Unmasking in neurons of the inferior colliculus of Eptesicus fuscus with binaural stimulation

171 single inferior colliculus neurons displaying basic auditory properties similar to those described previously were sampled, and 118 of those were tested to determine whether monaurally masked responses (band passed noise of ± 5 kHz around the best frequency of the pure tone) could be recovered if the masking noise was presented binaurally. 26% of the units tested showed such an improvement in signal detection i.e. what is called masking level difference (MLD) by psychoacoustician.s. Signal detection was improved by more than 20 dB in a few cases, but the usual improvement did not exceed 11 dB. The data suggest that MLD occurs only in units showing binaural facilitation in addition to inhibition (I, E/E type), in contrast to the more common binaural I/E types which may only provide basic azimuth information. The neurophysiological results are discussed in view of the findings described in the literature on psychoacoustical MLD and in terms of the biological importance these results have for the bat's acoustical space orientation (echolocation) and this system's excellent resistance to jamming. Since psychoacoustical explanations for the MLD effect appear to be of little relevance on a cellular level, possible neural mechanisms are discussed as well.