Area 3a: topographic organization and cortical connections in marmoset monkeys.

The functional organization of area 3a, a cortical field proposed to be involved in somato-motor-vestibular integration, has never been described for any primate. In the present investigation, the topographic organization and connections of area 3a were examined in marmosets using electrophysiological recording and anatomical tracing techniques. Multi-unit neuronal activity was recorded at a number of closely spaced sites; receptive fields (RFs) for neurons were determined, and the optimal stimulus was identified. In all cases, neurons in area 3a responded to the stimulation of deep receptors on the contralateral body. The representation of the body in area 3a was from the toes and foot, to the hindlimb, trunk, forelimb, hand and face in a mediolateral progression. In all cases electrophysiological results were related to myeloarchitecture, and the map in area 3a was found to be coextensive with a strip of lightly to moderately myelinated cortex just rostral to the darkly myelinated 3b. To examine the cortical connections of area 3a, injections of anatomical tracers were made into electrophysiologically identified body part representations. Area 3a has dense intrinsic connections and receives substantial inputs from the primary motor cortex (M1), the supplementary motor area (SMA), areas 1 and 2, the second somatosensory area (S2), and areas in posterior parietal cortex (PP). The connections of area 3a indicate that integration of cortical representations of body parts occurs both within area 3a and between area 3a and other somatosensory and motor areas. In addition, there are differential patterns of interconnections between behaviorally relevant body part representations of area 3a, such as the forelimb, compared to other body part representations (hindlimb/ trunk), especially with 'higher order' cortical fields. This suggests that 3a may be an important component in a network that generates a common frame of reference for hand and eye coordinated reaching tasks.