Corticopulvinar Connections of Areas V5, V4, and V3 in the Macaque Monkey: a Dual Model of Retinal and Cortical Topographies

The connection zones of cortical areas V3, V4, and V5 (MT) with the thalamic pulvinar nucleus in the macaque monkey were identified. A combination of single- and dual-tracer techniques was used to study their topography and to establish whether these zones occupy separate or overlapping pulvinar territories. In each case, the retinotopic distribution of tracer in the pulvinar was charted by reference to its parallel distribution within the maps of cortical areas V1 and V2. Each of the areas V3, V4, and V5 were found to connect with both the 1° and the 2° maps located within the inferior and lateral pulvinar nuclei and to respect the previously identified topographies of these maps. However, V5 connects to a narrow zone lining the rostrolateral margin of the lateral and inferior pulvinar and V4 to a broader zone within the body of these two nuclei, which is adjacent to but separate from the V5 zone; the V3 zone overlaps both. Focal injections into cortex produce columns of pulvinar label whose trajectory defines a line of isorepresentation. The lines of isorepresentation in the 1° and 2° maps are approximately linear and parallel and adopt a rostrolateral to caudomedial axis; in the 1° map, this axis is roughly perpendicular to the facet of the inferior pulvinar that lies adjacent to the lateral geniculate nucleus. The connections of V5 and V4 can be modelled as successive zones along the axis of isorepresentation, with registered visual topographies. The scheme is extended by existing reports that inferotemporal cortex connects to the caudomedial pole of this axis-reflecting an occipitotemporal cortical gradient, in that V1 and other prestriate areas, e.g., V3, connect to the opposite pole. Thus a simple model of the mapped volume in the pulvinar arises, in which a unidimensional cortical topography is represented orthogonally to retinal topography. Adjoining this volume medially, within the inferior and medial pulvinar, is a second, heavier zone of V5 connectivity, which is poorly topographic. Both the medial and the rostrolateral zones of V5 connectivity may overlap with previously identified regions of tectal input to the pulvinar.