Directional sensitivity of bat inferior collicular neurons determined under normal and monaurally plugged ear...

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 degrees contralateral in azimuth, 0 degrees 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 degrees 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.