The separability of temporal frequency and velocity.

McKee, Silverman and Nakayama (1986; Vision Research, 26, 609-619) have shown that velocity discrimination performance is little affected by quite large random changes in the spatial frequency, and hence temporal frequency, of the grating patterns to be discriminated. We show that the converse is also true: temporal frequency discrimination can be performed despite random changes in velocity. This casts doubt on McKee et al.'s conclusion that velocity is the fundamental dimension of the temporal mechanisms mediating discrimination and that temporal frequency has to be inferred indirectly from velocity and spatial frequency. We suggest instead that both velocity and temporal frequency are represented in the visual system but that the two dimensions are only partially separable at the perceptual level.