Volman S F, Konishi M
Division of Biology, California Institute of Technology, Pasadena.
Brain Behav Evol. 1990;36(4):196-215. doi: 10.1159/000115307.
Bilateral ear asymmetry is found in some, but not all, species of owls. We investigated the neural basis of sound localization in symmetrical and asymmetrical species, to deduce how ear asymmetry might have evolved from the ancestral condition, by comparing the response properties of neurons in the external nucleus of the inferior colliculus (ICx) of the symmetrical burrowing owl and asymmetrical long-eared owl with previous findings in the symmetrical great horned owl and asymmetrical barn owl. In the ICx of all of these owls, the neurons had spatially restricted receptive fields, and auditory space was topographically mapped. In the symmetrical owls, ICx units were not restricted in elevation, and only azimuth was mapped in ICx. In the barn owl, the space map is two-dimensional, with elevation forming the second dimension. Receptive fields in the long-eared owl were somewhat restricted in elevation, but their tuning was not sharp enough to determine if elevation is mapped. In every species, the primary cue for azimuth was interaural time difference, although ICx units were also tuned for interaural intensity difference (IID). In the barn owl, the IIDs of sounds with frequencies between about 5 and 8 kHz vary systematically with elevation, and the IID selectivity of ICx neurons primarily encodes elevation. In the symmetrical owls, whose ICx neurons do not respond to frequencies above about 5 kHz, IID appears to be a supplementary cue for azimuth. We hypothesize that ear asymmetry can be exploited by owls that have evolved the higher-frequency hearing necessary to generate elevation cues. Thus, the IID selectivity of ICx neurons in symmetrical owls may preadapt them for asymmetry; the neural circuitry that underlies IID selectivity is already present in symmetrical owls, but because IID is not absolutely required to encode azimuth it can come to encode elevation in asymmetrical owls.
在部分（而非全部）猫头鹰物种中发现了双耳不对称的现象。我们研究了对称和不对称物种中声音定位的神经基础，通过比较穴小鸮（对称）和长耳鸮（不对称）下丘臂外侧核（ICx）中神经元的反应特性与之前在大角鸮（对称）和仓鸮（不对称）中的研究结果，来推断耳朵不对称可能是如何从祖先状态演变而来的。在所有这些猫头鹰的ICx中，神经元具有空间受限的感受野，并且听觉空间呈拓扑映射。在对称的猫头鹰中，ICx单元在高度上不受限制，ICx中仅映射方位角。在仓鸮中，空间图谱是二维的，高度构成第二维度。长耳鸮中的感受野在高度上有所受限，但其调谐不够尖锐，无法确定是否映射了高度。在每个物种中，方位角的主要线索是双耳时间差，尽管ICx单元也对方位强度差（IID）进行调谐。在仓鸮中，频率在约5至8kHz之间的声音的IID随高度系统变化，并且ICx神经元的IID选择性主要编码高度。在对称的猫头鹰中，其ICx神经元对高于约5kHz的频率没有反应，IID似乎是方位角的辅助线索。我们推测，已经进化出产生高度线索所需的高频听力的猫头鹰可以利用耳朵不对称。因此，对称猫头鹰中ICx神经元的IID选择性可能使它们预先适应不对称；对称猫头鹰中已经存在作为IID选择性基础的神经回路，但由于编码方位角并非绝对需要IID，所以在不对称猫头鹰中它可以转而编码高度。
