Do component motions recombine into a moving plaid percept?

Abstract

Two superimposed gratings, which differ in orientation and move independently, combine into a coherently moving plaid if the component gratings are similar. The effect on the plaid motion percept of the addition of texture to each grating was investigated. The texture disambiguates the motion of each component grating. Under the assumption of recombination of first-stage motion detectors into a second stage, which becomes available to perception, one would expect the perceived motion direction of the plaid to change as a result of texture addition. Subjects perceived the oblique motion direction of textured bars of a single orientation correctly. This occurred for texture details with dimensions down to the resolution limit in the fovea (1 min of arc). Two bar patterns with fine texture (1 min of arc details) which differ in orientation were perceived to cohere into a plaid. The plaid's motion direction, however, was independent of the parallel motion components of the bars. For coarser textures (2 and 4 min of arc details) the bar patterns were perceived to slide past one another. In addition, we found that the plaid motion percept occurred less frequently for longer motion sequences, wider bars and for a combination of the component textures at the intersections which is compatible with partial transparency of the bars. These results do not support the two-stage model of Adelson and Movshon (1982), where only the motion component perpendicular to each grating orientation is encoded and where the perception of the plaid motion results from "recombination" of these perpendicular motion components. The data are more in line with a model where first-stage motion detectors are orientation selective but without the restriction that their preferred direction of motion is perpendicular to their preferred orientation. In the second stage it is proposed that combination occurs across the orientation dimension only. This preserves the direction of motion "labels" at the output of the second stage and allows for representation of transparent as well as coherent plaid motion at this stage of processing.