The horizontal location of a sound source that contains low-frequency components is largely determined by the difference of arrival times of the sound reaching the two ears – the interaural time difference (ITD). The medial superior olive (MSO) in mammals is the primary central auditory site whose neurons are differentially responsive to ITDs. ITD sensitivity in the MSO is constructed via phase-locked inputs converging on coincidence-detecting neurons. Using a biophysical model of MSO, I show that the dynamic properties of the low-threshold potassium current increases the precision of both phase-locking and coincidence detection in response to realistic inputs. The pathways of input from each ear to the MSO are subject to differential internal delays, creating neurons tuned to different ITDs. Using data from single-unit recordings in gerbil MSO, I show evidence that slight frequency mismatches in the cochlear origin of inputs onto the MSO from each ear (cochlear disparity) contribute to internal delay, in addition to previously-studied delay mechanisms. Further, I show evidence that cochlear disparity produces MSO neurons that are not tuned to a consistent ITD across frequency (e.g., "trough-type" units). ITD tuning in MSO neurons was found to scale with sound level, even in cases where the firing rate decreased. I demonstrate that this observation is inconsistent with the current model of how inhibition contributes to internal delay. The distribution of internal delays across the MSO population has been proposed to be optimized for encoding sound azimuth by being organized as a spatial map across ITD (creating a place code) or broad tuning to one spatial channel (a slope code). I found no evidence for a spatial map of ITD in the gerbil MSO. The distribution of internal delays was not optimized for a place or slope code, although subpopulations existed which would be ideal for each code. Further, there were many units that would not be relevant to sound localization, but would be relevant to signal detection. MSO units were tuned to ITDs of contralateral-leading sounds at lower frequencies, similar to previously-published reports, however there were many ipsilaterally-tuned units at high phase-locking frequencies.
|Advisor:||Semple, Malcolm N.|
|Commitee:||Carr, Catherine E., Reyes, Alex D., Rinzel, John, Sanes, Dan H.|
|School:||New York University|
|Department:||Center for Neural Science|
|School Location:||United States -- New York|
|Source:||DAI-B 70/12, Dissertation Abstracts International|
|Keywords:||Cochlear disparity, Coincidence detection, Interaural time difference, Medial superior olive, Neural coding, Sound localization|
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