Motion Perception in High-Pass Filtered Random-Dot Patterns: Motion Energy,Element-Matching,or Both?

Abstract

We have measured perception of the direction of displacement of two-frame random-dot patterns (50% light/dark pixels) that have been spatially high-pass filtered. In the ‘standard’ condition, pairs of high-pass filtered images, identical apart from the displacement, were presented in succession. The displacement could be in either of two opposite directions and the task was to identify the direction. The ‘reverse’ condition was the same except that image contrast was inverted between the two frames. Various element sizes and filter cut-offs were used. Two distinct patterns of results were obtained. For small check sizes, performance alternated cyclically between veridical direction perception and incorrect direction perception (aliasing) as displacement size was increased over a wide range. The period of the cycle was close to one period of the lowest spatial frequency remaining in the image after filtering, ie performance was as would be expected for a grating of that spatial frequency. In the ‘reverse’ condition the cyclical psychometric functions were inverted, ie reversed-phi motion occurred. For large check sizes, and particularly for high filter cut-offs, there was no cyclical alternation of direction perception and reversed phi did not occur in the ‘reverse’ condition. The results suggest that two mechanisms are at play. In most circumstances, detection appears to be based on motion energy since the cyclical alternation is predicted by a consideration of the spatiotemporal energy of the stimulus but not by element-matching theories. But for large elements, particularly when most of the low spatial frequencies have been removed, element-matching takes over. Elements are matched without regard to their contrast polarity. The results are thus inconsistent with the single front-end filter mechanism which Morgan [1992 Nature (London) 355 344] has advanced to explain performance in this type of task.