On the half-cycle displacement limit of sampled directional motion.

We employed filtered random-dot kinematograms to determine the maximum displacement (dmax) at which sampled directional motion was reliably detected. The images were produced using ideal band-pass filters that varied in lower cut-off frequency (f1), in bandwidth, and in range (alpha) of component orientations that were passed. Results showed that dmax, expressed in cycles of f1, increased with f1 and alpha, and decreased with bandwidth. In many conditions, dmax exceeded half a cycle of f1, a result that appears to contradict predictions from quadrature models of motion detection. However, an account that does not violate the half-cycle limit can be given on two assumptions. First, motion perception is mediated by a population of orientation and frequency-selective sensors that respond correctly to displacements up to half a cycle in the preferred direction. Second, the outputs from all sensors (notably including off-axis sensors) are linearly summated to yield perception of motion. A computer simulation based on these assumptions provided a remarkably close fit to the psychophysical data.