Features of cortically evoked swallowing in the awake primate (Macaca fascicularis)

Although the cerebral cortex has been implicated in the control of swallowi ng, the output organization of the cortical swallowing representation, and features of cortically evoked swallowing, remain unclear. The present study defined the output features of the primate "cortical swallowing representa tion" with intracortical microstimulation (ICMS) applied within the lateral sensorimotor cortex. In four hemispheres of two awake monkeys, microelectr ode penetrations were made at less than or equal to 1-mm intervals, initial ly within the face primary motor cortex (face-MI), and subsequently within the cortical regions immediately rostral, lateral, and caudal to MI. Two IC MS pulse trains [35-ms train, 0.2-ms pulses at 333 Hz, less than or equal t o 30 mu A (short train stimulus, T/S); 3- to 4-s train, 0.2-ms pulses at 50 Hi, less than or equal to 60 mu A (continuous stimulus, C/S)] were applied at less than or equal to 500-mu m intervals along each microelectrode pene tration to a depth of 8-10 mm, and electromyographic (EMG) activity was rec orded simultaneously from various orofacial and laryngeal muscles. Evoked o rofacial movements, including swallowing, were verified by EMG analysis, an d T/S and C/S movement thresholds were determined. Effects of varying ICMS intensity on swallow-related EMG properties were examined by applying supra threshold C/S at selected intracortical sites. EMG patterns of swallows evo ked from various cortical regions were compared with those of natural swall ows recorded as the monkeys swallowed liquid and solid material. Results in dicated that swallowing was evoked by C/S at similar to 20% of 1,569 intrac ortical sites where ICMS elicited an orofacial motor response in both hemis pheres of the two monkeys, typically at C/S intensities less than or equal to 30 mu A. In contrast, swallowing was not evoked by T/S in either monkey. Swallowing was evoked from four cortical regions: the ICMS-defined face-MI , the face primary somatosensory cortex (face-SI), the region lateral and a nterior to face-MI corresponding to the cortical masticatory area (CMA), an d an area >5 mm deep to the cortical surface corresponding to both the whit e matter underlying the CMA and the frontal operculum; EMG patterns of swal lows elicited from these four cortical regions showed some statistically si gnificant differences. Whereas swallowing ONLY was evoked at some sites, pa rticularly within the deep cortical area, swallowing was more frequently ev oked together with other orofacial responses including rhythmic jaw movemen ts. Increasing ICMS intensity increased the magnitude, and decreased the la tency, of the swallow-related EMG burst in the genioglossus muscle at some sites. These findings suggest that a number of distinct cortical foci may p articipate in the initiation and modulation of the swallowing synergy as we ll as in integrating the swallow within the masticatory sequence.