RETINOTOPIC ORGANIZATION OF MACAQUE PULVINAR

The retinotopic organization of the inferior and lateral pulvinar in rhesus monkeys [M. mulatta] was investigated using .OMEGA. microelectrodes and extracellular recordings. The animals were immobilized with gallamine triethiodide and anesthetized with a N2O-O2 mixture. Both the inferior and lateral nuclei were visually responsive. Two complete representations of the contralateral hemifield occupy most of this territory. One representation lies mainly within the inferior pulvinar but extends into the adjacent lateral pulvinar. The vertical meridian lies in the dorsal and lateral margins of the inferior pulvinar while the periphery is found in the medial margin adjacent to the medial geniculate body. The fovea and central vision are represented laterally and posteriorly. A 2nd representation of the hemifield lies entirely within the lateral pulvinar and partially surrounds the 1st along its dorsal, lateral, ventral and caudal aspects. The lower quadrant of the 2nd map lies in the dorsal half of the lateral pulvinar; the upper quadrant is almost completely separated in the ventral half. A double representation of the horizontal meridian forms the outer boundary of the 2nd representation and lies in the external margin of the pulvinar. The 2 maps share a common representation of the vertical meridian and the pattern of topography reverses in crossing this joint boundary from 1 map to the other. The physiologically defined boundary between the 2 representations, i.e., the representation of the vertical meridian, does not coincide with the cytoarchitectonic boundary between the inferior and lateral nuclei, but coincides with the medial edge of a fiber system that fills the external half of the lateral pulvinar. Both visuotopic maps encompass virtually the entire visual hemifield. Receptive-field size for cell clusters in the 2 visuotopic zones is the same and, for both regions, increases with increasing eccentricity.