Sensitivity to interaural time differences across sound frequency: models of auditory-brainstem neurons
Brughera, Andrew Robert
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Normal-hearing listeners can locate sound sources, using binaural cues for azimuth angle. These binaural differences in the timing and intensity of sound arriving at the two ears, interaural time differences (ITDs) and interaural intensity differences (IIDs), also support selective listening in multi-talker environments. Auditory-brainstem neurons of the medial superior olive (MSO) and lateral superior olive (LSO) encode ITD in the envelope of sound (ITDENV) and in the temporal fine structure of low-frequency sound (ITDTFS); LSO neurons encode IID. Bilateral-cochlear-implant (bCI) listeners generally receive only IID and ITDENV. Experimental bCI pulse-bursts overcome adaptation, and convey electrical ITDTFS. Improving the understanding of mechanisms for ITD sensitivity can help bCI developers convey acoustic ITDTFS. In this dissertation, models for auditory-brainstem neurons are developed that explain human ability to detect small differences in ITD, as neuronal and MSO population mechanisms. Promoting binaural-coincidence detection and limiting backpropagation, model MSO ion-channels set resting potentials that reproduce dendritic and somatic KLT activation, somatic Na+ inactivation, and a lower amount of axonal Na+ inactivation. Sensitivity to ITDTFS in moderately fast and very fast model MSO neurons collectively match physiological data from 150 to 2000 Hz. The best-ITD (the ITD of highest spike rate) can be made contralateral-leading, by contralateral inhibition of moderate speed, or by asymmetric axon location, leveraging dendritic filtering. Leveraging standard binaural-display models, neuronal populations based on these model MSO neurons match normal-hearing human discrimination thresholds for ITDTFS in sine tones from 39 to 1500 Hz. Adaptation before binaural interaction helps model MSO neurons glimpse the ITDTFS of sound direct from a source, before reflected sound arrives from different directions. With inputs from adapting model spherical bushy cells, a moderately fast model MSO neuron reproduces in vivo responses to amplitude-modulated binaural beats, with a frequency-dependent emphasis of rising vs. peak sound-pressure for ITDTFS encoding, which reflects human ITD detection and reverberation times in outdoor environments. Distinct populations of model LSO neurons, spanning the range of electrical membrane impedance as a function of frequency in LSO neurons, collectively reflect discrimination thresholds for ITDENV in transposed tones across carrier frequency (4-10 kHz) and modulation rate (32-800 Hz).
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