Neural encoding in the human brainstem relevant to the pitch of complex tones.

Psychoacoustic studies have shown that complex tones containing resolved harmonics evoke stronger pitches than complex tones with only unresolved harmonics. Also, unresolved harmonics presented in alternating sine and cosine (ALT) phase produce a doubling of pitch. We examine here whether the temporal pattern of phase-locked neural activity reflected in the scalp recorded human frequency following response (FFR) preserves information relevant to pitch strength, and to the doubling of pitch for ALT stimuli. Results revealed stronger neural periodicity strength for resolved stimuli, although the effect of resolvability was weak compared to the effect observed behaviorally; autocorrelation functions and FFR spectra suggest a different pattern of phase-locked neural activity for ALT stimuli with resolved and unresolved harmonics consistent with the doubling of pitch observed in our behavioral estimates; and the temporal pattern of neural activity underlying pitch encoding appears to be similar at the auditory nerve (auditory nerve model response) and the rostral brainstem level (FFR). These findings suggest that the phase-locked neural activity reflected in the scalp recorded FFR preserves neural information relevant to pitch that could serve as an electrophysiological correlate of the behavioral pitch measure. The scalp recorded FFR may provide for a non-invasive analytic tool to evaluate neural encoding of complex sounds in humans.