8 - Parallel Streams in the Corticogeniculate Pathway: Intrinsic and Extrinsic Projections of Neurons in Layer VI of Striate Cortex

Abstract

Publisher Summary One of the major advances in the understanding of functional organization of the visual system has been the identification of parallel geniculocortical pathways that link different populations of retinal ganglion cells with different layers of the striate cortex. Despite variation across species in the features that distinguish these pathways, two general classes can be identified in all: those that terminate in the granular layer of striate cortex, layer IV; and those that terminate in the supragranular layers, layers I-III. It has also been found that the projections from the lateral geniculate nucleus (LGN) to the striate cortex, like all thalamocortical pathways, are reciprocated by a dense feedback projection that originates from neurons in cortical layer VI. This chapter also discusses experiments to understand how this descending pathway is organized in relation to the parallel granular and supragranular ascending systems. The results suggest that neurons in layer VI of striate cortex are in a position to selectively influence the transmission of activity in these two pathways by virtue of their descending projections to the LGN as well as their local axonal projections to the granular and supragranular layers of the striate cortex. To examine the organization of the corticogeniculate pathway, small extracellular injections of biocytin are made into layer VI of tree shrew striate cortex and then the pattern of labeled axons in the LGN are examined.