The dorsomedial visual areas in New World and Old World monkeys: homology and function

The extrastriate cortex near the dorsal midline has been described as part of an 'express' pathway that provides visual input to the premotor cortex. This pathway is considered important for the integration of sensory informa tion about the visual field periphery and the skeletomotor system, especial ly in relation to the control of arm movements. However, a better understan ding of the functional contributions of different parts of this complex has been hampered by the lack of data on the extent and boundaries of its cons tituent visual areas. Recent studies in macaques have provided the first de tailed view of the topographical organization of this region in Old World m onkeys. Despite differences in nomenclature, a comparison of the visuotopic organization, myeloarchitecture and connections of the relevant visual are as with those previously studied in New World monkeys reveals a remarkable degree of similarity and helps to clarify the subdivision of function betwe en different areas of the dorsomedial complex. A caudal visual area, named DM or V6, appears to be important for the detection of coherent patterns of movement across wide regions of the visual field, such as those induced du ring self-motion. A rostral area, named M or V6A, is more directly involved with visuomotor integration. This area receives projections both from DM/V 6 and from a separate motion analysis channel, centred on the middle tempor al visual area (or V5), which detects the movement of objects in extraperso nal space. These results support the suggestion, made earlier on the basis of more fragmentary evidence, that the areas rostral to the second visual a rea in dorsal cortex are homologous in all simian primates. Moreover, they emphasize the importance of determining the anatomical organization of the cortex as a prerequisite for elucidating the function of different cortical areas.