Reduced gamma-band coherence to distorted feedback during speech when what you say is not what you hear

Communication between the frontal lobes, where speech is generated, and the temporal lobes, where it is perceived, may occur through the action of an efference copy/corollary discharge mechanism that prepares the temporal lobes for the expected sound. We suggest that coherence of EEG in gamma-band between frontal and temporal lobes may reflect the successful action of such a mechanism. We tested the hypothesis that there would be a disruption of gamma-band coherence when the expected auditory consequence of speech does not match the auditory experience.

EEG was recorded from 21 healthy adult subjects as they uttered the sound [a:] (Talking condition) and as they heard these recorded sounds played back (Listening condition). As they spoke, subjects heard real-time feedback of the sounds that were: (1) pitch-shifted down one semi-tone, (2) pitch-shifted down one-half of a semi-tone, or (3) not pitch-shifted (veridical), each in separate runs. Event-related gamma coherence between frontal and temporal sites was calculated relative to the onset of the sound, as it was being spoken during Talking, and as it was being played back during Listening.

Frontal-temporal gamma-band coherence spanning 33-43 Hz was greater during Talking than Listening and greater when speech was veridical than when it was distorted a whole semi-tone.

Gamma-band fronto-temporal synchrony may reflect a "binding of expectation with experience." Disruption of this synchrony may provide feedback to the frontal lobes, particularly regions subserving vocalization, to implement sensorimotor adaptations to either adjust motor programs for speech production in the short run, or to reorganize expectations in the long run.