Cortical gamma-oscillations modulated by listening and overt repetition of phonemes

Both superior temporal gyrus and inferior Rolandic area have been reported to be involved in perception and production of speech in humans. Here, we determined how these cortical structures were activated by listening and subsequent overt articulation of syllables, by measuring event-related gamma-oscillations as quantitative measures of cortical activation. Fifteen subjects were presented an auditory syllable consisting of either "fee [fi:]," "faa [falpha:]," "hee [hi:]," or "haa [halpha:]," and were instructed to overtly repeat each given syllable. Gamma-oscillations in the superior temporal gyrus were highly augmented during syllable-presentation, least augmented at the onset of syllable-articulation, and again highly augmented following syllable-articulation. Gamma-oscillations were augmented in the inferior Rolandic area prior to and during syllable-articulation with the onset and peak occurring earlier in the left side. Subsets of the inferior Rolandic sites, more frequently on the left side, showed differential gamma-augmentation elicited by articulation of phoneme [f] more than [h] or phoneme [i:] more than [alpha:]. Our observations suggest that the superior temporal gyrus may be active when externally presented or articulated auditory stimuli are present, and may be minimally active when articulation is about to be initiated. Our novel observation of phoneme-specific differential gamma-augmentation in the inferior Rolandic area may be partially attributed to the mouth position during phoneme-articulation. Our observations support the hypothesis that positioning of the mouth to articulate phonemes is predominantly driven and/or monitored by the primary sensorimotor area on the left side.