Abstract
Localization of sound source by acoustic cues is basing on the time and the intensity differences of sound captured by two ears. The sound source localization in azimuth depends on the resolution of interaural time differences (ITD). Temporal information coded in the auditory nerve fibers is first extracted in the nucleus magnocellularis (NM) and ITD is then calculated in the nucleus laminaris (NL) in birds. We investigated the accuracy of synaptic transmission in NM and estimated the limiting accuracy of coincidence detection in NL by using slice preparations of chicken's brain stem. NM neurons showed a tonotopic differentiation, due mostly to the expression of low threshold K current (LTK, esp. Kv1.1). NM neurons fire with a minimum jitter (10 μs) when driven trans-synaptically. The coincidence of bilateral inputs from NM generates action potentials in NL, and the time window of coincidence detection (CD) was narrow; 340 μs. Accuracy of CD had a strong positive correlation with the time course of EPSP. EPSP time course was shortened by two currents: H current and LTK current. The time course of EPSP became close to or faster than that of EPSC. Assuming the limiting time course of EPSP as that of mEPSC gave a limiting time window of CD as narrow as 150 μs in the chicken. This limiting accuracy of CD was nearly the accuracy achieved by barn owl NL, in which the difference of 1 degree azimuth angle produces 4 μs ITD producing nearly 5% changes of the spike rate. Barn owl and man have the ability of localizing the sound source at 1 degree azimuth angle, most precise among terrestrial animal. [Jpn J Physiol 54 Suppl:S23 (2004)]
