Brainstem correlates of concurrent speech identification in adverse listening conditions.

When two voices compete, listeners can segregate and identify concurrent speech sounds using pitch (fundamental frequency, F0) and timbre (harmonic) cues. Speech perception is also hindered by the signal-to-noise ratio (SNR). How clear and degraded concurrent speech sounds are represented at early, pre-attentive stages of the auditory system is not well understood. To this end, we measured scalp-recorded frequency-following responses (FFR) from the EEG while human listeners heard two concurrently presented, steady-state (time-invariant) vowels whose F0 differed by zero or four semitones (ST) presented diotically in either clean (no noise) or noise-degraded (+5dB SNR) conditions. Listeners also performed a speeded double vowel identification task in which they were required to identify both vowels correctly. Behavioral results showed that speech identification accuracy increased with F0 differences between vowels, and this perceptual F0 benefit was larger for clean compared to noise degraded (+5dB SNR) stimuli. Neurophysiological data demonstrated more robust FFR F0 amplitudes for single compared to double vowels and considerably weaker responses in noise. F0 amplitudes showed speech-on-speech masking effects, along with a non-linear constructive interference at 0ST, and suppression effects at 4ST. Correlations showed that FFR F0 amplitudes failed to predict listeners' identification accuracy. In contrast, FFR F1 amplitudes were associated with faster reaction times, although this correlation was limited to noise conditions. The limited number of brain-behavior associations suggests subcortical activity mainly reflects exogenous processing rather than perceptual correlates of concurrent speech perception. Collectively, our results demonstrate that FFRs reflect pre-attentive coding of concurrent auditory stimuli that only weakly predict the success of identifying concurrent speech.