The interaural level difference (ILD) is a sound localization cue first computed in the lateral superior olive (LSO) by comparing the loudness of sounds between the two ears. In the auditory cortex, one class of neurons is excited by contralateral but not ipsilateral monaural sounds. These "EO" neurons prefer ILDs where contralateral sounds are louder than ipsilateral sounds. Another class, the "PB" neurons, are unresponsive to monaural sounds but respond predominantly to binaural ILDs, when both ears receive simultaneous sounds of roughly equal loudness (0 ILD). Behavioral studies show that ILD sensitivity is invariant to increasing sound levels. However, in the LSO, ILD response functions shift towards the excitatory ear as sound level increases, indicating level-dependence. Thus, changes in firing rate can indicate either a change in sound location or sound level, or both. This suggests a transformation in level-sensitivity between the LSO and the perception of sound sources, yet the location of this transformation remains unknown. I performed recordings in the auditory cortex of the rat to test whether neurons were invariant to overall sound level. I found that with increasing sound levels, ILD responses were level-dependent, suggesting that level invariance of ILD sensitivity is not present in the rat auditory cortex. In general, neurons follow one of two processing strategies. The tuning of cortical cells typically follows the "inheritance strategy", such that the spiking output of the cell matches that of the excitatory synaptic input. However, cortical tuning can be modified by inhibition in the "local processing strategy". In this case, neurons are prevented from spiking at non-preferred stimuli by inhibition that overwhelms excitation. The tuning strategy of cortical neurons to ILD remains unknown. I performed whole-cell recordings in the anesthetized rat and compared the spiking output with synaptic inputs to ILDs within the same neurons. I found that the PB neurons showed evidence of the local processing strategy, which is a novel role for cortical inhibition, whereas the EO neurons utilized the inheritance strategy. This result suggests that an auditory cortical circuit computes sensitivity for midline ILDs. This dissertation includes previously published/unpublished co-authored material.

This paper explores how vision affects spatial audio perception in virtual reality. We created four virtual environments with different reverb and room sizes, and recorded binaural clicks in each one. We conducted two experiments: one where participants judged the audio-visual match, and another where they pointed to the click direction. We found that vision influences spatial audio perception and that congruent audio-visual cues improve accuracy. We suggest some implications for virtual reality design and evaluation.

Spatial hearing is crucial in real life but deteriorates in participants with severe sensorineural hearing loss or single-sided deafness. This ability can potentially be improved with a unilateral cochlear implant (CI). The present study investigated measures of sound localization in participants with single-sided deafness provided with a CI. Sound localization was measured separately at eight loudspeaker positions (4°, 30°, 60°, and 90°) on the CI side and on the normal-hearing side. Low- and high-frequency noise bursts were used in the tests to investigate possible differences in the processing of interaural time and level differences. Data were compared to normal-hearing adults aged between 20 and 83. In addition, the benefit of the CI in speech understanding in noise was compared to the localization ability. Fifteen out of 18 participants were able to localize signals on the CI side and on the normal-hearing side, although performance was highly variable across participants. Three participants always pointed to the normal-hearing side, irrespective of the location of the signal. The comparison with control data showed that participants had particular difficulties localizing sounds at frontal locations and on the CI side. In contrast to most previous results, participants were able to localize low-frequency signals, although they localized high-frequency signals more accurately. Speech understanding in noise was better with the CI compared to testing without CI, but only at a position where the CI also improved sound localization. Our data suggest that a CI can, to a large extent, restore localization in participants with single-sided deafness. Difficulties may remain at frontal locations and on the CI side. However, speech understanding in noise improves when wearing the CI. The treatment with a CI in these participants might provide real-world benefits, such as improved orientation in traffic and speech understanding in difficult listening situations.

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Sound localization is a vital aspect of hearing for safe navigation of everyday environments. It is also an important factor in speech intelligibility. This ability is facilitated by the interaural level difference (ILD) cue, which arises from binaural hearing: a sound will be more intense at the nearer ear than the farther. In a hearing-impaired listener, this binaural cue may not be available for use and localization may be diminished. While conventional, bilateral, wide dynamic range compression (WDRC) hearing aids distort the interaural level difference by independently altering sound intensities in each ear, wirelessly-linked devices have been suggested to benefit this task by matching amplification in order to preserve ILD. However, this technology has been shown to have varying degrees of success in aiding speech intelligibility and sound localization. As hearing impairment has wide-ranging adverse impacts to physical and mental health, social activity, and cognition, the task of localization improvement must be urgently addressed. Toward this end, neural modelling techniques are used to determine neural representations of ILD cues for linked and unlinked bilateral WDRC hearing aids. Findings suggest that wirelessly-linked WDRC is preferable over unlinked hearing aids or unaided, hearing-impaired listening, although parameters for optimal benefit are dependent on sound level, frequency content, and preceding sounds. / Thesis / Master of Applied Science (MASc)