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-potentials (NPs) in NL of anesthetized chicken, and pharmacologically demonstrated that NPs originated from EPSCs and action potentials in NL. Because of the robust NPs, recording single unit from NL is difficult. However, occasionally recorded NL unit activities showed the same properties as NPs in the sensitivity to frequency and ITD of stimulus sound. Plot of NL activity against ITDs (ITD tuning curve) has peaks and troughs alternately, corresponding to the favorable and unfavorable ITDs. In the high- and middle-best frequency (BF) neurons, the increase of sound intensity shifted the whole ITD tuning curve upward, but in the low-BF neurons, the troughs of ITD tuning curve was depressed compared to the peaks, creating a large contrast between peaks and troughs. This proposes the effect of inhibitory input from superior olivary nucleus (SON). For testing this hypothesis, we lesioned SON electrolytically, and recorded well-isolated and stable single-unit activities with a loose patch technique. After lesioning SON, the whole ITD tuning curve shifted upward even in the low BF neuron when sound intensity was increased. This might support the idea that inhibitory input from SON improves coincidence detection in NL. [J Physiol Sci. 2006;56 Suppl:S186]
