Li L, Frost B J
Psychology Department, Queen's University, Kingston, Ont., Canada.
Hear Res. 1996 Oct;100(1-2):192-200. doi: 10.1016/0378-5955(96)00119-0.
Electromyographic (EMG) responses of the cervicoauricular muscles (CAM) to free-field sounds were recorded in two groups of rats whose brainstems were dissected transversely either at a pretectal or transtectal level. After the rat recovered from anesthesia, wide-band noise pulses were presented and speaker positions were varied systematically in azimuth. Sound levels were set at 10-15 dB above empirically determined threshold for an EMG response to a sound from 0 degree azimuth. In both animal groups, transient CAM EMGs with short latency were produced and three main types of azimuthal sensitivity of CAM EMG response were observed. (1) For the majority of the cases, an inverted "U' type of azimuthal sensitivity was identified: the maximum activity occurred around 0 degree azimuth, but as the speaker was moved toward either the ipsilateral or contralateral fields, the sound-evoked activity declined systematically. This directional tuning is quite different from the passive pinna directionality which is very lateral in the resting positions used in this study. (2) In a small number of cases, the spatial sensitivity curves were not symmetrical about the midline (0 degree azimuth): the EMG response was vigorous in one hemifield and dropped off systematically as the speaker was moved toward extreme positions of the other hemifield. Regardless of shapes of EMG spatial tuning curves, obstruction of either the ipsilateral or contralateral meatus reduced the sound-elicited response dramatically and eliminated the spatial sensitivity. (3) Some cases exhibited an omnidirectional function: the EMG spike rate had no or minor systematical variation as the speaker position was changed in azimuth. The results of this study indicate that with either pretectal or transtectal decerebrate preparations, the acoustically evoked CAM EMG can exhibit an azimuthal sensitivity which is based on binaural processing.
在两组大鼠中记录了颈耳肌（CAM）对自由场声音的肌电图（EMG）反应，这两组大鼠的脑干分别在顶盖前或横切水平进行横向解剖。大鼠从麻醉中恢复后，呈现宽带噪声脉冲，并在方位角上系统地改变扬声器的位置。声级设置为比根据经验确定的对来自0度方位角声音的EMG反应阈值高10 - 15 dB。在两组动物中，均产生了潜伏期短的瞬态CAM EMG，并观察到CAM EMG反应的三种主要方位敏感性类型。（1）在大多数情况下，确定了一种倒“U”型方位敏感性：最大活动出现在0度方位角附近，但随着扬声器向同侧或对侧场移动，声音诱发的活动会系统性下降。这种方向调谐与本研究中使用的静息位置非常外侧的被动耳廓方向性有很大不同。（2）在少数情况下，空间敏感性曲线关于中线（0度方位角）不对称：EMG反应在一个半侧场中强烈，并且随着扬声器向另一个半侧场的极端位置移动而系统性下降。无论EMG空间调谐曲线的形状如何，同侧或对侧耳道的阻塞都会显著降低声音诱发的反应并消除空间敏感性。（3）一些情况表现出全向性功能：随着扬声器在方位角上位置的改变，EMG尖峰率没有或仅有微小的系统性变化。本研究结果表明，无论是顶盖前还是横切去大脑制备，听觉诱发的CAM EMG都可以表现出基于双耳处理的方位敏感性。
