Interactions among motion-sensitive mechanisms in human vision

Physiological and psychophysical evidence indicate that the human visual system contains mechanisms that respond strongly to a stimulus moving in a preferred direction, but weakly or not at all to that same stimulus moving in other directions. The experiments to be reported here investigate how these simple direction-selective mechanisms interact when more than one direction of motion is present at the same time and place. To this end, contrast discrimination thresholds were measured among stimuli composed of the sum of a leftward and rightward drifting sine grating of equal spatial frequency and equal but opposite drift rate. The data are well fit by an opponent model in which outputs of mechanisms sensitive to opposite directions of motion inhibit each other with a division-like operation. It is shown that these inhibitory interactions are useful for sharpening the response of simple direction-selective mechanisms to their preferred signals, in the presence of other signals and noise. Similar interactions can sharpen responses of mechanisms within other stimulus dimensions, such as spatial orientation or frequency. The opponent motion model can thus be seen as one member of a general class of models useful for enhancing the response of simple mechanisms in vision