The present investigation was designed to determine the organization of somatosensory fields in the lateral sulcus of macaque monkeys using standard microelectrode recording techniques. Our results provide evidence for two complete representations of the body surface. We term these fields the second somatosensory area (SII) and the parietal ventral area (PV) because of their similarities in position, internal organization, and relationship to anterior parietal fields, as described for SII and PV in other mammals. Areas SII and PV are mirror- symmetrical representations of the body surface, sharing a common boundary at the representations of the digits of the hand and foot, lips, and mouth. These fields are located adjacent to the face representations of anterior parietal fields (areas 3b, 1, and 2), and are bounded ventrally and caudally by other regions of cortex in which neurons are responsive to somatic or multimodal stimulation. The finding of a double representation of the body surface in the region of cortex traditionally designated as SII may explain conflicting descriptions of SII organization in macaque monkeys. In addition, the present study raises some questions regarding the designation of serial processing pathways in Old World monkeys, by suggesting that fields may have been confused in studies demonstrating such pathways. We propose that SII and PV are components of a common plan of organization, and are present in many eutherian mammals.

In this paper, we review evidence from comparative studies of primate cortical organization, highlighting recent findings and hypotheses that may help us to understand the rules governing evolutionary changes of the cortical map and the process of formation of areas during development. We argue that clear unequivocal views of cortical areas and their homologies are more likely to emerge for "core" fields, including the primary sensory areas, which are specified early in development by precise molecular identification steps. In primates, the middle temporal area is probably one of these primordial cortical fields. Areas that form at progressively later stages of development correspond to progressively more recent evolutionary events, their development being less firmly anchored in molecular specification. The certainty with which areal boundaries can be delimited, and likely homologies can be assigned, becomes increasingly blurred in parallel with this evolutionary/developmental sequence. For example, while current concepts for the definition of cortical areas have been vindicated in allowing a clarification of the organization of the New World monkey "third tier" visual cortex (the third and dorsomedial areas, V3 and DM), our analyses suggest that more flexible mapping criteria may be needed to unravel the organization of higher-order visual association and polysensory areas.

The dorsomedial area (DM), a subdivision of extrastriate cortex characterized by heavy myelination and relative emphasis on peripheral vision, remains the least understood of the main targets of striate cortex (V1) projections in primates. Here we placed retrograde tracer injections encompassing the full extent of this area in marmoset monkeys, and performed quantitative analyses of the numerical strengths and laminar patterns of its afferent connections. We found that feedforward projections from V1 and from the second visual area (V2) account for over half of the inputs to DM, and that the vast majority of the remaining connections come from other topographically organized visual cortices. Extrastriate projections to DM originate in approximately equal proportions from adjacent medial occipitoparietal areas, from the superior temporal motion-sensitive complex centered on the middle temporal area (MT), and from ventral stream-associated areas. Feedback from the posterior parietal cortex and other association areas accounts for Ͻ10% of the connections. These results do not support the hypothesis that DM is specifically associated with a medial subcircuit of the dorsal stream, important for visuomotor integration. Instead, they suggest an early-stage visual-processing node capable of contributing across cortical streams, much as V1 and V2 do. Thus, although DM may be important for providing visual inputs for guided body movements (which often depend on information contained in peripheral vision), this area is also likely to participate in other functions that require integration across wide expanses of visual space, such as perception of self-motion and contour completion.

The somatosensory cortex of adult mammals has been shown to have a capacity to reorganize when inputs are removed by cutting afferent nerves or amputating a part of the body. The area of cortex that would normally respond to stimulation of the missing input can become responsive to inputs from other parts of the body surface. Although a few animals have been studied with repeat recording, no attempt has been made to follow the time-course of changes at cortical loci and the immediate effects of a small amputation have not been reported. We have followed the changes in response in the primary somatosensory cortex in the flying-fox following amputation of the single exposed digit on the forelimb. Immediately after amputation, neurons in the area of cortex receiving inputs from the missing digit were not silent but responded to stimulation of adjoining regions of the digit, hand, arm and wing. In the week following amputation, the enlarged receptive fields shrank until they covered only the skin around the amputation wound. The immediate response is interpreted as a removal of inhibition and the subsequent shrinking of the field may be due to re-establishment of the inhibitory balance in the affected cortex and its inputs.

The representation of the visual field in visual areas of the dorsolateral, lateral, and ventral cortices was studied by means of extracellular recordings and fluorescent tracer injections in anaesthetised marmoset monkeys. Two areas, forming mirror-symmetrical representations of the contralateral visual field, were found rostral to the second visual area (V2). These were termed the ventrolateral posterior (VLP) and the ventrolateral anterior (VLA) areas. In both areas, the representation of the lower quadrant is located dorsally, between the foveal representation of V2 and the middle temporal crescent (MTc), whereas the representation of the upper quadrant is located ventrally, in the supratentorial cortex. A representation of the vertical meridian forms the common border of areas VLP and VLA, whereas the horizontal meridian is represented both at the caudal border of area VLP (with V2) and at the rostral border of area VLA (with multiple extrastriate areas). The foveal representations of areas VLP and VLA are continuous with that of V2, being located at the lateral edge of the hemisphere. The topographic and laminar patterns of projections from dorsolateral and ventral cortices to the primary (V1) and dorsomedial (DM) visual areas both support the present definition of the borders of areas VLP and VLA. These results argue against a separation between dorsolateral and ventral extrastriate areas and provide clues for the likely homologies between extrastriate areas of different species.