AMPLIFICATION OF THALAMIC PROJECTIONS TO MIDDLE SUPRASYLVIAN CORTEX FOLLOWING ABLATION OF IMMATURE PRIMARY VISUAL-CORTEX IN THE CAT

The purpose of the present study was to identify expansions in thalami c projections to middle suprasylvian (MS) cortex that could be linked to the sparing of visually guided behaviors that follow the removal of visual cortex early in postnatal life. Injections of retrograde trace rs were made into the medial bank of the middle suprasylvian sulcus in intact, adult cats and in adult cats that had incurred ablations of a reas 17 and 18 on the day of birth (P1), P28, or greater than or equal to 6 months of age, and the numbers of labeled neurons in the thalamu s were counted. In the thalamus of the intact cat, the greatest number of labeled neurons are located in the lateral division of the lateral posterior nucleus and there are intermediate numbers in the medial di vision of the lateral posterior nucleus (LPm); and smaller numbers wit hin the medial interlaminar nucleus, the C-complex of the dorsal later al geniculate nucleus (dLGN), the geniculate wing, and the pulvinar nu cleus. Following the removal of areas 17 and 18 at different ages, tha lamic projections to MS cortex exhibit an age-dependent reorganization . Removals on P1, induce twice the normal number of neurons in LPm and three times the normal number of neurons in the C-complex of dLGN to project to MS cortex. Removals on P28 induce five times the normal num ber of neurons in the C-complex to project to MS cortex. In addition, removals at both ages resulted in projections from the A-laminae to MS cortex becoming permanently established. No changes in the pattern or number of neurons that project to MS cortex were detected when areas 17 and 18 were removed in adulthood. These results show that pathways through the C-complex of dLGN and through LPm expand substantially fol lowing ablation of immature areas 17 and 18, These expanded pathways a re linked to Y- and W-functional streams of visual signals that are re layed from the retina to extrastriate cortex either directly through d LGN or indirectly via the superior colliculus and LPm. These signals m ay be critical for the sparing of neural operations following ablation of areas 17 and 18 early in development.