Functional Organization in the Orofacial Region of the Postcentral Somatosensory Cortex

The first somatosensory cortexiSIjof primates is located in the postcentral gyrus of the parietal lobe. The orofacial structures are represented most laterally in SI. The inspection of neuronal properties there should lead to understanding of the neural basis that underlie dexterous orofacial functions, such as speech, mastication, manipulation of objects, and oral stereognosis. In the orofacial representation of SI, a substantial number of neurons have unique receptive fields (RFs) resulting from the spatiotemporal integration of converging somesthetic inputs. The relative incidences of those unique neurons increase on moving caudally from area 3 towards area 2. A neural process that binds spatiotemporal information arising from functionally-related portions might enable the brain to monitor the movement of objects in contact with orofacial structures or the kinematic trajectory of orofacial structures themselves. On the other hand, the modular organization of the neocortex is a widely documented concept, in which neural connectivity, composed of nearby cortical neurons, is considered to be the functional unit of integration. From this viewpoint, studies are needed to compare physiological properties among neurons localized in a small region of the orofacial representation. As for the RF extension, a small but substantial proportion of the pairs of nearby neurons are associated with discrete but functionally-related oral portions of different structures. As for the temporal aspects, a study is now underway in our laboratory to reveal the temporal relation between the activities of nearby neurons during sustained natural stimuli.