Modulation of the trigeminofacial pathway during syllabic speech

Abstract

The human orofacial system is richly endowed with low-threshold, slowly adapting mechanoreceptors that respond to self-generated movements and external loads. The functional linkage between these afferents and the recruitment of motor units in the lower face during the dynamics of speech is unknown. Mechanically evoked activity in the orbicularis oris muscles was studied in young human female adults (N=10) during a lip force recruitment task associated with the repetition of the nonsense speech utterance "ah-wah." This speech task involved the recruitment of perioral motor units against an elastic load. A skin contactor probe coupled to a servo-controlled linear motor delivered punctate ipsilateral mechanical inputs (25 ms duration, 1800 mum displacement) to the glabrous surface of the upper lip in order to index the modulation and specificity of the compound trigeminofacial response as a function of speech force recruitment threshold (Ft). Modulation of the early (Ft=0.2 N) and later (Ft=1.0 N) components of the evoked perioral response was found at the two force thresholds. Beginning at approximately 60 ms post-stimulus, a significant suppression response was found among lower lip EMG recording sites and its magnitude was greatest when the mechanical perturbation occurred during the early phase of lip force recruitment. Variation in the lip force trajectories was manifest by a greater difference in net interangle force associated with lip perturbations indexed to the early Ft. This was interpreted to reflect the operation of a feedforward mechanism which may play a more significant role during an evolving speech action. Thus, the application of servo-controlled mechanosensory inputs effectively indexed the excitability of the facial motor nucleus during production of a simple speech phrase. Future studies are needed to explore mechanisms of short-term adaptation and trigeminofacial modulation during propositional speech in health and disease.

Figure 1

R1/R2/S1 Trigeminofacial modulation NET response, Ft = 0.2N. Perioral reflex recorded in each quadrant of the orbicularis oris muscle. Each composite indicates the response to mechanical stimuli presented at the early force threshold across all subjects.

Figure 2

R1/R2/S1 Trigeminofacial modulation NET response, Ft = 1.0N. Perioral reflex recorded in each quadrant of the orbicularis oris muscle. Each composite indicates the response to mechanical stimuli presented at the mid-contraction force threshold across all subjects.

Figure 3

The pooled mean lip force trajectories for all 10 test subjects and associated 95 percent confidence intervals for each of the four conditions are shown graphically. Reference lines indicate trigger points for mechanical stimulation (STIM Conditions) and force threshold levels used for real-time waveform discrimination.

Figure 4

Net interangle lip force derived by subtracting pooled interangle force (NO STIM – control condition) from the pooled interangle force (STIM – experiment condition).

Figure 5

Suppression responses associated with early versus mid-contraction phase stimuli. Comparison of the suppression component observed in each muscle.

Figure 6

Lip force recruitment thresholds. Interangle lip force task representative of the two experimental conditions. The punctate mechanical stimulus was triggered using real-time waveform-threshold discrimination when the early (Ft = 0.2N) or mid-contraction (Ft = 1.0N) threshold event was met.