Second somatic sensory cortical area (SII) in a prosimian primate, Galago crassicaudatus.

The cortex adjacent to and along the upper bank of the lateral sulcus (UB-LS) of a prosimian primate, Galago crassicaudatus, was explored to determine the topographical representation of low-threshold cutaneous inputs to this region. The somatic sensory projections to this cortex were considered homologous to those defined in other species as the second somatosensory cortical area (SII). Multiple and single neuron recordings were obtained with tungsten microelectrodes in animals anesthetized with sodium pentobarbital or ketamine hydrochloride; receptive fields were determined by means of manually applied tactile stimuli. The area of SII was located approximately 1-1.5 mm rostral to the posterior limit of LS, extended rostrally ∼4 mm, and occupied nearly all of the upper bank of the sulcus throughout this region. Receptive fields (RFs) in SII were primarily contralateral except for some bilateral input in the cortex representing portions of the trunk, head, and face. The boundaries of RFs were well defined, especially where recordings were located in the middle layers of the cortex. The distribution of RFs across SII was somatotopically organized into a single, relatively erect representation of the body that involved inputs from the face rostral and medial (superficially along the UB-LS) surrounding an enlarged forelimb area; the latter, in turn, lies rostral and medial to the hindlimb zone. Projections from the tail and sacrum are located furthest caudal and lateral (deeper along UB-LS). Separate regions that were devoted to the glabrous skin surfaces of the distal limbs formed the rostral and lateral boundaries of the distal fore- and hindlimb representations, respectively. In the zone for the glabrous surfaces of the forelimb digits, individual digits dominated discrete components of the SII map, especially medially where digit 1 was represented. The glabrous tip of digit 5 was represented caudal and lateral to the tip of digit 1. A similar radial to ulnar medial to lateral sequence was noted in the area representing the palm. Except for a possible medially located tre 1 zone in the hindlimb representation, separated representations for the glabrous skin of individual toes were not noted. The dorsal hairy surfaces of the digits and toes were, respectively, amalgamated within the representations for the dorsal surfaces of the hand and foot. In these regions, which were found superficial and slightly caudal to their respective glabrous zones, some RFs were found that were devoted only to the distal extremities, but most RFs included more proximal portions of the hand or foot dorsum. The boundary between SII and SI was located just medial to the LS and in neighboring representations for the top of the head and most medial aspects of the ophthalmic and maxillary projections from the face. The cytoarchitectonic appearance of SII in Galago resembles that of other primates (Jones and Burton: J. Comp. Neurol. 168:197-248, '76) and of cortex previously described as prokoniocortex (Sanides and Krishnamurti: J. Hirnforschung. 9: 226–252, '67). SII was also shown to have corticocortical connections with SI that follow matching topographical components in both cortical areas. The thalamocortical connections for SII were shown to arise from the following nuclei: ventroposterior lateral (VPL), ventroposterior inferior, posterior nucleus (PO), and central lateral (CL). A portion of the area in VPL labeled from injections in SII also sends connections to SI. The connections from VPL to SII appear to be topographically organized.