Topographical distribution and functional properties of cortically induced rhythmical jaw movements in the monkey (Macaca fascicularis)

1. The lateral part of the pericentral cortex of both hemispheres in three awake monkeys was explored with intracortical microstimulation (ICMS) using short trains (T/S; 200-microseconds pulses at 333 Hz for 35 ms, less than or equal to microA) and long trains (C/S; 200-microseconds pulses at 50 Hz for 3 s, less than or equal to 60 microA). In both hemispheres of one of these monkeys, the responsiveness of single cortical neurons to stimulation of the orofacial region was tested at the same intracortical sites where ICMS was applied. 2. Movements were evoked from four physiologically defined cortical regions: the primary face motor cortex (MI), the primary face somatosensory cortex (SI), the principal part of the cortical masticatory area (CMAp) which was located in the precentral gyrus lateral to MI, and a deep part of the cortical masticatory area (CMAd) which was located in the inferior face of the frontal operculum. 3. Two types of cortically induced movements were observed: a single twitch movement and EMG activity of the orofacial muscles that was evoked by T/S at a short latency (10-45 ms) and rhythmical jaw movements (RJMs) which were only evoked by C/S. 4. RJMs were evoked at C/S frequencies ranging from 20 to 300 Hz. At movement threshold, the frequency of the cortically induced RJMs varied from 0.7 to 1.5 Hz and usually increased with the increase of C/S intensity up to 2 times movement threshold. The vertical amplitude of RJMs was also stimulus dependent, and at movement threshold it ranged from 3 to 9 mm. 5. The movement patterns of the cortically induced RJMs remained constant during the course of C/S but could be differentiated in the frontal plane into ipsilateral- (RJMi), vertical-(RJMv), and contralateral- (RJMc) directed movements. These three different patterns of RJMs were associated with different patterns of masticatory muscle activity. 6. Each cortical region contained many sites from which RJMs could be induced (so-called RJM sites). The RJMi sites were more numerous than RJMc sites in all regions except SI and were located anterolateral or lateral to the RJMc sites in each region; the RJMv sites were scattered throughout each cortical region. 7. In MI, C/S elicited RJMs from 94 intracortical sites from which short-latency twitch movements could also be evoked by T/S.(ABSTRACT TRUNCATED AT 400 WORDS)