Synchronization between overt speech envelope and EEG oscillations during imagined speech.

Neural oscillations synchronize with the periodicity of external stimuli such as the rhythm of the speech amplitude envelope. This synchronization induces a speech-specific, replicable neural phase pattern across trials and enables perceived speech to be classified. In this study, we hypothesized that neural oscillations during articulatory imagination of speech could also synchronize with the rhythm of speech imagery. To validate the hypothesis, after replacing the imagined speech with overt speech due to the physically unobservable nature of imagined speech, we investigated (1) whether the EEG-based regressed speech envelopes correlate with the overt speech envelope and (2) whether EEG during the imagined speech can classify speech stimuli with different envelopes. The variability of the duration of the imagined speech across trials was corrected using dynamic time warping. The classification was based on the distance between a test data and a template waveform of each class. Results showed a significant correlation between the EEG-based regressed envelope and the overt speech envelope. The average classification accuracy was 38.5%, which is significantly above the rate of chance (33.3%). These results demonstrate the synchronization between EEG during the imagined speech and the envelope of the overt counterpart.