Neural correlates and mechanisms of sound localization in everyday reverberant settings

Abstract

Nearly all listening environments-indoors and outdoors alike-are full of boundary surfaces (e.g., walls, trees, and rocks) that produce acoustic reflections. These reflections interfere with the direct sound arriving at a listener's ears, distorting the binaural cues for sound localization. Yet, human listeners have little difficulty localizing sounds in most settings. This thesis addresses fundamental questions regarding the neural basis of sound localization in everyday reverberant environments. In the first set of experiments, we investigate the effects of reverberation on the directional sensitivity of low-frequency auditory neurons sensitive to interaural time differences (ITD), the principal cue for localizing sound containing low frequency energy. Because reverberant energy builds up over time, the source location is represented relatively faithfully during the early portion of a sound, but this representation becomes increasingly degraded later in the stimulus. We show that the directional sensitivity of ITD-sensitive neurons in the auditory midbrain of anesthetized cats and awake rabbits follows a similar time course. However, the tendency of neurons to fire preferentially at the onset of a stimulus results in more robust directional sensitivity than expected, suggesting a simple mechanism for improving directional sensitivity in reverberation. To probe the role of temporal response dynamics, we use a conditioning paradigm to systematically alter temporal response patterns of single neurons. Results suggest that making temporal response patterns less onset-dominated typically leads to poorer directional sensitivity in reverberation. In parallel behavioral experiments, we show that human lateralization judgments are consistent with predictions from a population rate model for decoding the observed midbrain responses, suggesting a subcortical origin for robust sound localization in reverberant environments. In the second part of the thesis we examine the effects of reverberation on directional sensitivity of neurons across the tonotopic axis in the awake rabbit auditory midbrain. We find that reverberation degrades the directional sensitivity of single neurons, although the amount of degradation depends on the characteristic frequency and the type of binaural cues available. When ITD is the only available directional cue, low frequency neurons sensitive to ITD in the fine-time structure maintain better directional sensitivity in reverberation than high frequency neurons sensitive to ITD in the envelope. On the other hand, when both ITD and interaural level differences (ILD) cues are available, directional sensitivity is comparable throughout the tonotopic axis, suggesting that, at high frequencies, ILDs provide better directional information than envelope ITDs in reverberation. These findings can account for results from human psychophysical studies of spatial hearing in reverberant environments. This thesis marks fundamental progress towards elucidating the neural basis for spatial hearing in everyday settings. Overall, our results suggest that the information contained in the rate responses of neurons in the auditory midbrain is sufficient to account for human sound localization in reverberant environments.