Visual Motion Perception

Abstract

The brain carries out complementary motion integration and segmentation processes to compute unambiguous global motion percepts from ambiguous local motion signals. When, for example, an animal runs at variable speeds behind forest cover, the forest is an occluder that creates apertures through which fragments of the animal's motion signals are intermittently experienced. The brain groups these fragments into a trackable percept of the animal and its trajectory. Form and motion processes are needed to accomplish this using feedforward and feedback interactions both within and across cortical processing streams. All the cortical areas V1, V2, MT, and MST are involved in these interactions. Figure-ground processes in the stream form through V2, such as the separation of occluding boundaries of the forest cover from boundaries of the animal, select the motion signals, which determine global object motion percepts in the motion stream through MT. Sparse, but unambiguous, feature tracking signals are amplified before they propagate across position and either select consistent motion signals, or are integrated with far more numerous ambiguous motion signals. Figure-ground and integration processes together determine the global percept. A neural model is used to clarify and organize many perceptual and brain data about form and motion interactions, including data about motion grouping across apertures in response to a wide variety of displays, and probabilistic decision-making in parietal cortex in response to random dot displays.