Independence in the processing of first- and second-order motion signals at the local-motion-pooling level

The interaction of first- and second-order motion signals at the local-motion-pooling level were investigated using locally-paired dots that moved orthogonally to each other. Dots were either luminance-defined, which could, potentially drive both first- and second-order local-motion units, or contrast-defined, which only drive second-order local-motion units. The response measure used was the nature of the motion percept: either unidirectional or transparent motion. The likelihood of perceiving transparent motion was varied by adjusting the trajectory length of the dots. Increasing the trajectory length increased the likelihood that observers would perceive transparency. The results, taken as a whole, support the notion of independent first-order and second-order local-motion-pooling units, with the spatial extent of the second-order units being larger than that of the first-order units.