Magnocellular and parvocellular contributions to the responses of neurons in macaque striate cortex

Anatomical and physiological studies of the primate visual system have suggested that the signals relayed by the magnocellular and parvocellular subdivisions of the LGN remain segregated in visual cortex. It has been suggested that this segregation may account for the known differences in visual function between the parietal and temporal cortical processing streams in extrastriate visual cortex. To test directly the hypothesis that the temporal stream of processing receives predominantly parvocellular signals, we recorded visual responses from the superficial layers of V1 (striate cortex), which give rise to the temporal stream, while selectively inactivating either the magnocellular or parvocellular subdivisions of the LGN. Inactivation of the parvocellular subdivision reduced neuronal responses in the superficial layers of V1, but the effects of magnocellular blocks were generally as pronounced or slightly stronger. Individual neurons were found to receive contributions from both pathways. We furthermore found no evidence that magnocellular contributions were restricted to either the cytochrome oxidase blobs or interblobs in V1. Instead, magnocellular signals made substantial contributions to responses throughout the superficial layers. Thus, the regions within V1 that constitute the early stages of the temporal processing stream do not appear to contain isolated parvocellular signals. These results argue against a direct mapping of the subcortical magnocellular and parvocellular pathways onto the parietal and temporal streams of processing in cortex.