Thalamo‐cortical projections in the tree shrew (Tupaia glis)

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Abstract
Cortical lesions were placed in 18 hemispheres, and thalamic degeneration was studied after a survival period of at least six weeks. Very small lesions within the striate area produced complete degeneration of neurons in a column through the lateral geniculate, from medial to lateral borders and comprising all of the laminae. Lesions of various loci within the striate area reveal a precise topographic projection, with the rostral lateral geniculate sending fibers to the caudal extremity of the striate area and the caudal lateral geniculate projecting to the rostral extremity of the striate; further, the dorso‐ventral dimension in the lateral geniculate projects to the medio‐lateral dimension in the striate area. Finally, the evidence from striate area lesions suggests that the lateral geniculate projections are confined to the striate area as defined by cytoarchitecture, which in turn corresponds precisely with visual area I as defined by electrophysiological recording. This conclusion is supported by the failure to find retrograde degeneration after lesions of the belt of cortex adjacent to the striate area. The temporal area which occupies an extensive section from V II to the rhinal fissure and the auditory cortex and which has been shown to be a visual receiving area, is the target of essential projections from the pulvinar. The pulvinar also sends sustaining collaterals within the temporal area and probably outside as well, especially to V II. However, the very crude topographic organization apparent in the pulvinar projections does not seem to be sufficiently refined to account for the organization of V II. A suggestion was made in closing that V II may be the result of convergent evolution in different mammalian lines of descent.