EFFECTS ON NONHUMAN PRIMATE MASTICATION OF REVERSIBLE INACTIVATION BYCOOLING OF THE FACE PRIMARY SOMATOSENSORY CORTEX

Rhythmical jaw movements can be evoked by intracortical microstimulati on within four physiologically defined regions, one of which is the pr imary face somatosensory cortex (face SI). It has been proposed that t hese regions may be involved in the selection and/or control of mastic atory patterns generated at the brainstem level. The aim here was to d etermine if mastication is affected by reversible, cooling-induced ina ctivation of the face SI. Two cranial chambers were chronically implan ted in two monkeys (Macaca fascicularis) to allow access bilaterally t o the face SI. A thermode was placed on the dura or pia overlying each ST that had been shown with micro-electrode recordings to receive int raoral inputs. A hot or cold alcohol-water solution was pumped through the thermodes while the monkey chewed a small piece of apple or a sul tana during precool (thermode temperature, 37 degrees C), cool (2-4 de grees C), and postcool (37 degrees C) conditions. Electromyographic (E MG) activity was recorded intramuscularly from the masseter, genioglos sus, and anterior digastric. Cooling of SI impaired rhythmical jaw and tongue movements and EMG activity associated with mastication in one monkey (H5), and modified the pattern of EMG activity in the other (H6 ). The total masticatory time (i.e., time taken for chewing and manipu lation of the bolus before swallowing) was increased. This was due pri ncipally to an increase in the oral transport time (i.e., time taken f or manipulation of bolus after chewing and before swallowing: monkey H 6, control, 2.7 sec; cool, 5.2 sec, p < 0.05); the bolus was manipulat ed by the tongue during this period before swallowing. Within the chew ing time (i.e., time during which chewing occurred), cooling resulted in a significant increase in anterior digastric muscle duration, a sig nificant delay in the onset of masseter EMG activity, and a significan t increase in the variance of genioglossus EMG duration. The data supp ort the view that the face SI plays a part in modulating the central p attern generator for mastication. (C) 1998 Elsevier Science Ltd. All r ights reserved.