Event-related potentials and equivalent current dipoles in silent speech using a vowel and a word

Event-related potentials (ERPs) were recorded from six subjects who were prompted by one of two light-emitting diodes to perform one of three speech tasks: (1) silently speaking a vowel /a/, (2) silently speaking a word /asagao/ (the name of a Japanese flower with four syllables), and (3) withholding silent speech. The ERPs were averaged across subjects for each of the three speech tasks. The average ERPs in silent speech of a vowel and a word after subtracting those without silent speech showed three differences: a smaller positive difference at almost all scalp electrodes peaking at 200 ms latency (P200); a positive difference peaking at the parietal scalp electrode Pz at 400 ms latency (P400); and a negative difference in the frontal scalp area peaking at the scalp electrode Fz at 580 ms latency (N580). The peak amplitude of the difference ERPs for the word and those for the vowel differed significantly for N580, the former being larger than the latter, but not for P200 and P400. No significant hemispheric difference was observed for these peaks. Equivalent current dipoles were obtained for one subject, who showed the largest negative difference, by the dipole tracing method using the ERPs for the vowel and those for the word at the latencies when the frontal negative difference was observed. The dipoles were located at the cingulate gyrus and near the supplementary motor area whose functions are considered to be related to attention or motor planning and may be involved in the present task.