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.

No abstract

Extracellular recordings were made from single and multiple neurons in primary somatosensory cortex (area 3b) of macaque monkeys and flying foxes. When a small region of area 3b (or adjacent area 1) in the opposite hemisphere was cooled, thereby blocking activity that is normally transferred via the corpus callosum, larger receptive fields (RFs) were immediately unmasked for most neurons. RF expansion presumably reflects the expression of afferent inputs that are normally inhibited, suggesting that callosal inputs provide a source of tonic inhibition that contributes to the shaping of neuronal RFs. Quantitative analyses of single neuron responses revealed other effects that were consistent with a release from inhibition, such as increases in response magnitude to stimulation of points within the original RF and decreases in response latency. An unexpected finding was the reversal of these unmasking effects with extended periods of cooling: RFs returned to their original dimensions and within-field response magnitude decreased. In contrast to the initial effects, this reversal of disinhibition cannot be readily explained by an unmasking of previously unexpressed inputs. Any explanation for the reversal requires an increase in the efficacy of interneuron-mediated inhibition, and presumably occurs in response to ongoing, altered patterns of activity.

No abstract

The location and characteristics of the primary auditory cortex of the common marmoset, Callithrix jacchus jacchus, were determined in five anesthetized male adult animals by mapping the responses of cortical units and unit clusters to pure tone stimuli presented to the contralateral ear. The primary auditory cortex lies largely ventral to the lateral sulcus, the only major fissure on the lateral cortex of this smooth-brained primate, but in some animals it may extend significantly down the ventral bank of this sulcus. Responses are distributed such that low best frequencies are found rostroventrally whereas high best frequencies occur caudally. The disposition of frequency-band contours is fan-shaped, with contours separating low-frequency octaves nearly parallel to the lateral sulcus and high-frequency (greater than 8 kHz) contours perpendicular to that sulcus. Best frequencies range from 0.6 to 30 kHz across the primary field, but there is a disproportionate representation of the three octaves between 2 and 16 kHz. The most sensitive thresholds (as low as -2 dB SPL) are found between 7 and 9 kHz. The primary auditory cortex is similar in cytoarchitecture to that reported for the cat, showing a blurring of lamination in the middle layers (II-IV) and a preponderance of small cells in these merged layers, giving a highly granular appearance. The accessibility of the cochlear representation on the gyral surface makes the marmoset an attractive animal for studies of primate auditory cortex.