Visual corticogeniculate projections in the cat.

The anterograde axonal tracer Biocytin was injected into areas 17-19 of the cat visual cortex to determine the nature of the corticofugal projection from each area to the dorsal lateral geniculate nucleus (LGNd). Each projection had extremely fine terminal branches with even finer twigs that ended in single synaptic boutons. The terminal contacts of the projections from areas 17 and 18 extended throughout the LGNd while those from area 19 were found only in the A1 and CMAG laminae. Unlike earlier experiments, where injections were made into the optic radiations, no beaded axons were found, which adds support to the proposal that such fibres originate from the perigeniculate nucleus rather than the visual cortex. Within the LGNd, fibres of large calibre appeared only to arise from retrogradely labelled cells. All fibres of cortical origin were fine in diameter although there was a higher density of boutons contacting LGNd cell bodies for the area 17 terminals. The recurring fineness of the corticofugal projections indicates that corticofugal signals travel much more slowly than their geniculo-cortical counterparts while the greater bouton density of the area 17 projections points to a more independent effect from this cortical area.