Day Mitchell L
bioRxiv. 2023 Sep 17:2023.09.15.557943. doi: 10.1101/2023.09.15.557943.
The head-related transfer function (HRTF) is the direction-dependent acoustic filtering by the head that occurs between a source signal in free-field space and the signal at the tympanic membrane. HRTFs contain information on sound source location via interaural differences of their magnitude or phase spectra and via the shapes of their magnitude spectra. The present study characterized HRTFs for source locations in the front horizontal plane for nine rabbits, which are a species commonly used in studies of the central auditory system. HRTF magnitude spectra shared several features across individuals, including a broad spectral peak at 2.6 kHz that increased gain by 12 to 23 dB depending on source azimuth; and a notch at 7.6 kHz and peak at 9.8 kHz visible for most azimuths. Overall, frequencies above 4 kHz were amplified for sources ipsilateral to the ear and progressively attenuated for frontal and contralateral azimuths. The slope of the magnitude spectrum between 3 and 5 kHz was found to be an unambiguous monaural cue for source azimuths ipsilateral to the ear. Average interaural level difference (ILD) between 5 and 16 kHz varied monotonically with azimuth over ±31 dB despite a relatively small head size. Interaural time differences (ITDs) at 0.5 kHz and 1.5 kHz also varied monotonically with azimuth over ±358 μs and ±260 μs, respectively. Remeasurement of HRTFs after pinna removal revealed that the large pinnae of rabbits were responsible for all spectral peaks and notches in magnitude spectra and were the main contribution to high-frequency ILDs, whereas the rest of the head was the main contribution to ITDs and low-frequency ILDs. Lastly, inter-individual differences in magnitude spectra were found to be small enough that deviations of individual HRTFs from an average HRTF were comparable in size to measurement error. Therefore, the average HRTF may be acceptable for use in neural or behavioral studies of rabbits implementing virtual acoustic space when measurement of individualized HRTFs is not possible.
头部相关传递函数（HRTF）是头部在自由场空间中的源信号与鼓膜处信号之间进行的方向依赖性声学滤波。HRTF通过其幅度或相位谱的耳间差异以及幅度谱的形状包含有关声源位置的信息。本研究对九只兔子的前水平面声源位置的HRTF进行了表征，兔子是中枢听觉系统研究中常用的物种。HRTF幅度谱在个体间具有几个共同特征，包括在2.6 kHz处有一个宽谱峰，根据声源方位，增益增加12至23 dB；在大多数方位上，7.6 kHz处有一个凹陷，9.8 kHz处有一个峰值。总体而言，对于与耳朵同侧的声源，4 kHz以上的频率被放大，而对于前方和对侧方位则逐渐衰减。发现在3至5 kHz之间幅度谱的斜率是耳朵同侧声源方位的明确单耳线索。尽管头部尺寸相对较小，但5至16 kHz之间的平均耳间电平差（ILD）随方位在±31 dB范围内单调变化。0.5 kHz和1.5 kHz处的耳间时间差（ITD）也分别随方位在±358 μs和±260 μs范围内单调变化。去除耳廓后对HRTF的重新测量表明，兔子的大耳廓是幅度谱中所有谱峰和凹陷的原因，并且是高频ILD的主要贡献者，而头部的其余部分是ITD和低频ILD的主要贡献者。最后，发现幅度谱的个体间差异足够小，以至于个体HRTF与平均HRTF的偏差在大小上与测量误差相当。因此，当无法测量个体HRTF时，平均HRTF可用于兔子的神经或行为研究中实现虚拟声学空间。
