Euston David R, Takahashi Terry T
Institute of Neuroscience, University of Oregon, Eugene, Oregon 97403, USA.
J Neurosci. 2002 Jan 1;22(1):284-93. doi: 10.1523/JNEUROSCI.22-01-00284.2002.
Space-specific neurons in the owl's inferior colliculus have spatial receptive fields (RFs) computed from interaural time (ITD) and level (ILD) differences. Because of the shape of the owl's head, these cues vary with frequency in a manner specific for each location. We sought to determine the contribution of ILD to spatial selectivity. We measured the normal spatial receptive fields of space-specific neurons using virtual sound sources (i.e., noises filtered to simulate external sound sources, presented using headphones). The virtual-source filters were then altered so that ITD was fixed while frequency-specific ILDs varied according to location in the usual manner. The resulting "ILD-alone" RF typically revealed a horizontal band of excitation that included the normal RF. Above and below, the neurons were inhibited. Interestingly, the maxima of ILD-alone RFs were generally outside the normal RF, suggesting that space-specific neurons are not optimally tuned to the ILD spectrum occurring at the normal RF location. Congruously, frequency-specific ILD tuning, assessed with tones, better matched the ILDs at the peak of the ILD-alone RF than those at the peak of the normal RF. The firing evoked from the normal RF may thus reflect the balance of excitatory and inhibitory inputs needed to appropriately restrict the receptive field. Frequency-specific ILD tuning curves were combined with measured head-filtering characteristics to predict responses to the frequency-specific ILDs at each location. The predicted ILD-alone RFs, which are based on a simple sum of frequency-specific inputs, accounted for 56% of the variance in our measured ILD-alone RFs.
猫头鹰下丘中的空间特异性神经元具有根据双耳时间（ITD）和强度（ILD）差异计算得出的空间感受野（RFs）。由于猫头鹰头部的形状，这些线索随频率变化，且每个位置都有其特定的方式。我们试图确定ILD对空间选择性的贡献。我们使用虚拟声源（即经过滤波以模拟外部声源的噪声，通过耳机呈现）测量了空间特异性神经元的正常空间感受野。然后改变虚拟声源滤波器，使ITD固定，同时频率特异性ILD根据位置以通常方式变化。由此产生的“仅ILD”RF通常显示出一条水平的兴奋带，其中包括正常RF。在其上方和下方，神经元受到抑制。有趣的是，仅ILD的RF的最大值通常在正常RF之外，这表明空间特异性神经元并未针对正常RF位置出现的ILD频谱进行最佳调谐。相应地，用纯音评估的频率特异性ILD调谐与仅ILD的RF峰值处的ILD比与正常RF峰值处的ILD更匹配。因此，从正常RF诱发的放电可能反映了适当限制感受野所需的兴奋性和抑制性输入之间的平衡。频率特异性ILD调谐曲线与测量的头部滤波特性相结合，以预测每个位置对频率特异性ILD的反应。基于频率特异性输入简单求和的预测仅ILD的RF占我们测量的仅ILD的RF中56% 的方差。
