Mori K
Division of Biology, California Institute of Technology, Pasadena 91125, USA.
Hear Res. 1997 Sep;111(1-2):22-30. doi: 10.1016/s0378-5955(97)00090-7.
The barn owl uses the interaural time difference (ITD) to determine the azimuth of a sound source. Narrowband ITD-sensitive neurons cannot distinguish a given ITD from those that produce the same interaural phase difference (phase ambiguity). Neurons in the external nucleus of the inferior colliculus (ICx) resolve the ambiguity by gathering ITD information across many frequencies, thereby suppressing false responses (side peaks, SP) relative to the true ITD (the main peak, MP) in a response versus ITD curve. This process was quantitatively studied by comparing the ITD curve for a pair of tones presented simultaneously (two-tone curve) to the simple sum (predicted curve) of the individual ITD curves for the same tones presented separately. Sixteen of the 39 neurons tested did not show a significant difference in MP and SP responses between these curves (category I); 14 neurons showed significant SP suppression (category II). During iontophoretic application of bicuculline methiodide, a GABA(A) antagonist, most (n = 7/8) category II neurons lost nonlinear SP suppression and became linear, whereas category I neurons retained linear summation (n = 3/3). Thus, the nonlinear cross-frequency interaction of ITD responses in ICx neurons was mediated mostly by GABAergic inhibition, which enhanced SP suppression, and helped resolve phase ambiguity.
仓鸮利用双耳时间差（ITD）来确定声源的方位。对窄带ITD敏感的神经元无法将给定的ITD与产生相同双耳相位差（相位模糊）的ITD区分开来。下丘外侧核（ICx）中的神经元通过收集多个频率的ITD信息来解决这种模糊性，从而在响应与ITD曲线中抑制相对于真实ITD（主峰，MP）的错误响应（旁峰，SP）。通过将同时呈现的一对音调的ITD曲线（双音曲线）与单独呈现相同音调的各个ITD曲线的简单总和（预测曲线）进行比较，对这一过程进行了定量研究。在测试的39个神经元中，有16个在这些曲线之间的MP和SP响应上没有显示出显著差异（I类）；14个神经元表现出显著的SP抑制（II类）。在离子电渗法应用GABA(A)拮抗剂甲磺酸荷包牡丹碱期间，大多数（n = 7/8）II类神经元失去了非线性SP抑制并变得线性，而I类神经元保留了线性总和（n = 3/3）。因此，ICx神经元中ITD响应的非线性跨频率相互作用主要由GABA能抑制介导，这种抑制增强了SP抑制，并有助于解决相位模糊问题。
