Organization of the visual cortex: comparative anatomical and behavioral studies.

In all mammals there are two parallel visual pathways ascending to the cortex: the geniculostriate system and the tectopulvinar system. However, there is a difference among mammals in the locus and extent of the cortical target of the tectopulvinar path. In several diverse species such as squirrel and tree shrew the cortical target of the tectopulvinar system lies adjacent to area 17 and extends into the temporal lobe; but areas 18 and 19 do not receive fibers from the tectorecipient zone of the pulvinar in Galago and presumably in all primates. This anatomical difference may account for the drastic sensory effect of removing area 17 in primates, and the failure to find marked sensory changes in tree shrews or squirrels deprived of the geniculostriate system.