Effect of reverberation on the directional sensitivity of auditory neurons : peripheral factors

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student submitted PDF version of thesis. Page 41 blank.Includes bibliographical references (p. 39-40).Reverberation poses a challenge for theories of sound localization due to the interaction between the direct sound and the various acoustic reflections. These reflections corrupt binaural cues available to the receiver, resulting in a degradation of directional information available in the acoustic stimulus. Despite this interaction, directionally-sensitive neural responses in the auditory midbrain have been shown to be more robust to reverberation than predicted by a binaural model based on the long- term cross-correlation of the two ear-input signals (Devore et al., 2009, Neuron 63(1), pp 123-134). To determine the extent to which this robustness is central or peripheral in origin, and to quantitatively investigate whether peripheral adaptation contributes to this robustness, we recorded auditory nerve (AN) responses to tokens of noise with varying levels of simulated reverberation. We found many qualitatively similar trends in AN responses as have been previously observed in the midbrain, suggesting a peripheral origin of robust directional coding. In particular, we found that degradation of directional coding in the AN due to reverberation is stronger at high frequencies, and that this degradation is limited near the stimulus onset. We also show that peripheral adaptation plays a positive role in increasing robustness of directional representation in the presence of reverberation. We also investigated the nature of the frequency dependence observed in the degradation of directional coding due to reverberation. Based on our experimental results and the results of an auditory model, we argue that in addition to being affected by AN fibers' synchrony to stimulus fine structure, the frequency dependence is also produced by temporal structure of the reverberant room response.by Andrew H. Schwartz.S.M