The barn owls’ Minimum Audible Angle

Interaural time differences (ITD) and interaural level differences (ILD) are physical cues that enable the auditory system to pinpoint the position of a sound source in space. This ability is crucial for animal communication and predator-prey interactions. The barn owl has evolved an exceptional sense of hearing and shows abilities of sound localisation that outperform most other species. So far, behavioural studies in the barn owl often used reflexive responses to investigate aspects of sound localisation. Furthermore, they predominately probed the higher frequencies of the owl’s hearing range (> 3 kHz). In the present study we used a Go/NoGo paradigm to measure the barn owl’s behavioural sound localisation acuity (expressed as the Minimum Audible Angle, MAA) as a function of stimulus type (narrow-band noise centred at 500, 1000, 2000, 4000 and 8000 Hz, and broad-band noise) and sound source position. We found significant effects of both stimulus type and sound source position on the barn owls’ MAA. The MAA improved with increasing stimulus frequency, from 14° at 500 Hz to 6° at 8000 Hz. The smallest MAA of 4° was found for broadband noise stimuli. Comparing different sound source positions revealed smaller MAAs for frontal compared to lateral stimulus presentation, irrespective of stimulus type. These results are consistent with both the known variations in physical ITDs and variation in the width of neural ITD tuning curves with azimuth and frequency. Physical and neural characteristics combine to result in better spatial acuity for frontal compared to lateral sounds and reduced localisation acuity at lower frequencies.双耳时间差（Interaural Time Differences, ITD）与双耳声级差（Interaural Level Differences, ILD）是帮助听觉系统精准定位空间内声源位置的物理线索。该能力对动物交流及捕食者与猎物的互动均至关重要。仓鸮演化出了卓越的听觉感知能力，其声源定位性能远超绝大多数其他物种。迄今为止，针对仓鸮的行为学研究多通过反射性反应探究声源定位的相关特性，且大多仅探测了仓鸮听觉范围内的高频段（>3 kHz）。本研究采用Go/NoGo范式（Go/NoGo paradigm），测量了仓鸮的声源定位行为灵敏度（以最小可辨声源角Minimum Audible Angle, MAA表示），并分析其随刺激类型（中心频率分别为500、1000、2000、4000及8000 Hz的窄带噪声，以及宽带噪声）与声源位置的变化规律。研究发现，刺激类型与声源位置均对仓鸮的MAA存在显著影响：MAA随刺激频率升高而提升，从500 Hz时的14°降至8000 Hz时的6°；宽带噪声刺激下的最小MAA仅为4°。对比不同声源位置的结果显示，相较于侧方刺激呈现，正面刺激对应的MAA更小，且该规律不受刺激类型的影响。上述结果与已知的物理ITD变化规律，以及神经ITD调谐曲线宽度随方位角与频率的变化规律均保持一致。物理特性与神经特性共同作用，使得正面声源相较于侧方声源具备更优的空间定位灵敏度，而低频段的声源定位精度则相对更低。