Centrifugal organization of direction preferences in the cat's lateral suprasylvian visual cortex and its relation to flow field processing.

The cerebral cortex of the cat contains between 1 and 2 dozen representations of the visual field with different functional specializations. Six visual field maps lie along both banks of the suprasylvian sulcus, lateral and anterior to the visual areas in the occipital cortex. We have studied single-unit receptive field properties and their global organization across the visual field in 2 of these lateral suprasylvian areas, PMLS (essentially the Clare-Bishop area) and PLLS. Most neurons in PMLS and PLLS display selectivity for the direction of a light stimulus moving across their receptive fields with various degrees of directional tuning. We have used light spots of different size and velocity projected on a tangent screen in order to determine the direction preference of cells in these 2 areas. A strong tendency was found for neurons to respond best to centrifugal directions, i.e., to movement away from the area centralis. Thus, for these cells direction preference depends on the location of their receptive fields within the visual field. Velocity preference and binocular interaction in these neurons is also globally organized: Velocity preference increases with eccentricity, binocular synergism is strongest in the center of the visual field. Cluster analysis of recording tracks with respect to "radial" and "circular" cell categories reveals a grouping of cells with like properties in the lateral suprasylvian cortex. These new categories are formed by combining "centrifugal" and "centripetal" cells on the one hand and cells with direction preferences orthogonal to these on the other. The radial or centrifugal organization of direction preferences in conjunction with the global arrangement of velocity preference and binocular interaction suggests that PMLS and PLLS are involved in the processing of expanding visual flow fields of motion. Such flow fields are commonly encountered when a visual object moves towards an observer or during forward locomotion.