Raffi and Siegel define optic flow as the visual perception of the motion of the viewer. This perception is caused by the expansion or contraction of visual fields projected on the retina. As one moves toward an object, the visual field expands. It contracts when the observer moves back. Even if the observer is stationary, the expansion or contraction of the visual field can create the illusion of motion. The optic flow is one component (and, apparently, the primary one) of the perception of motion. It is the large-scale effect, processed first, onto which other visual and nonvisual cues are added.

At a neuronal level, optic flow is processed in different areas of the brain. These regions of the brain have different functions, but the perception of optic flow takes place as a foundation upon which each region of the brain perceives and evaluates other visual perceptions related to motion.

The paper summarizes the literature on the optical responses of six different areas of the macaque brain. The emphasis of the paper is on the biological function and anatomical relationships among these regions. The authors conclude that there is no support for a hierarchical model of the perception of optic flow. They hypothesize that there may be a nonhierarchical connectivity in the functioning of these regions in the brain.