Department of Physiology and Biophysics, University of Colorado School of Medicine, Aurora, CO 80045, USA.
Adv Exp Med Biol. 2013;787:273-82. doi: 10.1007/978-1-4614-1590-9_31.
For over a century, the Duplex theory has posited that low- and high-frequency sounds are localized using two different acoustical cues, interaural time (ITDs) and level (ILDs) differences, respectively. Psychophysical data have generally supported the theory for pure tones. Anatomically, ITDs and ILDs are separately encoded in two parallel brainstem pathways. Acoustically ILDs are a function of location and frequency such that lower and higher frequencies exhibit smaller and larger ILDs, respectively. It is well established that neurons throughout the auditory neuraxis encode high-frequency ILDs. Acoustically, low-frequency ILDs are negligible (∼1–2 dB); however, humans are still sensitive to them and physiological studies often report low-frequency ILD-sensitive neurons. These latter findings are at odds with the Duplex theory. We suggest that these discrepancies arise from an inadequate characterization of the acoustical environment. We hypothesize that low-frequency ILDs become large and useful when sources are located near the head. We tested this hypothesis by making measurements of the ILDs in chinchillas as a function of source distance and the sensitivity to ILDs in 103 neurons in the inferior colliculus (IC). The ILD sensitivity of IC neurons was found to be frequency independent even though far-field acoustical ILDs were frequency dependent. However, as source distance was decreased, the magnitudes of low-frequency ILDs increased. Using information theoretic methods, we demonstrate that a population of IC neurons can encode the full range of acoustic ILDs across frequency that would be experienced as a joint function of source location and distance.
一个多世纪以来，双工理论假设低频和高频声音分别使用两种不同的声学线索，即耳间时间（ITD）和水平（ILD）差异来定位。心理物理数据通常支持纯音的理论。从解剖学上讲，ITD 和 ILD 分别在两个平行的脑干途径中进行编码。从声学上讲，ILD 是位置和频率的函数，因此较低和较高的频率分别具有较小和较大的 ILD。已经确定听觉神经轴上的神经元都可以编码高频 ILD。从声学上讲，低频 ILD 可以忽略不计（约 1-2 dB）；但是，人类仍然对此敏感，生理研究经常报告低频 ILD 敏感神经元。这些后一种发现与双工理论不一致。我们认为，这些差异是由于对声学环境的描述不充分引起的。我们假设，当声源位于头部附近时，低频 ILD 会变得很大且有用。我们通过测量在 103 个下丘脑中的神经元的 ILD 灵敏度来测试此假设，这些神经元的位置与声源距离和 ILD 灵敏度有关。即使远场声学 ILD 是频率相关的，但是发现下丘脑中的神经元的 ILD 灵敏度与频率无关。但是，随着声源距离的减小，低频 ILD 的幅度增加。使用信息论方法，我们证明了一群下丘脑中的神经元可以对整个频率范围内的声学 ILD 进行编码，这些 ILD 将作为声源位置和距离的联合函数来体验。
