Effects of reversible bilateral inactivation of face primary motor cortex on mastication and swallowing

The effects of reversible cold block-induced bilateral inactivation of the face primary motor cortex (face MI) on mastication and swallowing were studied in awake monkeys. A warm or cold alcohol-water solution was pumped through thermodes placed bilaterally on the dura overlying the intracortical microstimulation-defined face MI while the monkey chewed and swallowed food during pre-cool (thermode temperature 37 degrees C), cold block (4 degrees C), and post-cool (37 degrees C) sessions. Vertical and horizontal jaw movements and electromyographic (EMG) activity of several muscles were monitored. Each masticatory sequence was divided into three masticatory phases (i.e. food preparatory, rhythmic chewing, preswallow). The cold block markedly affected the ability of the monkey to carry out mastication although it did not prevent mastication from occurring. The masticatory deficit was characterized by a significant elongation of the total masticatory time, including in particular elongation of the food preparatory phase. The coordination of the jaw- and tongue-muscle activities was severely disrupted during the food preparatory phase. Face MI cold block also significantly affected the duration of some masticatory-related EMG activities and had some limited effects on the temporal relationships of the EMG activities during mastication. Although cold block significantly affected the duration and some EMG parameters of the preswallow phase, it had no significant effect on swallow duration or the EMG parameters during swallowing. These findings provide further evidence that the primate face MI plays a critical role in the regulation of mastication and that it plays a role in the preparation for swallowing.