Department of Biology, University of Southern Denmark, Odense, Denmark.
Department of Biology, Institute for Theoretical Biology, Humboldt-Universität zu Berlin, Berlin, Germany.
J Neurophysiol. 2021 Mar 1;125(3):887-902. doi: 10.1152/jn.00576.2020. Epub 2021 Feb 3.
The configuration of lizard ears, where sound can reach both surfaces of the eardrums, produces a strongly directional ear, but the subsequent processing of sound direction by the auditory pathway is unknown. We report here on directional responses from the first stage, the auditory nerve. We used laser vibrometry to measure eardrum responses in Tokay geckos and in the same animals recorded 117 auditory nerve single fiber responses to free-field sound from radially distributed speakers. Responses from all fibers showed strongly lateralized activity at all frequencies, with an ovoidal directivity that resembled the eardrum directivity. Geckos are vocal and showed pronounced nerve fiber directionality to components of the call. To estimate the accuracy with which a gecko could discriminate between sound sources, we computed the Fisher information (FI) for each neuron. FI was highest just contralateral to the midline, front and back. Thus, the auditory nerve could provide a population code for sound source direction, and geckos should have a high capacity to differentiate between midline sound sources. In brain, binaural comparisons, for example, by IE (ipsilateral excitatory, contralateral inhibitory) neurons, should sharpen the lateralized responses and extend the dynamic range of directionality. In mammals, the two ears are unconnected pressure receivers, and sound direction is computed from binaural interactions in the brain, but in lizards, the eardrums interact acoustically, producing a strongly directional response. We show strongly lateralized responses from gecko auditory nerve fibers to directional sound stimulation and high Fisher information on either side of the midline. Thus, already the auditory nerve provides a population code for sound source direction in the gecko.
蜥蜴耳朵的结构可以让声音同时到达鼓膜的两面，产生一个非常定向的耳朵，但听觉通路对声音方向的后续处理还不得而知。我们在此报告来自第一阶段（听神经）的定向反应。我们使用激光测振仪测量壁虎的鼓膜反应，并在同一动物身上，从径向分布的扬声器向自由场发出声音，记录了 117 根听神经单个纤维的反应。所有纤维的反应在所有频率下都表现出强烈的侧向活动，具有与鼓膜定向性相似的卵形定向性。壁虎是发声动物，对叫声的组成部分表现出明显的神经纤维方向性。为了估计壁虎辨别声源的准确性，我们计算了每个神经元的 Fisher 信息 (FI)。FI 在中线的对侧、前后方最高。因此，听神经可以为声源方向提供群体编码，壁虎应该具有区分中线声源的高能力。在大脑中，例如通过 IE（同侧兴奋，对侧抑制）神经元进行双耳比较，应该会锐化侧向反应并扩展定向性的动态范围。在哺乳动物中，两只耳朵是不相连的压力接收器，声音方向是通过大脑中的双耳相互作用计算出来的，但在蜥蜴中，鼓膜在声学上相互作用，产生了非常定向的反应。我们发现壁虎听神经纤维对定向声音刺激有强烈的侧向反应，并且在中线的两侧都有很高的 Fisher 信息。因此，听神经已经为壁虎的声源方向提供了群体编码。
