Changes in excitability of the human motor cortex face area by observing speech

Introduction: The human mirror neuron system (MNS) plays an important role in acqusition of new motor skills. When one observes a particular movement performed by another person and imitates this movement, this area is activated and the motor cortex representation of this movement is also activated. In humans, the MNS is involved in observing not only limb movement but also speech. In the present study, we examined changes in human motor cortex excitability by observation of speech using transcranial magnetic stimulation (TMS). Methods: Twelve healthy right handed subjects participated in this study. TMS was applied to the face area and hand area of the left motor cortex, and we recorded motor evoked potential (MEP) and short-interval intracortical inhibition (SICI) from subjects' right orbicularis oris (OO) and first dorsal interosseous (FDI) muscles. In the main experiment, these measures were compared during viewing speech, fixating a small point, listening to speech, listening to non-verbal sounds, and viewing and listening to noise. In control experiment 1, we investigated to which extent changes in MEP and SICI were truly caused by viewing speech rather than non-speech lip conditions; lip smacking and lip resting conditions. Finally, in control experiment 2 we investigated to which extent changes in MEP and SICI were influenced by the difficulty of the speech conditions; viewing slow or fast speech conditions, and listening to speech with or without background noise were compared. For statistical analysis, one way rmANOVAs with task conditions as within-subject effect were performed. Post hoc testing was done using Fisher's Planned Least Significant Difference test. Results: MEPs of OO muscle increased only during viewing and listening to speech but not in any of the other conditions. The MEP increase was directly related to task difficulty. SICI was strongly facilitated only during viewing speech and listening to speech in noisy conditions. No significant MEP or SICI changes were observed in the FDI muscle. Conclusion: When subjects watch/listen to speech of another person, activation of the MNS results in a specific increase in excitability of motor cortical representations of the lip but not the hand. In addition, the high attentional load during viewing speech and listening to speech in noise may explain the enhancement of inhibitory mechanisms in motor cortex to finely tune motor cortical output to the lip musculature.