Organization of corticogeniculate projections in the turtle, Pseudemys scripta.

The efferent pathways from the visual cortex to the dorsal lateral geniculate complex of turtles have been studied by using the orthograde and retrograde transport of horseradish peroxidase (HRP). Injections of HRP in the lateral thalamus retrogradely label neurons throughout the visual cortex. The majority of labeled neurons have somata in layer 2 of the lateral part of dorsal cortex (D2); a minority have somata in layer 3. Labeled neurons in layer 2 tend to have vertically oriented, fusiform somata and dendrites that ascend into layer 1. Labeled neurons in layer 3 have fusiform somata and dendrites, both oriented horizontally. Injections of HRP in visual cortex orthogradely label corticofugal axons. Those projecting to the lateral geniculate complex course laterally from the visual cortex, pass through the striatum (occasionally bearing varicosities), and enter the diencephalon in the ventral peduncle of the lateral forebrain bundle. Individual axons leave the ventral peduncle and run dorsally in the transverse plane, entering the dorsal lateral geniculate complex from its ventral edge. They continue dorsally, principally in the cell plate of the geniculate complex, where they bear varicosities.