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An anterograde degeneration study of the tangential spread of axons in cortical areas 17 and 18 of the squirrel monkey (Saimiri Sciureus)

In area 17 of squirrel monkey, lamina IVB of Brodmann is divided into three sublaminae by a thin, dense row of cell bodies, the largest of which have apical poles. Study of the 17/18 border suggests that these are a group of pyramidal cells, continuous with the large border pyramids in the depth of layer III in area 18. In contrast, the densely packed granule cells of layers IVA and IVC appear to be continuous with those of layer IV of area 18. It is thus proposed that in area 17, the third and fourth neocortical layers not only divide into sublaminae, but that these in turn interdigitate and intermingle their cell types. The tangential extent of intracortical fibers has been studied by making perpendicular slits in both areas and measuring the spread of anterograde degeneration to each side of the lesion. In area 17, this spread is always the same whatever the orientation of the fibers with respect to visual field representation. Degenerating fibers are few in number and found mainly in layers I, IVB and VI. In layers IVB and VI they extend for 0.5–1.0 mm. In layer I, some reach as far as 2.0 mm. In area 18, degenerating fibers are more numerous and longer. They are plentiful in layer I, diminish considerably in layer II, increase again in number till the depth of III, are slightly less numerous in layer IV and are again plentiful in layers V and VI. Their length varies with orientation. In a direction corresponding to a visual field sector, they extend for 1.5–2.0 mm, a distance representing only a fraction of the total representation of a sector in the cortex. In a direction corresponding to a visual field zone, they extend further and the combined degeneration of the two sides spans the whole distance of area 18 from the vertical to the horizontal meridian (6 mm). It is suggested that at least some of the fibers of the stria of Gennari subserve binocular interaction and that the differential fiber lengths in area 18 are related to foveal acquisition or to a directional anisotropy of visual movement perception along sector‐zone axes.