DIRECTIONAL SENSITIVITY OF FREQUENCY TUNING CURVES OFINFERIOR COLLICULAR NEURONS IN BIG BROWN BAT, EPTESICUS FUSCUS
大棕蝠下丘听神经元频率调谐的方向敏感性

Eight big brown bats Eptesicus fuscus were used for this study. Under free field sound stimulation conditions, we studied the sound direction effect on frequency tuning curves of inferior collicular (IC) neurons. Five azimuthal angles were selected in this study: 80° and 40° contralateral relative to the recording site (C-80°, C-40°), 0°, 80° and 40° ipsilateral relative to the recording site(I-80°, I-40°). Among IC neurons studied, there were three kinds of frequency tuning curves: V shaped, enclosed and U shaped. We compared the sharpness of frequency tuning curves, minimum threshold, and best frequencies of 47 IC neurons determined at 5 azimuthal angles. The minimum threshold(MT) and sharpness of frequency tuning increased as the sound directions changed from contralateral to ipsilateral angles. QndB values at I-80° and I-40°were significantly higher than that at C-80°, C-40° and 0°(Repeated measures ANOVA, p<0.001). The average MT from high to low among 5 azimuth angles was as follows: MTI-80°>MTI-40°>MT0°>MTC-80°>MTC-40°. As sound direction changed, most neurons did not change their best frequencies. This study showed that the MT and sharpness of frequency tuning curves increased as the sound direction changed from contralateral to ipsilateral azimuthal angles. It suggests that sound direction effect on frequency tuning may be through the complex interplay between the excitatory and inhibitory input.