Effects of Contrast and Period on Perceived Coherence of Moving Square-Wave Plaids (evidence for a speed bias in the human visual system)

H. Farid, E.P. Simoncelli, M.J. Bravo and P.R. Schrater

Investigative Opthalmology and Visual Science (ARVO), Fort Lauderdale, FL, 1995

Purpose: The coherence of moving square-wave plaids depends on a number of stimulus parameters: plaid angle (theta), grating speed (Sg), contrast, and period. Last year at ARVO, we explored the dependence on the plaid angle and the grating speed. We found that coherence depended on both of these parameters: this dependence is best understood via a reparameterization in terms of pattern speed (Sp = Sg / cos(theta)). When Sp is below a critical speed (roughly 5 deg/sec), the plaid is more likely to be seen as coherent. Above this critical speed, the plaid has the appearance of two gratings sliding transparently over each other. This year, we examined the effect of contrast and component period on the coherence of square-wave plaids.

Methods: Subjects were presented with symmetric square-wave plaids of varying period and were asked whether the stimuli appeared transparent or coherent. In a second experiment, subjects judged the coherence of symmetric square-wave plaids of varying contrast.

Results: The experiments reveal that both contrast and period affect the perceived coherence of the stimuli: gratings of higher contrast and gratings of smaller period appear more coherent. For fixed period and contrast, the effect of varying plaid angle and grating speed is consistent with our previous experiments: coherence is determined by the pattern speed relative to a critical speed. However, the current experiments reveal that this the critical speed depends on the stimulus contrast and period.

Conclusions: These results suggest that the primary determinant of square-wave plaid coherence is the pattern speed. This behavior may be explained by a model for velocity perception with a built-in preference for slower speeds.