Laminar lesions made by the thermocoagulation in the lateral striate cortex of Suimiri reveal details of three distinct groups of descending fibers: interlaminar, cortico-cortical, and cortico-subcortical. The two most massive connections originate mainly from layer 111. These are the interlaminar fibers terminating i n the underlying layer V, and the systematically arranged projection upon area 18. Another cortical projection upon a narrow region i n 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 i n 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 111 which thus is subdivided into three distinct sublayers.

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Neurons of area 17, the primary visual cortex, project to various anatomically and physiologically different extrastriate areas and subcortical regions. In the present investigation, we addressed the question of whether the efferent neurons in area 17 can contribute to functional diversity between these regions. We approached this question by analyzing the dendritic morphology of neurons in area 17 projecting to area MT, area 19DM, and the superior colliculus in the new world simian primate Callithrix jacchus, because dendritic morphology is an important factor in determining physiological properties of nerve cells. Retrograde transport of fluorochromes injected into the target regions, and intracellular injections of Lucifer yellow in the prelabelled neurons, revealed the following. 1) Morphologically identical large pyramidal cells in layer VI of area 17 project to all three targets. Some of them possess axon collaterals to two or all three targets, suggesting that they provide common information to all three areas. 2) Pyramidal cells in layer IIIc projecting to area MT form a morphologically homogeneous population. 3) Three small to medium-sized pyramidal cell types in layers IIIa-c, spiny stellate cells in layer IIIc, and another large pyramidal cell type in layer VI project to area 19DM. 4) Pyramidal cells in the lower two-thirds of layer V in area 17 project to the superior colliculus. In conclusion, we have shown that in Callithrix one efferent pathway may originate from several cell types. However, with the exception of the large cells in layer VI, efferent cells projecting to area MT, area 19DM, and the superior colliculus were morphologically distinct. This suggests that functional differences between brain regions could arise in part from morphological heterogeneity between and within the efferent cell populations.

With the aid of the techniques of tracing axonal pathways by anterograde fiber degeneration, and by anterograde (autoradiography) and retrograde (HRP-histochemistry) axoplasmic transport, it could be shown that area 17 projects in a topographically and visuotopically organized manner onto the temporal visual area MT. The fibers of this association system originate from pyramidal cells in layer IIIc, and from the solitary cells of Meynert; they terminate in layers IV and III of area MT. A correspondingly organized system of countercurrent fibers originates from pyramidal cells in layers III/II and V/VI of area MT and terminates separately in layers VI, IIIc and I of area 17.