Innervation patterns of single physiologically identified geniculocortical axons in the striate cortex of the tree shrew.

We examined the termination patterns of single geniculocortical axons in the striate cortex of the tree shrew by using intracellular recording and horseradish peroxidase staining methods. Axons were classified by whether they responded to light onset (ON center) or light offset (OFF center) and whether they were driven by the ipsi- or contralateral eye. Afferents with ON-center responses end in the upper part of layer IV (IVa) whereas afferents with OFF-center responses end in the lower part of layer IV (IVb). Within each tier, axons driven by the ipsilateral and contralateral eye overlap. These results suggest that binocular convergence occurs within layer IV without mixing the information from the ON- and OFF-center pathways and we consider the significance of this arrangement for visual cortical function.