Visual cortex of the grey squirrel (Sciurus carolinensis): Architectonic subdivisions and connections from the visual thalamus

Thalamic projections to the visual cortex of the grey squirrel were studied by retrograde degeneration in the lateral geniculate and the pulvinar nuclei. The lateral geniculate was found to project to architectonic area 17 which also corresponds to visual area I as defined by its retinotopic organization. The projection is spatially organized in a precise way, and for every cortical point there is a corresponding column in the lateral geniculate which extends from border to border. For the binocular sector of area 17 the lateral geniculate column lies in the trilaminar part of the lateral geniculate, while for the uniocular sector the column lies in the bilaminar sector of the lateral geniculate. The pulvinar projects to several architectonic areas, areas 18 and 19 and to two or more temporal areas below area 19. This projection is roughly topographic but follows the sustaining pattern. When the squirrel is compared to the tree shrew and hedgehog what emerges is a conception of those changes in the visual system which arose as a result of adaptation to an arboreal habitat.