Motion detection is limited by element density not spatial frequency

Two-frame random-element kinematograms were used to study the matching algorithm employed by the visual system to keep track of moving elements. Previous data have shown that the maximum spatial displacement detectable (dmax) for random-dot kinematogram stimuli increases both with increasing dot size and with decreasing centre frequency for spatially band-pass kinematograms. Both of these findings could be explained by either (i) a matching algorithm sensitive to the number of false targets in the display (informational limit) or (ii) spatial-frequency tuned sensors hardwired for detecting displacements of a constant proportion of their preferred frequency (phase-based limit). The present experiment was designed to differentiate between these alternative explanations. The stimuli were band-pass filtered (difference-of-Gaussian) random-dot patterns. The combination of six dot densities and three filter sizes produced 18 experimental conditions and allowed independent control of the spectral content and filtered-element density of the stimuli. When the dot density was high, dmax was larger for the coarse-filtered stimuli, as predicted by both theories. There was also a critical dot density for each filter size, above which dmax was constant but below which dmax rose sharply. This critical density was higher for fine-filtered stimuli such that at the lowest dot density of 0.025%, dmax was constant for all filter sizes. In support of the informational limit model, dmax was found to be directly proportional to the two-dimensional spacing of filtered elements. In contrast, dmax varied from 0.6 to 8.5 cycles of the stimulus peak frequency, suggesting that a phase-based model of motion detection cannot account for the results.