Abstract
A substantial body of evidence has suggested that signals transmitted through the magnocellular and parvocellular subdivisions of the LGN remain largely segregated in visual cortex. This hypothesis can be tested directly by selectively blocking transmission through either the magnocellular or parvocellular layers with small injections of lidocaine or GABA while recording cortical responses to a visual stimulus. In a previous study, we found that responses in the middle temporal visual area (MT) were almost always greatly reduced by blocks of magnocellular LGN, but that few MT neurons were affected by parvocellular blocks. In the present study, we have examined magnocellular and parvocellular contributions to area V4, which lies at the same level of processing in the cortical hierarchy as does MT and is thought to be a major recipient of parvocellular input. We found that inactivation of parvocellular layers usually resulted in a moderate reduction of visual responses (median reduction, 36%). However, comparable reductions in V4 responses were also seen following magnocellular blocks (median reduction, 47%). Directionally selective responses in V4 were not found to depend specifically on either subdivision. We conclude that area V4, unlike MT, receives strong input from both subdivisions of the LGN. These results suggest that the relationship between the subcortical magnocellular and parvocellular pathways and the parietal and temporal streams of processing in cortex is not one-to-one.
