Plauška Andrius, van der Heijden Marcel, Borst J Gerard G
Department of Neuroscience, Erasmus MC, University Medical Center Rotterdam, 3000 CA, Rotterdam, The Netherlands.
Department of Neuroscience, Erasmus MC, University Medical Center Rotterdam, 3000 CA, Rotterdam, The Netherlands
J Neurosci. 2017 Jul 26;37(30):7278-7289. doi: 10.1523/JNEUROSCI.0233-17.2017. Epub 2017 Jun 28.
The relative arrival times of sounds at both ears constitute an important cue for localization of low-frequency sounds in the horizontal plane. The binaural neurons of the medial superior olive (MSO) act as coincidence detectors that fire when inputs from both ears arrive near simultaneously. Each principal neuron in the MSO is tuned to its own best interaural time difference (ITD), indicating the presence of an internal delay, a difference in the travel times from either ear to the MSO. According to the stereausis hypothesis, differences in wave propagation along the cochlea could provide the delays necessary for coincidence detection if the ipsilateral and contralateral inputs originated from different cochlear positions, with different frequency tuning. We therefore investigated the relation between interaural mismatches in frequency tuning and ITD tuning during loose-patch (juxtacellular) recordings from principal neurons of the MSO of anesthetized female gerbils. Cochlear delays can be bypassed by directly stimulating the auditory nerve; in agreement with the stereausis hypothesis, tuning for timing differences during bilateral electrical stimulation of the round windows differed markedly from ITD tuning in the same cells. Moreover, some neurons showed a frequency tuning mismatch that was sufficiently large to have a potential impact on ITD tuning. However, we did not find a correlation between frequency tuning mismatches and best ITDs. Our data thus suggest that axonal delays dominate ITD tuning. Neurons in the medial superior olive (MSO) play a unique role in sound localization because of their ability to compare the relative arrival time of low-frequency sounds at both ears. They fire maximally when the difference in sound arrival time exactly compensates for the internal delay: the difference in travel time from either ear to the MSO neuron. We tested whether differences in cochlear delay systematically contribute to the total travel time by comparing for individual MSO neurons the best difference in arrival times, as predicted from the frequency tuning for either ear, and the actual best difference. No systematic relation was observed, emphasizing the dominant contribution of axonal delays to the internal delay.
声音到达双耳的相对时间是在水平面定位低频声音的重要线索。内侧上橄榄核（MSO）的双耳神经元充当重合探测器，当来自双耳的输入几乎同时到达时就会放电。MSO中的每个主神经元都被调谐到其自身的最佳双耳时间差（ITD），这表明存在内部延迟，即从任一耳朵到MSO的传播时间差异。根据立体听觉假设，如果同侧和对侧输入源自不同的耳蜗位置且具有不同的频率调谐，那么沿耳蜗的波传播差异可以提供重合检测所需的延迟。因此，我们在对麻醉的雌性沙鼠的MSO主神经元进行松散膜片（近细胞）记录期间，研究了频率调谐中的双耳失配与ITD调谐之间的关系。通过直接刺激听神经可以绕过耳蜗延迟；与立体听觉假设一致，在双侧电刺激圆窗期间对时间差异的调谐与同一细胞中的ITD调谐明显不同。此外，一些神经元表现出频率调谐失配，其幅度足够大，可能会对ITD调谐产生潜在影响。然而，我们没有发现频率调谐失配与最佳ITD之间存在相关性。因此，我们的数据表明轴突延迟主导ITD调谐。内侧上橄榄核（MSO）中的神经元在声音定位中发挥着独特作用，因为它们能够比较低频声音到达双耳的相对时间。当声音到达时间的差异恰好补偿内部延迟（即从任一耳朵到MSO神经元的传播时间差异）时，它们会产生最大放电。我们通过比较单个MSO神经元的最佳到达时间差异（根据任一耳朵的频率调谐预测）和实际最佳差异，测试了耳蜗延迟差异是否系统地影响总传播时间。未观察到系统关系，这强调了轴突延迟对内部延迟的主要贡献。
