Berson_1983_Neurosci_9_337

The extrageniculate visual thalamus of the cat is divisible into several major subdivisions but only one receives dense fiber projections from the striate cortex. In the present study, modern axon transport techniques and acetylcholinesterase histochemistry were used to examine the internal organization of this striate-recipient zone and some of its afferent and efferent connections. A detailed study of the corticothalamic fiber projections of the striate cortex clarified the topographic organization and boundaries of the striate-recipient zone. The nature and course of "projection lines" within the zone were defined and the subdivision was shown to correspond closely to a region of relatively weak acetylcholinesterase staining. Corticothalamic projections from two regions of the extrastriate visual cortex, area 19 and the medial division of the Clare-Bishop complex, converge with those from area 17 in the striate-recipient zone, but these extrastriate areas have more widespread projections to the extrageniculate thalamus than does the striate cortex. A weak subcortical projection to the striate-recipient zone was demonstrated, apparently originating in the superior colliculus. Retrograde tracing experiments indicated that the corticothalamic inputs of the striate-recipient zone are precisely reciprocal by thalamocortical projections. Extrageniculate thalamic projections to area 17 arise exclusively from this thalamic subdivision and are highly topographically ordered. The striate-recipient zone projects massively and apparently retinotopically to area 19 and to the medial division of the Clarc-Bishop area, as well as area 21(a), but these extrastriate areas receive additional afferents from other subdivisions of the extrageniculate thalamus. These findings appear to rule out a "non-specific" functional role for the striate-recipient zone. In its topographic organization, its reciprocal connections with areas of the visual cortex, and its sheer volume, the zone seems comparable to the dorsal lateral geniculate nucleus and may be fairly considered a satellite of the geniculocortical system. Certain of the zone's organizational and connectional features may be clues to its functional role and its possible homologues in other mammalian forms.