in humans and in macaque monkeys for sinusoidally modulated shearing motion of a random dot display. It was found that humans and macaques were very similar in their spatial frequency sensitivity profiles for shearing motion. These profiles were of a U-shape for all human and monkey subjects tested. Temporal frequency, varied over a wide range, did not influence the shape of the spatial frequency sensitivity curve, but only the threshold amplitudes. The above results held both for single and multiple temporal cycles of shearing motion. Previous reports for the human, using these same shearing motion stimuli, indicated no increase in threshold at the lower spatial frequencies. The reason for this discrepancy is that thresholds in the previous studies were not determined at a low enough spatial frequency to see clearly this increase in thresholds. Because of the striking similarity of the data for man and macaque, it is suggested that similar neural mechanisms underly the shearing motion sensitivity of the two species.