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

ObjectiveThis work investigates auditory speech processing in normal listeners through measurement of brainstem responses to the synthetic vowel /a/.MethodsThe vowel is presented in quiet and in continuous white noise with different signal-to-noise ratios (SNR) of +5, 0, −5, and −10 dB.ResultsIn 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).ConclusionsResults 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.SignificanceThe 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.

► A physiological signal-to-noise gain in the auditory processing of a vowel presented in noise can be quantified. ► Concurrent mechanisms of signal enhancement and noise suppression are at work in the auditory processing of a vowel presented in noise. ► Top-down active mechanisms are probably involved in achieving high positive signal-to-noise gains in normal listeners.