The effect of reverberation on subcortical neural encoding of speech stimuli in normal-hearing adults

Abstract

There is limited objective research on the brainstem representation of speech in degraded listening environments. The aim of this study was to determine the effect of reverberation on brainstem neural encoding in six normal hearing individuals using the Frequency Following Response (FFR). The FFR was recorded in response to a /u/(F0=120 Hz, F1=360)stimulus in four different levels of reverberation(clean, mild: 0.6 RT, moderate: 0.8 RT, severe: 1.1 RT). FFR data was divided into two types, a qualitative data analysis and a quantitative data analysis. Qualitative indices of the neural response at each RT were provided through a visual analysis of the periodicity and root mean square (RMS) amplitude of the grand averaged waveforms obtained at each reverberation condition. FFT spectral analysis was utilized to measure the amplitudes of both the F0 and F1 components for all participants in each condition. As expected, as reverberation severity was increased, F0 encoding ability decreased. This trend was relatively consistent across participants, especially in the severe reverberation condition. In contrast, there was an unexpected increase in F1 encoding ability with increased reverberation. Results from this study suggests that reverberation, regardless of severity causes a decrease in F0 neural encoding ability. However, reverberation effects are less substantial for F1 encoding. These differences in neural encoding ability between F0 and F1 components appear to be related to differences in the encoding of these two frequency components at the physiological level of cochlea.