Postcentral neurons with covert receptive fields in conscious macaque monkeys: their selective responsiveness to simultaneous two-point stimuli applied to discrete oral portions.

The representation of the oral structures in the postcentral somatosensory cortex was studied in conscious macaque monkeys by recording the activity of single neurons. A total of 2,807 neurons were isolated in the oral regions of three hemispheres in two animals. Of these, 375 neurons (area 3a, 3; area 3b, 123; area 1, 99; area 2, 150) lacked an apparent receptive field (RF), and their relative frequency was significantly higher in area 2 (19%) than in more rostral areas (area 3a, 8%; area 3b, 10%; area 1, 12%). We tested the responsiveness of these neurons to stimuli applied simultaneously to two discrete, but functionally related, oral structures (interstructural two-point stimuli: iTPS). Neurons in areas 3a, 3b, and 1 that lacked an apparent RF were not responsive to iTPS. However, 35 neurons in area 2 responded stably to iTPS applied to either of the following sets of oral structures: the tongue and incisors (n=18), incisors and lip (n=9), lip and tongue (n=12), or upper and lower lips (n=8). Of them, 19 neurons were activated during self-movements such as tongue protrusion, lip licking, and food manipulation. The neurons selectively responsive to iTPS might detect converging inputs from different oral structures and play a pivotal role in detecting objects straddling different oral structures and the mutual contact of oral structures.