Subject-averaged time-frequency power results.

(a) The time-frequency representation (TFR) of the event-related EEG signals (speech stimulation /a/, /e/, /i/, /o/, or /u/) over all subjects on the right and left anterior auditory fields. TFRs were plotted for the frequency range of 4 to 40 Hz, 0.2 to 0.8 s after each stimulus. One can observe high power activation in the low frequency band (especially in the delta, theta, and alpha band) between 0.3 and 0.6 s regardless of the sound stimulation. (b) Time-frequency regions with significant differences after the Bonferroni correction (p < 0.01 on the ANOVA test) are plotted. The color scale represents the F-values and non-significant regions are colored in white. Note that most of the brain responses between 0.2 and 0.8 s after the stimuli showed distinct neural responses across each vowel sound.