Neurophysiological representation of speech fundamental frequency and formant structure in the auditory system using frequency-following responses

Neural encoding of sounds starts from the auditory nerve and moves on to the auditory brainstem. What is less defined is how the brainstem responds to complex stimuli. The current study quantified brainstem encoding of fundamental frequencies and first formant amplitude of the spoken syllable /da/ using periodic frequency-following responses. Auditory brainstem responses to spoken syllables in both ears were monaurally presented under quiet conditions for 48 normal adults (25 female) with a mean age of 22.7 ± 2.05 years. The magnitudes of the sustained periodic frequency-following responses (FFR) were assessed by fundamental frequency, first formant amplitude, and higher frequency harmonics of the first formant. Measures of the FFR components of brainstem response to the spoken syllables were robustly obtained. Responses occurred earlier in right ear presentation than those in left, approaching significance for the FFR second peak (p =. 004). A significant difference in interpeak latency of DE was observed (p =. 019), but not for EF interpeak latency. Right ear presentation evoked greater spectral amplitude for F0, F1, and high frequency harmonic ranges than the left ear, but there were no significant differences between ears. Overall, waveform morphology for both sides was quite similar. It is evident that response elements reflecting F0 and F1 are symmetrically processed in the brainstem, suggesting the pathway is not primed for the prosodic elements of speech.