In the well-developed primate brain, the operculum has a unique structural feature that overlays the insula located within the lateral sulcus. Physiological and anatomical experiments in macaque monkeys have shown that the secondary somatosensory cortex (SII) lying in the parietal operculum (PO) is essentially engaged in the processing of somatosensory information ascending from wide range of bodily surface, whereas the postcentral primary somatosensory area processes rather localized information. Recent human brain imaging studies have revealed that SII receives various neural inputs from different sensory modalities suggesting multisensory integration and even other cognitive functions such as empathy. To explore the multisensory nature of SII at neuronal level, we recorded single-unit activity from SII and its surrounding PO cortices in awake macaque monkeys and examined how they respond to various visual and tactile stimuli. We recorded single unit activities from SII and the cortical regions surrounding SII from 8 hemispheres of 6 awake monkeys. Among a total of 1157 neurons which we could test for somatosensory RF identification, active movement trials and visual stimulation, we found 306 neurons responded to visual stimuli. Most of those visual neurons responded to complex visual stimuli such as stimulation to peripersonal space (n=123, 40.0%), observation of human action (n=88, 28.7%), and convergence type(n=10, 3.3%) followed by those responding to moving object stimulation outside the reach range of the monkey (n=71, 23.1%) and simple object presentation (n=9, 2.9%). Those visual neurons were distributed widely along the rostrocaudal axis together with neurons responding only to somatosensory stimuli. We could not find a specific region where visual neurons were clustered.
The neural properties responding to visual stimuli found in the present study were quite similar to those reported earlier in other cortical areas such as ventral premotor and intraparietal areas. These results suggest that the primate SII receives signals from more than one sensory cortices and governs multisensory integration, which may contribute to conducting attentive behaviors to the external world.