FUNCTIONAL-PROPERTIES OF SINGLE NEURONS IN THE PRIMATE FACE PRIMARY SOMATOSENSORY CORTEX .1. RELATIONS WITH TRAINED OROFACIAL MOTOR BEHAVIORS

1. We have demonstrated recently that reversible, cooling-induced inac tivation of the face primary somatosensory cortex (SI) severely impair s the successful performance of a tongue-protrusion task but has relat ively minor effects on the performance of a biting task. In an attempt to establish a neuronal correlate for these different behavioral rela tions, the present study was initiated to document the mechanoreceptiv e field properties of a population of face SI neurons and their activi ty during the tongue-protrusion and biting tasks. 2. Within SI, the re presentation of the face was found immediately lateral to that of the hand, and there was a clear somatotopic pattern of organization within face SI: the periorbital or nose region was located most medially int he face SI, then followed laterally in sequence the representation of the upper lip, lower lip, and intraoral area. A mechanoreceptive fiel d (RF) was identified for 253 neurons, which included 162 ''lip RF'' n eurons receiving mechanosensitive afferent inputs from the upper lip, lower lip, or both; 72 ''tongue RF'' neurons that received mechanosens itive afferent inputs from the tongue; 11 ''periodontium RF'' neurons receiving periodontal inputs; and 8 neurons that received inputs from other orofacial regions. 3. Nearly all (249/253) of the face SI neuron s responded to light tactile stimuli, and most of them received contra lateral inputs (78%) and showed a rapidly adapting (RA) response to ta ctile stimulation (82%). There was no significant difference in the ra tio of slowly adapting (SA) to RA neurons in areas 3b and 1. 4. For 19 3 neurons studied in one or both of the orofacial tasks, 113 were foun d, on the basis of histological reconstruction, to be distributed in a rea 1, 61 in area 3b, and 19 in an area 2. 5. The firing rate of most tongue RF (79% of 56) neurons and lip RF (60% of 93) neurons tested wa s significantly altered during the tongue-protrusion task. Only some ( 14% of 36 tongue RF neurons and 34% of the 92 lip RF neurons tested) s howed a significant change in firing rate during the biting task. Thre e of 7 periodontium RF neurons studied in the tongue-protrusion task a ltered their firing rate and 5 of 10 altered their firing rate during the biting task. 6. Most of the 116 face SI neurons studied during bot h tasks exhibited a preferential relation to the tongue-protrusion tas k as distinct from the biting task, and non showed task-related activi ty during the biting task only. 7. The proportion of SA neurons that a ltered their firing rate during the tongue-protrusion task was signifi cantly higher than that of RA neurons. A significantly higher proporti on of the tongue-protrusion task-related neurons also occurred in area 1 than in area 3b. 8. Tongue and lip RF neurons exhibited a variety o f different neuronal activity patterns during different phases of the tongue-protrusion task; some of these neurons and periodontium RF neur ons also exhibited a variety of different neuronal activity patterns d uring the biting task. The findings suggest that these neurons may be concerned with the subtleties of control over the tongue-protrusion an d jaw-closing movements. The adaptation characteristics of a neuron co uld not predict its activity pattern during the tasks.