Mu wave suppression during the perception of meaningless syllables: EEG evidence of motor recruitment

Motor involvement in speech perception has been recently studied using a variety of techniques. In the current study, EEG measurements from Cz, C3 and C4 electrodes were used to examine the relative power of the mu rhythm (i.e., 8-13 Hz) in response to various audio-visual speech and non-speech stimuli, as suppression of these rhythms is considered an index of 'mirror neuron' (i.e., motor) activity. Fourteen adult native English speaking females watched and listened to nine audio-video stimuli clips assembled from three different auditory stimuli (speech, noise, and pure tone) combined with three different video stimuli (speech, noise, and kaleidoscope-made from scrambling an image from the visual speech). Relative to the noise-noise (baseline condition), all visual speech conditions resulted in significant levels of suppression, a finding that is consistent with previous reports of mirror activity to visual speech and mu suppression to 'biological' stimuli. None of the non-speech conditions or conditions in which speech was presented via audition only resulted in any significant suppression of the mu rhythm in this population. Thus, visual speech perception appears to be more closely associated with motor activity than acoustic speech perception. It is postulated that in this study, the processing demands incurred by the task were insufficient for inducing significant mu suppression via acoustic speech only. The findings are discussed in theoretical contexts of speech perception and the mirror system. We suggest that this technique may offer a cost-efficient, non-invasive technique for measuring motor activity during speech perception.