How is complex second-order motion processed?

Abstract

Converging psychophysical and electrophysiological evidence suggests that first-order (luminance-defined) complex motion types i.e., radial and rotational motion, are processed by specialized extrastriate motion mechanisms. We ask whether radial and rotational second-order (texture-defined) motion patterns are processed in a similar manner. The motion sensitivity to translating, radiating and rotating motion patterns of both first-order (luminance-modulated noise) and second-order (contrast-modulated noise) were measured for patterns presented at four different exposure durations (106, 240, 500 and 750 ms). No significant difference in motion sensitivity was found across motion type for the first-order motion class across exposure duration (i.e., from 240 to 750 ms) whereas direction-identification thresholds for radiating and rotating second-order motion were significantly greater than that of the second-order translational stimuli. Furthermore, thresholds to all second-order motion stimuli increased at a significantly faster rate with decreasing exposure duration compared to those of first-order motion. Interestingly, simple and complex second-order thresholds increased at similar rates. Taken together, the results suggest that complex second-order motion is not analyzed in a sequential manner. Rather, it seems that the same ‘hard-wired’ mechanisms responsible for complex first-order motion processing also mediate complex second-order motion, but not before the pre-processing (i.e., rectification) of local second-order motion signals.