In the mammalian auditory system, perception of the azimuthal location of a sound is associated with disparities in the timing and level of a sound reaching the two ears: the interaural time difference (ITD) and the interaural level difference (ILD). ITD and ILD information are first encoded in the brainstem nuclei, the medial superior olive (MSO) and the lateral superior olive (LSO), respectively. In the past, it has been assumed that ITD and ILD sensitivity in the brainstem nuclei result from timing and level differences in activities of bilateral afferent inputs. However, several experimental observations suggest that cellular factors may also contribute to encoding ITDs and ILDs in the MSO and LSO. These observations include (1) the rate-ITD sensitivity of an MSO cell depends on sound level (Goldberg and Brown, 1969), which is likely related to level-dependent inhibition; (2) Synaptic inhibition can alter best ITDs in the MSO (Brand et al., 2002); (3) LSO cells show distinct chopper discharge patterns in response to tone-burst stimuli (Tsuchitani and Johnson, 1985), which is likely related to specific membrane properties.;In this thesis, a single-neuron modeling approach is used to provide quantitative explanations for these three observations. Simulations of the level dependence of the ITD sensitivity in the MSO indicate that inhibition adjusts the net excitation input level and allows a cell to maintain a robust ITD-tuning to sound level. Simulations of the shift of the best ITD by inhibition show that the interplay between depolarizing sodium currents and hyperpolarizing inhibitory currents affects the ITD-tuning of an MSO cell. Simulations of the chopper discharge pattern in the LSO indicate that LSO cells are heterogeneous in their membrane properties, and that this heterogeneity leads to differences in their rate-ILD responses and in their sensitivities to envelope-modulated stimuli. Together, the model results lead to the conclusion that ITD and ILD-dependent responses in the MSO and LSO are influenced by their synaptic and membrane properties, which may allow dynamic tuning of the auditory system in natural acoustic environments. Further, model results make testable predictions, which upon experimental testing will enrich our understanding of the neural basis of sound localization.
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机译:在哺乳动物的听觉系统中,对声音方位角位置的感知与到达两只耳朵的声音的时间和水平的差异相关:耳间时间差(ITD)和耳间水平差(ILD)。 ITD和ILD信息首先分别编码在脑干核,内侧上橄榄(MSO)和外侧上橄榄(LSO)中。过去,已经假定脑干核中的ITD和ILD敏感性是由于双边传入输入的活动的时间和水平差异所致。但是,一些实验观察表明,细胞因子也可能有助于在MSO和LSO中编码ITD和ILD。这些观察结果包括:(1)MSO细胞的ITD速率敏感性取决于声级(Goldberg and Brown,1969),这可能与水平依赖性抑制有关; (2)突触抑制可以改变MSO中的最佳ITD(Brand等,2002); (3)LSO细胞表现出独特的斩波放电模式(Tsuchitani and Johnson,1985),可能与特定的膜特性有关。(Tsuchitani and Johnson,1985);本文采用单神经元建模方法进行定量分析。对这三个观察结果的解释。对MSO中ITD灵敏度的水平依赖性的仿真表明,抑制作用会调节净激励输入水平,并使单元能够将鲁棒的ITD调谐保持在声音水平。通过抑制对最佳ITD转移的模拟显示,去极化钠电流和超极化抑制电流之间的相互作用会影响MSO细胞的ITD调节。 LSO中的斩波放电模式的模拟表明LSO细胞的膜特性是异质的,并且这种异质性会导致ILD反应速率和对包膜调制刺激的敏感性不同。总之,模型结果得出结论,即MSO和LSO中ITD和ILD依赖性响应受其突触和膜特性的影响,这可能允许在自然声学环境中动态调整听觉系统。此外,模型结果可以进行可预测的预测,通过实验测试可以丰富我们对声音定位的神经基础的理解。
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