Postcentral neurons with covert receptive fields in conscious macaque monkeys: their selective responsiveness to...

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.