Interaction between first- and second-order motion signals at the local motion scale

Motion perception appears to be mediated by, at least, two systems: a first-order and a second-order system. These two systems appear to be independent at the local-motion extraction and global-motion pooling stages. However interaction, in the form of inhibitory links between opponent directions of motion, could exist at the local-motion scale. Such an interaction would account for the failure to perceive coherent motion in a standard global-motion stimulus (random dot pattern in which signal dots move in the same direction and noise dots in random directions) when the signal dots reverse their luminance contrast (go from light to dark) as they move. Second-order motion units would signal motion in the displacement direction of the signal dots and the first-order motion units would signal motion in the opposite direction (reverse-phi motion). If the first- and second-order motion responses for each signal dot were of equal strength, then opponent inhibition would result in no net motion response to the signal dots. The possibility of opponent inhibition at the local-motion scale was investigated by manipulating the contrast-reversing global-motion stimulus to reduce the second-order response relative to the first-order. This was achieved by: decreasing dot contrast; increasing stimulus eccentricity; and increasing dot speed. These manipulations resulted in an increase in the perception of (first-order mediated) reverse-phi motion. We conclude that opponent inhibition exists between first-order and second-order units at the local-motion scale.