Joris P X, Yin T C
Department of Neurophysiology, University of Wisconsin Medical School, Madison, Wisconsin 53706, USA.
J Neurophysiol. 1998 Jan;79(1):253-69. doi: 10.1152/jn.1998.79.1.253.
Binaural cues for spatial localization of complex high-frequency sounds are interaural level and time differences (ILDs and ITDs). We previously showed that cells in the lateral superior olive (LSO) are sensitive to ITDs in the envelope of sinusoidally amplitude-modulated (AM) signals up to a modulation frequency of only approximately 800 Hz. To understand the limitations in this ITD-sensitivity, we here compare responses to monaural modulation in LSO and its input pathways, derived from cochlear nucleus and medial nucleus of the trapezoid body. These pathways have marked functional and morphological specializations, suggestive of adaptations for timing. Afferent cell populations were identified on the basis of electrophysiological signatures, and for each population, average firing rate and synchronization to AM tones were compared with auditory-nerve fibers and LSO cells. Except for an increase in modulation gain in some subpopulations, synchronization of LSO afferents was very similar to that in auditory nerve fibers in its dependency on sound pressure level (SPL), modulation depth, and modulation frequency. Distributions of cutoff frequencies of modulation transfer functions were largely coextensive with the distribution in auditory nerve. Group delays, measured from the phase of the response modulation as a function of modulation frequency, showed an orderly dependence on characteristic frequency and cell type and little dependence on SPL. Similar responses were obtained to a modulated broadband carrier. Compared with their afferents, LSO cells synchronized to monaurally modulated stimuli with a higher gain but often over a narrower range of modulation frequencies. Considering the scatter in afferent and LSO cell populations, ipsi- and contralateral responses were well matched in cutoff frequency and magnitude of delays. In contrast to their afferents, LSO cells show a decrease in average firing rate at high modulation frequencies. We conclude that the restricted modulation frequency range over which LSO cells show ITD-sensitivity does not result from loss of envelope information along the afferent pathway but is due to convergence or postsynaptic effects at the level of the LSO. The faithful transmission of envelope phase-locking in LSO afferents is consistent with their physiological and morphological adaptations, but these adaptations are not commensurate with the rather small effects of physiological ITDs reported previously, especially when compared with effects of ILDs. We suggest that these adaptations have evolved to allow a comparison of instantaneous amplitude fluctuations at the two ears rather than to extract interaural timing information per se.
用于复杂高频声音空间定位的双耳线索是耳间声级差和时间差（ILDs和ITDs）。我们之前表明，外侧上橄榄核（LSO）中的细胞对高达约800Hz调制频率的正弦调幅（AM）信号包络中的ITDs敏感。为了理解这种ITD敏感性的局限性，我们在此比较LSO及其输入通路（源自耳蜗核和梯形体内侧核）对单耳调制的反应。这些通路具有明显的功能和形态学特化，表明对时间的适应性。根据电生理特征识别传入细胞群体，并将每个群体的平均放电率和与AM音调的同步性与听神经纤维和LSO细胞进行比较。除了一些亚群的调制增益增加外，LSO传入神经的同步性在对声压级（SPL）、调制深度和调制频率的依赖性方面与听神经纤维非常相似。调制传递函数截止频率的分布在很大程度上与听神经中的分布共存。从响应调制的相位作为调制频率的函数测量的群延迟显示出对特征频率和细胞类型的有序依赖性，而对SPL的依赖性很小。对调制宽带载波也获得了类似的反应。与它们的传入神经相比，LSO细胞以更高的增益同步到单耳调制刺激，但通常在更窄的调制频率范围内。考虑到传入神经和LSO细胞群体中的分散性，同侧和对侧反应在截止频率和延迟幅度方面匹配良好。与它们的传入神经相反，LSO细胞在高调制频率下平均放电率降低。我们得出结论，LSO细胞表现出ITD敏感性的受限调制频率范围不是由于沿传入通路的包络信息丢失，而是由于LSO水平的汇聚或突触后效应。LSO传入神经中包络锁相的忠实传递与其生理和形态学适应性一致，但这些适应性与先前报道的生理ITDs的相当小的效应不相称，特别是与ILDs的效应相比。我们认为，这些适应性的进化是为了允许比较两只耳朵的瞬时幅度波动，而不是提取耳间时间信息本身。
