Takahashi T T
Institute of Neuroscience, University of Oregon, Eugene 97403.
J Exp Biol. 1989 Sep;146:307-22. doi: 10.1242/jeb.146.1.307.
The barn owl's auditory system computes interaural differences in time and amplitude and derives from them the horizontal and vertical coordinates of the sound source, respectively. Within the external nucleus of its inferior colliculus are auditory neurones, called 'space-specific neurones', that have spatial receptive fields. To activate a space-specific neurone, a sound must originate from a circumscribed region of space, or, if the sounds are delivered to each ear separately, using earphones, the stimuli must have the combination of interaural time and amplitude difference that simulates a sound broadcast from their receptive field. The sound-localization cues are processed in parallel, non-overlapping pathways extending from the cochlear nuclei to the subdivision of the inferior colliculus that innervates the space-specific neurones. Processing in the time pathway involves the coding of monaural phase angle, the derivation of sensitivity for interaural phase difference, and the calculation of interaural time difference (ITD) from interaural phase difference. The last process involves groups of neurones in the inferior colliculus whose collective firing signals a unique ITD, even though the activity of each constituent neurone signals multiple ITDs. The projections of these ensembles to the space-specific neurone endow the latter with a selectivity for ITD. Processing in the amplitude channel, about which less is known, initially involves an inhibitory process that sharpens the directional sensitivity of neurones in a lateral lemniscal nucleus. The inhibition is mediated by a commissural projection from the same lemniscal nucleus of the opposite side. At higher levels of the auditory system, neurones that are tuned to a limited range of interaural amplitude differences are found. It is proposed that at these higher stages, interaural amplitude difference, like ITD, is coded amidst an ensemble of neurones.
仓鸮的听觉系统会计算双耳在时间和振幅上的差异，并分别从中得出声源的水平和垂直坐标。在其下丘的外侧核中有被称为“空间特异性神经元”的听觉神经元，它们具有空间感受野。要激活一个空间特异性神经元，声音必须来自空间中的一个限定区域，或者，如果使用耳机将声音分别输送到每只耳朵，那么刺激必须具有模拟从其感受野发出的声音的双耳时间和振幅差异的组合。声音定位线索在从耳蜗核延伸到支配空间特异性神经元的下丘亚区的平行、不重叠的通路中进行处理。时间通路中的处理涉及单耳相位角的编码、双耳相位差敏感度的推导，以及从双耳相位差计算双耳时间差（ITD）。最后一个过程涉及下丘中的神经元群，其集体放电信号表示一个独特的ITD，尽管每个组成神经元的活动信号表示多个ITD。这些神经元群向空间特异性神经元的投射赋予了后者对ITD的选择性。在振幅通道中的处理，人们了解得较少，最初涉及一个抑制过程，该过程会增强外侧丘系核中神经元的方向敏感性。这种抑制是由来自对侧同一丘系核的连合投射介导的。在听觉系统的更高层次上，发现了调谐到有限范围的双耳振幅差异的神经元。有人提出，在这些更高的阶段，双耳振幅差异与ITD一样，是在一群神经元中进行编码的。
