Connectional and architectonic evidence for dorsal and ventral V3, and dorsomedial area in marmoset monkeys

The existence of a third visual area, V3, along the outer margin of V2 was originally proposed for primates on the basis of projections from V1. The e vidence for V3 was never totally convincing because investigators failed to demonstrate V1 projections to ventral V3, and projections to dorsal V3 cou ld be attributed to the dorsomedial visual area (DM). We have reexamined th e issue by placing large injections into both dorsal and ventral portions o f V1 and subsequently processing flattened cortex for myelin and cytochrome oxidase so that borders of V1 and V2 could be determined accurately. The i njections were in small-brained marmosets, where ventral V1 was most access ible and cortex could be flattened easily. The results indicate that dorsal V1 (representing the lower visual quadrant) projects to a narrow "dorsal V 3" located between DM and dorsal V2, whereas ventral V1 (representing the u pper visual quadrant) projects to a narrow "ventral V3." Architectonic bord ers for these dorsal and ventral strips were clearly apparent. In addition, all parts of V1 project to DM, whereas ventral V1 connections indicate tha t the dorsolateral area (DL) extends more ventral than has been established previously. We also placed injections within dorsal V2, dorsal and ventral DM, and dorsal, central, and ventral middle temporal (MT) area. Results fr om these injections were consistent with the proposed retinotopic organizat ions of V3, DM, and DL. We provide compelling evidence for the existence of areas V3, DM, and DL in marmosets and suggest that these areas are likely to be found in all primates.