HIGH-FREQUENCY OSCILLATIONS AS INDICATORS OF NEURAL CONTROL MECHANISMS IN HUMAN RESPIRATION, MASTICATION, AND SPEECH

Electromyograms (EMGs) were recorded from the right and left ventrolateral surfaces of the chest wall to study the activity of the diaphragm during three tasks: moderately deep breathing, speech production, and speechlike breathing in which subjects mimicked their speech breathing pattern but did not vocalize. In a second experimental protocol, activity of right and left masseter muscles was recorded in three conditions: chewing, rhythmic clenching, and speech. EMGs were analyzed in the frequency domain by computing the average spectrum of 128-ms intervals selected from the rising phase of the EMG in 30 repetitions of each task. Coherence functions between bilateral pairs of EMGs were computed and examined for significant coherence in the range of 20-230 Hz. In the deep breathing condition, significant coherence was present in two ranges, 20-60 Hz and 60-110 Hz. Oscillations in the 60- to 110-Hz range have long been associated with the operation of the primary respiratory pattern generator, and the present results replicate earlier reports of correlated oscillations in this range in bilateral recordings of human respiratory muscles during voluntarily controlled breathing. In speech and speechlike breathing, correlated oscillations in the 60- to 110-Hz range were significantly reduced. This result supports the hypothesis that the neural circuitry characterized by high-frequency oscillations (HFOs) in the 60- to 110-Hz range, possibly a central pattern generator (CPG), is not the primary source of drive to respiratory motoneuron pools in speech. During chewing, highly correlated oscillations were observed in right and left masseter activity. Significant coherence occurred in the range of 20-140 Hz, with very large peaks in the 20- to 60-Hz band. This result is interpreted in light of studies of rhythmic jaw movements elicited by cortical stimulation in experimental animals and studies of the reflex organization of the jaw-closing muscles. It is possible that highly correlated oscillations in the 20- to 60-Hz band arise from a central command system, such as a pattern generator, from reflexes, or from an interaction of central and reflex inputs. In most subjects, nonzero coherence in the 20- to 60-Hz band also was observed during clenching and speech; however, levels of coherence in the 20- to 60-Hz band were significantly reduced in these conditions compared with chewing. Taken together, the present results suggest that coherent HFOs observed in consistent frequency ranges across subjects are characteristic of human motor behaviors that are likely to involve pattern generation circuitry.