Projections of the visual cortex to the lateral geniculate and posterior thalamic nuclei in the cat.

Abstract

AbstractFiber degeneration in the cat following lesions of various parts of areas 17, 18 and 19 was traced to the lateral geniculate and posterior thalamic nuclei by the method of Nauta and Gygax. Area 17 lying in the lateral and posterolateral gyri projects to the most medial parts of laminae A, A1 and central interlaminar nucleus (NIC). Area 17 in the suprasplenial gyrus sends fibers to the medial and middle parts of laminae A and A1 and the underlying part of NIC, while that in the splenial gyrus gives off fibers to the dorsolateral and ventrolateral parts of laminae A, A1 and NIC. These data indicate that there is a mediolateral organization in the projection from area 17 to the dorsal lateral geniculate nucleus. A topical organization exists also in the anteroposterior direction; the anterior, middle and posterior parts of area 17 project to the corresponding portion of the dorsal nucleus. Area 18 gives rise to fibers that pass between the medial interlaminar nucleus (NIM) and lamina A1 into the dorsolateral part of NIM, the medial part of NIC and the medial ends of laminae A and A1. The projections of area 18 to the dorsal nucleus are also organized in the anteroposterior dimension. Area 19 sends fibers to the major part of NIM, except to its most ventral and dorsolateral parts, and to the medialmost parts of NIC and lamina B. The majority of these fibers run along the medial aspect of NIM, but a few fibers pass ventrolaterally between NIM and lamina A1. It should be mentioned that NIM receives fibers not only from area 19 in the lateral and the posterior suprasylvian gyri, but also from area 19 in the medial cortex. As with areas 17 and 18, the corticogeniculate projections of area 19 are organized in an anteroposterior dimension.The ventral lateral geniculate nucleus also receives fibers from areas 17 and 19. The anterior part of area 17 projects only very few fibers to the ventral nucleus, but the posterior part gives off many fibers to the external layer of the nucleus. Area 17 fibers from the posterolateral gyrus are distributed in the dorso‐medial part of the external layer and those from the suprasplenial gyrus enter the ventral and medial portion of the layer. In contrast most of area 18 and that part of area 19 in the lateral gyrus project no fibers to the ventral nucleus. The other parts of area 19 give off fibers to the internal layer and the adjoining portion of the external layer of the ventral nucleus. There appears to be an anteroposterior organization of the projection of the visual cortex on the ventral nucleus. The posterior thalamic nucleus receives fibers from areas 18 and 19 and part of area 17. Area 18 sends fibers to the medial and ventromedial part of the posterior nucleus, while most of area 19 projects fibers to the dorsal and middle part of the nucleus. Area 17 lying in the upper portion of the posterolateral gyrus and the suprasplenial and splenial gyri projects to the dorsomedial and medial part of the posterior nucleus.