Pollak G D
Department of Zoology, University of Texas, Austin 78712.
Hear Res. 1988 Nov;36(2-3):107-24. doi: 10.1016/0378-5955(88)90054-8.
Disparities in time and intensity are the two chief cues animals use for localizing a sound source in space. Echolocating bats belonging to the family Molossidae emit brief, ultrasonic signals for orientation that sweep downward about an octave over the duration of the pulse. Due to acoustic shadowing and the directional properties of the ears, pronounced interaural intensity disparities are created that vary as a function of azimuth. However, due to the small headwidth of these animals, azimuthal changes create small interaural time disparities that are at most 30 microseconds. The experiments in this report are concerned with the binaural processing of time and intensity disparities using brief FM signals that simulate the animal's natural echolocation calls. Binaural neurons receiving excitation from one ear and inhibition from the other (E-I neurons) were recorded from the inferior colliculus of Mexican free-tailed bats. The majority of units sampled were highly sensitive for temporal disparities of 100-300 microseconds, and a few had significant changes in discharge probability when interaural time was changed by 10-20 microseconds. However, all E-I neurons were also sensitive to intensity disparities. With only one exception, all E-I neurons traded time for intensity. On the average, each decibel difference in intensity could be compensated for by advancing or delaying the inhibitory sound by 47 microseconds. The main conclusion is that the auditory system processes interaural disparities by transforming level differences at the two ears into latency differences. Thus the discharge probability of each binaural neuron is determined largely by the arrival times of the discharges from the excitatory and inhibitory ears. In view of the substantial time-intensity trading ratios, the small interaural time disparities produced by azimuthal locations off the midline play no role in shaping the response properties of these neurons. Specific examples of how time-intensity trades can translate into a high spatial selectivity are presented.
时间和强度的差异是动物在空间中定位声源所使用的两个主要线索。犬吻蝠科的回声定位蝙蝠发出短暂的超声波信号用于定向，这些信号在脉冲持续时间内向下扫过约一个八度。由于声影和耳朵的方向性，会产生明显的双耳强度差异，其随方位角而变化。然而，由于这些动物的头部宽度较小，方位角变化产生的双耳时间差异很小，最多为30微秒。本报告中的实验涉及使用模拟动物自然回声定位叫声的短暂调频信号对时间和强度差异进行双耳处理。从墨西哥无尾蝠的下丘记录到了接受一只耳朵的兴奋和另一只耳朵的抑制的双耳神经元（E-I神经元）。大多数采样单元对100 - 300微秒的时间差异高度敏感，当双耳时间改变10 - 20微秒时，少数单元的放电概率有显著变化。然而，所有E-I神经元也对强度差异敏感。除了一个例外，所有E-I神经元都用时间换取强度。平均而言，强度每相差一分贝，可以通过将抑制性声音提前或延迟47微秒来补偿。主要结论是，听觉系统通过将两耳的电平差异转化为潜伏期差异来处理双耳差异。因此，每个双耳神经元的放电概率在很大程度上取决于来自兴奋耳和抑制耳的放电到达时间。鉴于存在大量的时间 - 强度交换比率，由中线以外的方位位置产生的小双耳时间差异在塑造这些神经元的反应特性方面不起作用。文中给出了时间 - 强度交换如何转化为高空间选择性的具体例子。
