Speed selectivity for optic flow in area 7a of the behaving macaque.

Area 7a, in the inferior parietal lobe, has been implicated in optic flow processing to obtain spatial information about the environment. Optic flow, angle-of-gaze and center-of-motion dependencies are already documented, but the selectivity of area 7a to speed is unknown. Such information is crucial as area 7a provides the final step in visual motion analysis that begins at the lateral geniculate nucleus and passes through MT, MST and LIP/VIP. Macaque area 7a neurons were tested with optic flows with speeds of 0.5-128 degrees /s. Of 161 neurons tested in four hemispheres of two adult male macaques, 53% (86/161) were speed selective at either the time of stimulus onset, at the end of the trial, or at both times. Speed selec- tivities resembling the basic filter types (band-pass, band-reject, high-pass, low-pass, broadband) were found. Area 7a neurons exhibited two novel properties not previously reported elsewhere. Speed selectivity was found to be dynamic in that many cells gained, lost or changed speed tuning over the course of a trial. In addition, speed dependence and optic flow selectivity interacted. For example, a cell could preferentially respond to one type of naviga- tional optic flow at a slow speed and a different navigational optic flow at a fast speed. The presence of speed selectivity combined with other properties of area 7a neurons indicates that these neurons may have a role in the concurrent representation of heading as well as multiple object speeds and directions.