NEURONAL-ACTIVITY IN THE 2ND SOMATOSENSORY CORTEX OF MONKEYS (MACACA-MULATTA) DURING ACTIVE TOUCH OF GRATINGS

1. In penetrations made into the upper bank of the lateral sulcus in t wo monkeys (Macaca mulatta), cells were isolated from the second somat osensory cortex (SII). During single-cell recordings, animals performe d an active touch task in which they rubbed their fingertips over pair s of gratings differing in groove width and indicated which was the sm oother surface. Hand motion and downward applied force were measured a nd recorded during these strokes. 2. In this survey, 151 penetrations provided observations on 352 cells that responded to passive stimulati on of the digits or during performance of the active touch task. Consi stent with previous reports, receptive fields (RFs) in SII were large, often multidigit, and frequently included a portion or all of the han d and occasionally the arm. Modality was determined for 92 of 127 full y characterized cells, and included 70 cutaneous, 5 deep, 11 Pacinian corpuscle, and 6 joint cells. Characteristic of SII, modality could no t be defined in 35 cells that were unresponsive to passive stimulation or whose responses varied widely over time. 3. Response properties of a subgroup of 79 cells in SII resembled those previously studied in t he primary somatosensory cortex (SI) and ventroposterior lateral nucle us of the thalamus (VPL) using identical procedures. Correlation analy sis revealed that 29 of these cells, like a portion of cells in SI, re sponded to changes in groove width independent of force or velocity. T his selectivity could be considered a form of feature specificity. 4. In contrast to SI and VPL, transient responses to the fingertips conta cting small elevated metal bars, which demarcated the beginning, middl e, and end of strokes across the gratings, were seen in a majority of SII cells (109/127). During contact with bars, 89 cells displayed exci tatory responses and 20 cells showed suppressed activity. Twelve cells , which responded to bars in isolation from gratings, provided a possi ble example of increased stimulus selectivity. 5. Passive stimulation failed to activate 16 cells that responded, in some cases differential ly to gratings or force, during the task. Responses of nine other cell s demonstrated task-dependent modulation in the form of response reduc tion or enhancement during selected portions of the stroke. In these s ame cells, response changes did not occur under comparable stimulus co nditions in other portions of the stroke that differed only in behavio ral context. These types of selective response modulations, not noted in our previous studies of VPL or SI, suggest that mechanisms regulati ng sensory inputs may affect SII. 6. Greater stimulus selectivity, lar ger RFs, loss of modality specificity, and selective modulation observ ed in SII are discussed in reference to the possibility that they char acterize SII as being ''upstream'' in proposed somatosensory hierarchi es.