Perceptual Integration and Differentiation of Directions in Moving Patterns

摘要

Two or more directions of motion cannot be discriminated from one direction over separations of direction greater than 16 and less than 60 deg at threshold contrast. The multiple directions produce a percept indistinguishable from a single direction. When the same stimuli are presented at 6 or more times threshold contrast, another kind of perception error occurs: the component directions are seen separately but their perceived directions repel one another. In a series of experiments, contrast, directional content, pattern content and velocity determined the conditions under which directions of motion summate or repel one another. Stimulus changes increasing inhibition enhance repulsion; changes decreasing inhibition enhance summation. A 3 stage, vector model can explain these errors in directional perception. Stage 1 of direction processing is excitation of directionally sensitive units. Such units may be portrayed in polar coordinates as vectors whose orientation is their optimal direction of sensitivity and whose length indicates the amount of activation. Stage 2 of motion perception is an inhibitory process limiting the distribution of excitation to units whose sensitivity is close to the direction of motion and suppressing activity in units sensitive to other directions. In stage 3 all remaining excitation is processed by a decision mechanism that determines the perceived direction of motion. Two decision processes, a peak detection and a vector summing process, are discussed.