Neuronal responses to optic flow in the monkey parietal area PEc.

Area PEc, a high order association area, is located in the dorsocaudal portion of the superior parietal cortex. PEc neurons encode visual motion signals, especially the direction of stimulus motion. The present study tested if PEc neurons also process visual correlates of self-motion. The extracellular activity of single neurons in response to optic flow stimuli was recorded in two monkeys (Macaca fascicularis) trained in a fixation task. The stimuli were produced by random dots simulating planar motion, radial expansion and radial contraction. A substantial number of PEc neurons were specifically activated by radial optic flow and were selective for the position of the focus of expansion with respect to the fovea. Eccentric positions of the focus of expansion were preferred. Almost all neurons showed opponent excitatory-inhibitory activity to expanding-contracting visual fields. Planar motion elicited very weak responses. Optic flow responsiveness is not entirely explained by classical bar sensitivity in PEc neurons, suggesting that optic flow and classical bar responses could serve different mechanisms in the integration of visuo-motor signals to prepare body movements.