抄録
Interaural time difference (ITD) is a major cue for sound source localization. However, the ITDs experienced by animals with a small head are extremely small, making ITD detection difficult particularly for the low-frequency sound. A precisely-timed inhibition is proposed to overcome this disadvantage in some mammals. Here, we found a sound intensity dependent mechanism of compensation for the small ITD cue in the coincidence detector neurons in nucleus laminaris (NL) of the chicken by in vivo unit recordings and the mechanism was confirmed by simulation. Loud sound made the contrast of ITD tuning robust in the low-frequency NL neurons by suppressing the firing activity at the worst-ITD. In contrast, the suppression was weaker in the middle- to high-frequency NL neurons. The contrast of ITD tuning was maximal at the intermediate sound intensity and was lost at the loud sound because of the increase of firing activity at the worst-ITD. The suppression of firing activity was through the activation of superior olivary nucleus (SON) and was eliminated by electrolytic lesion of SON. The frequency dependency of suppression reflected the robust projection of SON to low-frequency NL. Consequently, the small ITD cue in the low-frequency sound was compensated by the sound intensity dependent inhibition from SON and the ITD sensitivity was made nearly constant across frequencies. [J Physiol Sci. 2008;58 Suppl:S121]
