1. The goal of this study was to quantitatively characterize the response properties of somatosensory and multisensory neurons in cortical area 7b (or PF) of monkeys that were behaviorally trained to perform an appetitive tolerance-escape task. Particular emphasis was given to characterizing nociceptive thermal responses and correlating such responses to thermal pain tolerance as measured by escape frequency. 2. A total of 244 neurons that responded to somatosensory stimulation alone or to both somatosensory and visual stimulation (multisensory) were isolated and studied in the trigeminal region of cortical area 7b. Thirty neurons responded only to visual stimulation. Thermoreceptive neurons formed similar to 13% (31 of 244) of the neurons that had somatosensory response properties. Thermal nociceptive neurons made up similar to 9% (21 of 244) of the neurons that had somatosensory response properties or similar to 68% (21 of 31) of the neurons that had thermoreceptive response properties. Thermal nociceptive neurons responded either exclusively to noxious thermal stimuli (high-threshold thermoreceptive, HTT) or differentially to nonnoxious and noxious thermal stimuli (wide-range thermoreceptive, WRT). Multimodal HTT neurons had nonnociceptive (low-threshold mechanoreceptive, LTM) and/or nociceptive (nociceptive-specific, wide-dynamic-range) mechanical receptive fields, whereas multimodal WRT neurons had only nonnociceptive (LTM) mechanical receptive fields. Thermal nonnociceptive neurons (low-threshold thermoreceptive, LTT) made up similar to 3% (8 of 244) of the neurons that had somatosensory properties or similar to 26% (8 of 31) of the neurons that were thermoreceptive. The background discharge of two thermoreceptive neurons (6%, 2 of 31) was inhibited by innocuous thermal stimulation. 3. Thermal nociceptive neurons (HTT and WRT) were functionally differentiated by statistical analyses into subpopulations that did encode (HTT-EN, WRT-EN) and did not encode (HTT-NE, WRT-NE) the magnitude of noxious thermal stimulus intensities. The mean slopes and median regression coefficients for the stimulus-response (S-R) functions of HTT-EN and WRT-EN neurons, respectively, were significantly greater than those for the S-R functions of HTT-NE and WRT-NE neurons. In contrast to HTT-NE and WRT-NE neurons, HTT-EN and WRT-EN neurons reliably encoded the magnitude of noxious thermal intensity by grading their mean discharge frequency. 4. The S-R functions of HTT-EN and WRT-EN neurons, unlike those of HTT-NE and WRT-NE neurons, closely approximated stimulus intensity-escape frequency functions. The S-R function of an individual HTT-EN or WRT-EN neuron, in contrast to the S-R function of an HTT-NE or WRT-NE neuron, was found to be significantly correlated to the behavioral S-R function from each respective monkey. 5. Some HTT and WRT neurons (7 of 21) were multisensory and responded to stimulation of spatially aligned visual and cutaneous receptive fields. Threatening or novel visuosensory stimuli that approached the face along a trajectory aligned with the most sensitive portion of the cutaneous receptive field evoked the highest mean discharge frequency. Visuosensory responses were often sustained by holding the visual targets close to the cutaneous receptive field. The maximum peak discharge frequencies in response to noxious thermal and visual stimulation were approximately equal. 6. A total of 36 task-related neurons were either inhibited or excited during performance of the appetitive tolerance-escape task without thermal stimulation applied to the face. Inhibition of activity in some task-related neurons (11 of 17 tested) was reversed by innocuous or noxious heating of the face. 7. Histological reconstruction of recording sites in the area 7b revealed no somatotopic representation of the trigeminal nerve subdivisions, no visuotopic representation of visual space, and no functional segregation of cell types (i.e., HTT, WRT, LTT). 8. The sources of afferent nociceptive input to the first and second somatosensory cortices and area 7b as well as the response properties of nociceptive neurons in these cortical areas are discussed with respect to current concepts of serial and parallel processing of somatosensory information in the primate cortex. Converging lines of evidence from both clinical and experimental studies support a role for area 7b in nociception and pain perception and in spatially directed attention. 9. A synthesis of evidence from many fields of study indicates that area 7b is an important site for multisensory convergence and integration of somatosensory and visual inputs. The functional and behavioral significance of multisensory integration in area 7b is discussed with a special emphasis on nociceptive information processing.
