Directional Sensitivity of Bat Inferior Collicular Neurons Determined under Normal and Monaurally Plugged Ear Conditions

蝙蝠下丘神經元在正常耳及單耳堵塞條件下的方向敏感性

任晃蓀(Philip H.-S. Jen) ； 吳敏(Min Wu) ； 皮亥魯·但尼爾(A. Daniel Pinheiro)

《The Chinese Journal of Physiology》 34卷4期 (1991/12) Pp. 371-386

摘要

爲進一步探討雙耳强度差別及耳廓位置對聽神經元方向敏感性的影響，我們應用自由聲場刺激研究了大灰褐色蝙蝠在三種不同耳朵條件下，下丘神經元的方向敏感性。三種耳朵條件爲正常耳、單耳堵塞、單耳堵塞及耳廓向後彎曲。我們首先記錄每一神經元對來自水平方位對側40度，垂直方位0度聲刺激（聲刺激期間爲4毫秒，上升及下降期間各0.5毫秒）反應的最佳頻率及最低閾値。然後記錄此神經元對7個不同水平方位最佳頻率聲刺激反應的最低閾値以及對兩個不同强度（大於最低閾値10及20分貝）的最佳頻率聲刺激的神經脈衝發放數目。雖然不同下丘神經元的最低閾値及脈衝發放數目會隨聲源方位產生不等的差異，但不論在什麼耳朵條件下，大部分(90%)神經元均對來自對側水平方位的聲刺激反應出最低閾値及最高脈衝發放數目。然而在117個神經元中，有112個神經元對聲刺激方向不敏感，因此它們的神經脈衝發放數目不因聲刺激方向的改變而產生顯著的變化。單耳堵塞（用濕棉球）或同時把耳廓向後彎曲，不僅提高神經元的閾値，也減少了它的脈衝發放數目。因此，此神經元的方向敏感性也產生了變化。單耳堵塞對每一神經元的閾値及其方向敏感性變化的影響程度因不同神經元而異。因此，同樣的單耳堵塞固然可加强某些下丘神經元的方向敏感性，卻也會同時減弱其它下丘神經元的方向敏感性。不管在那一種耳朵條件下，下丘神經元的方向敏感性均取決於聲刺激的頻率。高頻神經元的方向敏感性通常要比低頻神經元的方向敏感性敏銳。同時其最低閾値角度及最大脈衝發放角度亦隨著頻率的增加而漸趨動物的正前方。

並列摘要

Jen, P.H.-S., M. Wu, and A. D. Pinheiro. Directional sensitivity of bat inferior collicular neurons determined under normal and monaurally plugged ear conditions. Chinese J. Physiol. 34(4) 371-386, 1991. In an effort to further understand the combined effect of binaural intensity difference and pinna position on the directional sensitivity of an auditory neuron, we used free field stimulation to study the directional sensitivity of inferior collicular (IC) neurons of the big brown bat, Eptesicus fuscus, under the following three ear conditions: normal ears, monaurally plugged ear and monaurally plugged plus pinna bending backward. The best frequency (BF) and minimum threshold (MT) of each neuron were first determined for a sound (4 ms duration, 0.5 ms rise-decay times) delivered from 40° contralateral in azimuth, 0° in elevation relative to the recording site. Then, the neuron's MT to a BF sound delivered from 7 selected azimuthal angles with respect to the bat's head were determined. The number of impulses to a BF sound delivered at two intensities (10 and 20 dB re MT determined at 40° contralateral) from each of 7 azimuthal angles was also subsequently determined. Although different IC neurons showed different variations in MT and number of impulses with sound source azimuth, most had a lowest MT and/or maximal number of impulses to BF sounds delivered from contralateral azimuthal angles regardless of ear conditions. However, among 117 neurons studied, there were 12 neurons in which the number of impulses did not vary significantly with sound direction thus showing nondirectional characteristics. Monaural plugging (a wet cotton ball) with or without backward bending of the pinna not only increased the MT of a recorded neuron but also greatly reduced the number of impulses of the neuron so that the directional sensitivity of the neuron was also modified. The effect of monaural plugging on the threshold of a neuron and the degree of modification on directional sensitivity varied among individual IC neurons. Thus, while the directional sensitivity of some IC neurons became sharper, for other neurons the same monaural plugging reduced the sharpness of their directional sensitivity curve. Directional sensitivity of IC neurons is dependent upon stimulus frequency regardless of ear conditions. High frequency neurons generally have sharper directional sensitivity than low frequency neurons. The angle of lowest MT and maximal number of impulses tended to move toward the middle portion of a bat's frontal auditory space with increasing BF.

並列關鍵字

bat ； directional sensitivity ； inferior colliculus ； monaural plugging ； pinna bending