Two-dimensional map of direction selectivity in cortical visual area MT of Cebus monkey.

We studied the spatial organization of direction of motion in visual area MT of the Cebus apella monkey. We used arrays of 6 (700 micro m apart) parallel electrodes in penetrations tangential to the cortical layers to record multi-unit responses to moving bars, at 200 micro m steps. We determined the direction selectivity at each recording site. The data from single penetrations showed cyclic and gradual changes in the direction selectivity of clusters of cells, intermixed with abrupt 180 degrees discontinuities along the electrode track. In order to obtain maps of direction of motion selectivity, we examined the spatial distribution of direction of motion in MT and we applied a method to determine the location of the centers of radial arrangements of direction selectivity. This tangential organization is characterized by slow continuous changes in direction of motion, interrupted by discontinuities. The changes in direction selectivity are organized radially in a pinwheel fashion and in slabs of linear variation. The pinwheel arrangements have 800-1400 micro m in diameter. The size of the radial arrangement is comparable to the point image size in area MT at each eccentricity.