Phase-locked cortical responses to a human speech sound and low-frequency tones in the monkey

Concurrent recordings of average evoked potentials (AEP) and multiple unit activity (MUA) in monkey primary cortex to the syllable /da/, low-frequency tones, and clicks were performed. The AEP in response to the syllable consisted of a periodic alternation superimposed upon slower phasic deflections. All components inverted across the superior temporal plane, indicating their auditory cortical origin. The periodic activity was phase-locked to the syllable's fundamental frequency at a latency of approximately 11 msec. MUA displayed a similar pattern of periodic activity, but with a shorter interval between stimulus and response peaks. This phase-locked MUA occurred only at regions of AEP polarity inversion. Phase-locked activity was also observed in the cortical AEP to 100 and 250 Hz, but not to 500 Hz tonal stimulation. MUA phase-locked to the stimulus frequency only occurred at 100 Hz. Both the periodic and slow components of the AEP were volume-conducted to the dorsal cortical surface. This finding suggests the possibility that similar cortical responses to speech sounds can be recorded from the human scalp.