Grant Simon, Binns K Esther
Division of Neurophysiology, National Institute for Medical Research, Mill Hill, NW7 1AA, London, UK.
Exp Brain Res. 2003 Aug;151(4):478-88. doi: 10.1007/s00221-003-1495-4. Epub 2003 Jun 27.
Auditory brainstem abnormalities affecting decussation patterns and nuclei involved in the acoustic chiasm exist in a variety of albino mammals, suggesting that binaural processes underlying spatial hearing may be disrupted in these mutants. To evaluate this we have compared the contribution of the two ears in albino and normally pigmented guinea pigs to the spatial tuning of auditory neurons in the deep layers of the superior colliculus (SC). Broadband noise stimuli at threshold and at suprathreshold intensities were presented from different azimuthal loudspeaker locations under free-field anechoic conditions, and auditory receptive fields were plotted before, during and after occluding the ipsilateral ear. We show that the deep layers of the albino SC contain a map of contralateral auditory azimuth along its anteroposterior axis, which is aligned with the visual map in the superficial layers above, just as in normal animals. We also show that threshold spatial responses are elicited only via the contralateral ear and at similar stimulus intensities (mean approximately 30 dB SPL) in the two pigmentation phenotypes. The mechanisms that maintain spatial tuning at sound intensities of 10-40 dB above threshold, however, differ markedly in these animals. Plugging the ipsilateral ear in normal guinea pigs caused significant expansions of their auditory receptive fields and loss of directional tuning, but in the albinos occlusion had little effect on these spatial properties. The results suggest that while spatial selectivity for relatively loud sounds among SC neurons is normally maintained via the binaural combination of contralateral excitatory drive and ipsilateral inhibition, it is achieved in albinos almost exclusively by monaural input from the contralateral ear. This finding is consistent with an excessive contralateral ear dominance of higher levels of the albino auditory system caused by anomalies of their acoustic chiasm, analogous to the monocular dominance of the visual system that results from excessive axon crossing at the optic chiasm in these animals.
多种白化哺乳动物存在影响交叉模式和参与听觉交叉的核团的听觉脑干异常，这表明这些突变体中空间听觉的双耳处理过程可能受到干扰。为了评估这一点，我们比较了白化豚鼠和正常色素沉着豚鼠的两只耳朵对中脑上丘（SC）深层听觉神经元空间调谐的贡献。在自由场消声条件下，从不同方位角的扬声器位置呈现阈值和阈上强度的宽带噪声刺激，并在堵塞同侧耳之前、期间和之后绘制听觉感受野。我们发现，白化SC深层沿其前后轴包含对侧听觉方位图，与上方浅层的视觉图对齐，就像在正常动物中一样。我们还表明，阈值空间反应仅通过对侧耳引发，并且在两种色素沉着表型中，刺激强度相似（平均约30 dB SPL）。然而，在这些动物中，将声音强度维持在阈值以上10 - 40 dB时保持空间调谐的机制明显不同。堵塞正常豚鼠同侧耳会导致其听觉感受野显著扩大和方向调谐丧失，但在白化豚鼠中，堵塞对这些空间特性影响很小。结果表明，虽然SC神经元对相对响亮声音的空间选择性通常通过对侧兴奋性驱动和同侧抑制的双耳组合来维持，但在白化豚鼠中几乎完全通过对侧耳的单耳输入来实现。这一发现与白化听觉系统较高水平对侧耳过度占主导地位相一致，这是由其听觉交叉异常引起的，类似于这些动物中视觉系统因视交叉处轴突过度交叉而导致单眼占主导地位。
