Electrocorticographic correlates of overt articulation of 44 English phonemes: intracranial recording in children with focal epilepsy.

ABSTRACT

OBJECTIVE: We determined the temporal-spatial patterns of electrocorticography (ECoG) signal modulation during overt articulation of 44 American English phonemes. METHODS: We studied two children with focal epilepsy who underwent extraoperative ECoG recording. Using animation movies, we delineated 'when' and 'where' gamma- (70-110 Hz) and low-frequency-band activities (10-30 Hz) were modulated during self-paced articulation. RESULTS: Regardless of the classes of phoneme articulated, gamma-augmentation initially involved a common site within the left inferior Rolandic area. Subsequently, gamma-augmentation and/or attenuation involved distinct sites within the left oral-sensorimotor area with a timing variable across phonemes. Finally, gamma-augmentation in a larynx-sensorimotor area took place uniformly at the onset of sound generation, and effectively distinguished voiced and voiceless phonemes. Gamma-attenuation involved the left inferior-frontal and superior-temporal regions simultaneously during articulation. Low-frequency band attenuation involved widespread regions including the frontal, temporal, and parietal regions. CONCLUSIONS: Our preliminary results support the notion that articulation of distinct phonemes recruits specific sensorimotor activation and deactivation. Gamma attenuation in the left inferior-frontal and superior-temporal regions may reflect transient functional suppression in these cortical regions during automatic, self-paced vocalization of phonemes containing no semantic or syntactic information. SIGNIFICANCE: Further studies are warranted to determine if measurement of event-related modulations of gamma-band activity, compared to that of the low-frequency-band, is more useful for decoding the underlying articulatory functions.