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.

Projections from the lateral division of the lateral posterior-pulvinar complex to area 21a and the striate cortex in the cat.

The axonal tracer HRP-WGA, injected into area 21a of the cat, was used in conjunction with AChE histochemistry to demonstrate that a reciprocal pathway links area 21a with the LPl (striate recipient zone) in the lateral posterior-pulvinar complex. Separate injections of the fluorescent tracers, Fast blue (FB) and Diamidino yellow (DY), were then placed simultaneously in the striate cortex and area 21a, respectively, to identify the relative position of cells projecting to both cortical areas and to pinpoint the location of efferent terminals in the LPl. Both FB and DY labelled cells and terminals are located together in the LPl but none of the neurons projecting to the striate cortex and area 21a was double labelled which would have indicated the presence of cells with axons projecting to both areas. Nonetheless, the intermingling of cells projecting to the striate cortex and area 21a, and the reciprocal character of both pathways, is consistent with a model in which the two cortical areas exchange signals via the LPl.