Mokri Yasamin, Worland Kate, Ford Mark, Rajan Ramesh
Department of Physiology, Monash University, Monash, Vic., 3800, Australia.
Ear Sciences Institute of Australia, Perth, WA, Australia.
Eur J Neurosci. 2015 Jul;42(1):1685-704. doi: 10.1111/ejn.12914. Epub 2015 May 6.
Humans can accurately localize sounds even in unfavourable signal-to-noise conditions. To investigate the neural mechanisms underlying this, we studied the effect of background wide-band noise on neural sensitivity to variations in interaural level difference (ILD), the predominant cue for sound localization in azimuth for high-frequency sounds, at the characteristic frequency of cells in rat inferior colliculus (IC). Binaural noise at high levels generally resulted in suppression of responses (55.8%), but at lower levels resulted in enhancement (34.8%) as well as suppression (30.3%). When recording conditions permitted, we then examined if any binaural noise effects were related to selective noise effects at each of the two ears, which we interpreted in light of well-known differences in input type (excitation and inhibition) from each ear shaping particular forms of ILD sensitivity in the IC. At high signal-to-noise ratios (SNR), in most ILD functions (41%), the effect of background noise appeared to be due to effects on inputs from both ears, while for a large percentage (35.8%) appeared to be accounted for by effects on excitatory input. However, as SNR decreased, change in excitation became the dominant contributor to the change due to binaural background noise (63.6%). These novel findings shed light on the IC neural mechanisms for sound localization in the presence of continuous background noise. They also suggest that some effects of background noise on encoding of sound location reported to be emergent in upstream auditory areas can also be observed at the level of the midbrain.
即使在不利的信噪比条件下，人类也能准确地定位声音。为了探究其背后的神经机制，我们研究了背景宽带噪声对大鼠下丘（IC）神经元特征频率处双耳声级差（ILD）变化的神经敏感性的影响，ILD是高频声音在水平方位定位的主要线索。高强度的双耳噪声通常会导致反应抑制（55.8%），但低强度时也会导致增强（34.8%）以及抑制（30.3%）。当记录条件允许时，我们接着检查双耳噪声效应是否与两只耳朵各自的选择性噪声效应有关，我们根据已知的每只耳朵输入类型（兴奋和抑制）的差异来解释这一点，这些差异塑造了IC中特定形式的ILD敏感性。在高信噪比（SNR）时，在大多数ILD功能中（41%），背景噪声的影响似乎是由于对来自两只耳朵的输入产生了作用，而对于很大一部分（35.8%）来说，似乎是由对兴奋性输入的作用导致的。然而，随着SNR降低，兴奋的变化成为双耳背景噪声导致变化的主要因素（63.6%）。这些新发现揭示了在存在连续背景噪声的情况下IC中声音定位的神经机制。它们还表明，一些据报道在上游听觉区域中出现的背景噪声对声音位置编码的影响，在中脑水平也能观察到。
