Connections of the visual cortex in the hedgehog (Paraechinus hypomelas). I. Halamocorticalprojections

Visual thalamocortical projections were studied in the Pakistani hedgehog with anterograde degeneration techniques following large aspiration lesions and discrete electrolytic lesions of visual thalamic nuclei. After 3‐ to 9‐day survival periods, the brains were processed and stained with the Fink‐Heimer technique, and examined with the light microscope. The resutls of these studies show that there are at least four separate thalamic nuclei that project to visual cortex in the hedgehog. First, the dorsal lateral geniculate nucleus projects with a topographic order to layer IV and the adjacent portion of layer III of striate cortex. Second, a thalamic transition zone, adjacent to the dorsal lateral geniculate nucleus, projects widely to layer VI throughout striate cortex and possibly to all laminae of striate cortex in the area lying just medial to the striate‐parastriate border. The laminar pattern of termination of this transition zone is similar to that of the lateral posterior nucleus. Third, the lateral posterior nucleus projecs with little discernible topographic localization to layer VI of both striate and parastriate cortex, and in addition, to the remaining laminae of parastriate cortex except the outer one‐half of layer I. Fourth, the anterior intralaminar region projects to the outer one‐third to one‐half of layer I in both striate and parastriate cortex. Evidence derived from ablations of the entire thalamus on one side suggests that there may be still another thalamic nucleus that projects to visual cortex in the hedgehog.