Abstract
Interaural time difference (ITD) is an essential cue for the sound source localization along the horizontal axis. In birds, ITDs are first calculated in neurons of nucleus laminaris (NL) by detecting the coincidence of binaural synaptic inputs. Previously, we recorded neurophonic potential (NP) in NL of anesthetized chicken (posthatch 1-6 days), using a tungsten electrode inserted into a glass pipette: The NP changed periodically with ITDs. The characteristic frequencies were tonotopically arranged, and the ITD sensitivity varied orderly as the recording site shifted. NP was specifically reduced by CNQX, and this may indicate that NP originates in EPSCs and action potentials from NL. However, the relationship between the NPs in NL and the activities of NL neurons is not known. In this study, we made single-unit recordings with a sharp electrode to understand the ITD processing in NL. The basic properties of NL spikes agreed with those of NP, in that they were phase-locked to pure-tone stimulus, and sensitive to frequency and ITD of stimulus sound. This suggests that the NPs recorded in NL would reflect the activities of NL neurons. As the sound intensity increases, binaural responses for favorable ITD largely increased, but those for unfavorable ITD did not. Furthermore, responses for unfavorable ITD were sometimes smaller than spontaneous ones. These results suggest a presence of some inhibitory system in NL. This inhibition may play an essential role for accurate ITD detection. [Jpn J Physiol 55 Suppl:S154 (2005)]
