Objective measurement of physiological signal-to-noise gain in the brainstem response to a synthetic vowel

Objective: This work investigates auditory speech processing in normal listeners through measurement of brainstem responses to the synthetic vowel /a/. Methods: The vowel is presented in quiet and in continuous white noise with different signal-to-noise ratios (SNR) of + 5, 0, - 5, and - 10 dB. Results: In the presence of noise, transient response waves V and A are delayed when compared to those evoked in quiet, whereas the amplitude of wave V and the steepness of the slope between waves V and A are strongly reduced. The spectral component of the steady-state evoked response corresponding to the fundamental frequency (F0) of the vowel shows significantly greater amplitude and local SNR in the less severe noise conditions compared to the quiet condition. Such increases of the amplitude and SNR were not observed for the spectral component corresponding to the first formant of the vowel (F1). Conclusions: Results suggest that, at F0, both local noise suppression and signal enhancement contribute to the SNR gain. There is suppression of local noise near F1, but no signal enhancement. Significance: The physiological SNR gain was estimated to be approximately + 12 dB at both F0 and F1, as stimulus SNR was reduced from + 5 to - 10 dB. (C) 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.