VISUAL CORTICOGENICULATE PROJECTIONS IN THE CAT

The anterograde axonal tracer Biocytin was injected into areas 17-19 o f the cat visual cortex to determine the nature of the corticofugal pr ojection from each area to the dorsal lateral geniculate nucleus (LGNd ). Each projection had extremely fine terminal branches with even fine r twigs that ended in single synaptic boutons. The terminal contacts o f the projections from areas 17 and 18 extended throughout the LGNd wh ile those from area 19 were found only in the A(1) and C-MAG laminae. Unlike earlier experiments, where injections were made into the optic radiations, no beaded axons were found, which adds support to the prop osal that such fibres originate from the perigeniculate nucleus rather than the visual cortex. Within the LGNd, fibres of large calibre appe ared only to arise from retrogradely labelled cells. All fibres of cor tical 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 cor ticofugal signals travel much more slowly than their geniculo-cortical counterparts while the greater bouton density of the area 17 projecti ons points to a more independent effect from this cortical area.