Subcortical projections, cortical associations, and some intrinsic interlaminar connections of the striate cortex in the squirrel monkey ( Saimiri )

Abstract Laminar lesions made by the thermocoagulation in the lateral striate cortex of Saimiri reveal details of three distinct groups of descending fibers: interlaminar, cortico‐cortical, and cortico‐subcortical. The two most massive connections originate mainly from layer III. These are the interlaminar fibers terminating in the underlying layer V, and the systematically arranged projection upon area 18. Another cortical projection upon a narrow region in the superior temporal sulcus originates mainly from the infragranular layers, from which originate also the fibers passing to three subcortical structures, viz. the colliculus superior, the medial pulvinar and the griseum pontis. These subcortical projections are sparse compared to the interlaminar and cortico‐cortical connections. In particular, no evidence was obtained that the lateral geniculate nucleus receives fibers from the lateral striate cortex. Lesions injuring the white matter, however, produced, in addition, anterograde degeneration in the nucleus praetectalis, three pulvinar nuclei, the lateral geniculate nucleus and the pregeniculate nucleus. This degeneration must have resulted from the interruption of fibers originating in areas other than the lateral striate cortex, and passing beneath the site of the lesion. The origin of the interlaminar fibers and of the association fibers upon area 18 provided strong evidence that, in Saimiri's striate cortex, the two sublayers traditionally referred to as sublayers IVa and IVb, actually form part of the conspicuously enlarged layer III which thus is subdivided into three distinct sublayers.