Model parameters.

Having two ears enables us to localize sound sources by exploiting interaural time differences (ITDs) in sound arrival. Principal neurons of the medial superior olive (MSO) are sensitive to ITD, and each MSO neuron responds optimally to a best ITD (bITD). In many cells, especially those tuned to low sound frequencies, these bITDs correspond to ITDs for which the contralateral ear leads, and are often larger than the ecologically relevant range, defined by the ratio of the interaural distance and the speed of sound. Using in vivo recordings in gerbils, we found that shortly after hearing onset the bITDs were even more contralaterally leading than found in adult gerbils, and travel latencies for contralateral sound-evoked activity clearly exceeded those for ipsilateral sounds. During the following weeks, both these latencies and their interaural difference decreased. A computational model indicated that spike timing-dependent plasticity can underlie this fine-tuning. Our results suggest that MSO neurons start out with a strong predisposition toward contralateral sounds due to their longer neural travel latencies, but that, especially in high-frequency neurons, this predisposition is subsequently mitigated by differential developmental fine-tuning of the travel latencies.

双耳赋予我们利用声音到达的耳间时间差（interaural time differences, ITDs）定位声源的能力。内侧上橄榄核（medial superior olive, MSO）的主神经元对ITD具有敏感性，且每个MSO神经元均会对其最佳耳间时间差（best ITD, bITD）产生最优响应。在诸多细胞（尤其针对低声频率调谐的细胞）中，这些bITD对应于对侧耳领先的ITD，且通常超出由耳间距与声速之比所定义的生态学相关范围。我们通过沙鼠体内记录实验发现，在听觉发育初期，bITD的对侧领先程度较成年沙鼠更为显著，且对侧声音诱发活动的传导潜伏期显著长于同侧声音。在后续的数周内，上述两类潜伏期及其耳间差值均出现下降。计算模型表明，脉冲时序依赖可塑性可作为这一精细调控过程的基础。我们的研究结果提示，MSO神经元最初会因更长的神经传导潜伏期而表现出对侧声音偏好，但在高频神经元中，这种偏好会随后通过传导潜伏期的差异性发育精细调控得到削弱。