Converging patterns of inputs from oral structures in the postcentral somatosensory cortex of conscious macaque monkeys.

Single neuronal activities were recorded in the oral region of the postcentral gyrus in conscious Japanese monkeys. Among 5,756 neurons isolated, receptive fields (RFs) and submodalities were identified in 1,502 neurons in area 3b, 970 in area 1, and 1,461 in area 2. The relative incidence of neurons that had bilateral RFs increased gradually upon moving caudally from area 3b to area 2 (bilateral integration). A total of 276 neurons had bimaxillary RFs covering both the maxillary and mandibular divisions of the trigeminal nerve, such as the upper and lower lips, upper and lower teeth, palate and tongue, or combinations thereof. There was also a tendency for the relative incidence of neurons with bimaxillary RFs to increase across the postcentral gyrus but with an abrupt change in area 2 (bimaxillary integration). A total of 382 neurons had composite RFs covering more than one of five oral structures: lip, cheek mucosa, teeth/gingiva, tongue, and palate. The relative incidence of neurons with composite RFs was significantly higher in area 2 than in areas 3b and 1 (interstructural integration). These results indicate that the convergence of inputs from oral structures proceeds in a hierarchical manner across the postcentral gyrus, but chiefly in area 2 for the bimaxillary and interstructural integrations. The relative incidence of neurons with composite RFs was higher among neurons associated with the teeth/gingiva or palate than among neurons associated with the tongue or lip in all three areas. We interpret this to mean that anatomical or functional differences between oral structures might be reflected in the converging patterns in the oral representation.