| Adaptation is commonly defined as a decrease in response to a constant stimulus. In the auditory system, this type of adaptation is seen at multiple levels. However, in the interaural time difference (ITD) detection circuit, the first-order central neurons encode information in the timing of spikes rather than the overall firing rate. Here we described a novel type of adaptation in the nucleus magnocellularis (NM) neurons in response to prolonged current injections in the acute chick brainstem slices in vitro. NM adaptation (Adslo) manifested as an increase in firing rate and a decrease in spike time precision that developed over approximately 20 seconds. Adslo depended on sustained depolarization, was independent of firing, and was eliminated by alpha-Dendrotoxin (0.1 microM), implicating slow inactivation of low-threshold voltage-activated K + channels as its mechanism. We found that NM neurons increased firing rate and decreased spike timing precision during ongoing simulated physiological inputs, and we used NM responses to construct simulated binaural inputs to nucleus laminaris (NL) in acute brainstem slices in vitro. NL neurons showed reduced response to changes in simulated ITD when stimulated with adapted NM inputs. The reduction in ITD sensitivity of NL neurons was caused by the degraded spike timing precision in NM. We predicted that Ad slo can cause location-independent frequency-specific increase in ITD detection thresholds in vivo. We found that human listeners showed increased ITD thresholds after exposure to an adapter stimulus, and this reduction in ITD detection occurred in a location-independent frequency-specific manner. This finding supports the role of Adslo in localization of ongoing auditory input. |
