Hierarchical somesthetic processing of tongue inputs in the postcentral somatosensory cortex of conscious macaque monkeys.

The representation of the oral structures in the lateralmost part of the postcentral somatosensory cortex in conscious macaque monkeys was studied by recording the activities of single neurons. A total of 104 penetrations were made in the oral regions of six hemispheres in four animals and 2,292 neurons were isolated. The characteristics of the receptive fields (RF) of 1,598 neurons were identified. Of them, 513 neurons (area 3b, 196; area 1, 104; area 2, 213) along 44 penetrations responded to mechanical stimulation of the tongue (tongue neurons). The relative incidence of tongue neurons that had bilateral RFs increased gradually (bilateral integration) on moving caudally from area 3b to area 2. There was also a tendency for the RFs on the tongue to expand in the anteroposterior axis of the tongue (anteroposterior integration). Furthermore, the relative incidence of tongue neurons with composite RFs covering both the tongue and other surrounding oral structures was significantly higher in area 2 than in areas 3b and 1 (interstructural integration). As a result of the bilateral, anteroposterior and interstructural integration, the extent of the RFs of tongue neurons increased progressively from area 3b to area 2. We therefore concluded that hierarchical somatosensory processing, which has been established in the postcentral somatosensory cortex representing other body parts, is also present in the oral representation. We speculate that the hierarchical scheme in the oral representation might be a prerequisite neural process for the oral stereognosis that eventually takes place in the association cortices.