Department of Communication Sciences and Disorders, University of South Florida, Tampa, Florida.
Department of Chemical and Biomedical Engineering, University of South Florida, Tampa, Florida.
J Neurophysiol. 2019 Aug 1;122(2):737-748. doi: 10.1152/jn.00090.2019. Epub 2019 Jun 26.
Cortical encoding of auditory space relies on two major peripheral cues, interaural time difference (ITD) and interaural level difference (ILD) of the sounds arriving at a listener's ears. In much of the precortical auditory pathway, ITD and ILD cues are processed independently, and it is assumed that cue integration is a higher order process. However, there remains debate on how ITDs and ILDs are encoded in the cortex and whether they share a common mechanism. The present study used electroencephalography (EEG) to measure evoked cortical potentials from narrowband noise stimuli with imposed binaural cue changes. Previous studies have similarly tested ITD shifts to demonstrate that neural populations broadly favor one spatial hemifield over the other, which is consistent with an opponent-channel model that computes the relative activity between broadly tuned neural populations. However, it is still a matter of debate whether the same coding scheme applies to ILDs and, if so, whether processing the two binaural cues is distributed across similar regions of the cortex. The results indicate that ITD and ILD cues have similar neural signatures with respect to the monotonic responses to shift magnitude; however, the direction of the shift did not elicit responses equally across cues. Specifically, ITD shifts evoked greater responses for outward than inward shifts, independently of the spatial hemifield of the shift, whereas ILD-shift responses were dependent on the hemifield in which the shift occurred. Active cortical structures showed only minor overlap between responses to cues, suggesting the two are not represented by the same pathway. Interaural time differences (ITDs) and interaural level differences (ILDs) are critical to locating auditory sources in the horizontal plane. The higher order perceptual feature of auditory space is thought to be encoded together by these binaural differences, yet evidence of their integration in cortex remains elusive. Although present results show some common effects between the two cues, key differences were observed that are not consistent with an ITD-like opponent-channel process for ILD encoding.
听觉空间的皮质编码依赖于到达听众耳朵的声音的两个主要的外围线索，即两耳时间差（ITD）和两耳强度差（ILD）。在大部分的皮质前听觉通路中，ITD 和 ILD 线索是独立处理的，并且假设线索整合是一个更高阶的过程。然而，关于皮质中如何编码 ITD 和 ILD，以及它们是否共享一个共同的机制，仍然存在争议。本研究使用脑电图（EEG）来测量窄带噪声刺激引起的诱发性皮质电位，这些刺激施加了双耳线索变化。以前的研究也同样测试了 ITD 变化，以证明神经元群体普遍偏向于另一个空间半球，这与一种计算广泛调谐神经元群体之间相对活动的对侧通道模型是一致的。然而，同样的编码方案是否适用于 ILD，以及如果是这样，处理这两个双耳线索是否分布在皮质的相似区域，这仍然是一个争论的问题。结果表明，ITD 和 ILD 线索在单调响应的幅度变化方面具有相似的神经特征；然而，线索的方向并不平等地引起所有线索的反应。具体来说，ITD 变化引起的反应对于向外的变化大于向内的变化，而与变化的空间半球无关，而 ILD 变化的反应则取决于变化发生的半球。活性皮质结构对线索的反应只有很小的重叠，这表明两者不是由相同的通路表示的。两耳时间差（ITD）和两耳强度差（ILD）对水平面上的听觉源定位至关重要。听觉空间的高阶感知特征被认为是由这些双耳差异共同编码的，但皮质中它们的整合证据仍然难以捉摸。尽管目前的结果显示出两种线索之间存在一些共同的影响，但观察到的关键差异与 ILD 编码的 ITD 样对侧通道过程不一致。
