Thalamic projections to layer I of striate cortex shown by retrograde transport of horseradish peroxidase.

The diffusion of horseradish peroxidase was restricted to layers I and II of the striate cortex in Tupaia glis and Galago senegalensis. In the lateral geniculate body of Tupaia labeled cells were found only in layer 3; some labeled cells were also found in the lateral nucleus. In Galago labeled cells were found only in layers 4 and 5 of the lateral geniculate body; a band of cells was also found in the pulvinar nucleus. These results support the distinction between two overlapping thalamic systems, a layer I and a layer IV system.