Information Processing in the Interaural Time Difference Pathway of the Barn Owl

The interaural time difference (ITD) is one of two primary binaural cues used to compute the position of a sound source in space. In the barn owl, the ITD is processed in a dedicated neural pathway that terminates at the core area of the central nucleus of the inferior colliculus (ICcc). The actual locus of the computation of the ITD is prior to ICcc in the nucleus laminaris (NL), and ICcc receives no feed-forward inputs carrying information that did not originate in NL. Here, we compare single-unit responses in these two nuclei. The neurons of both nuclei encode spectrotemporal properties of the stimulus with high resolution in both frequency and time domains, and their response to ITD cues has a spectral profile given by the square of their spectral tuning, a relationship consistent with the prediction that these neurons behave as if computing the cross-correlation of the signals at the two ears. However, the ITD tuning curves of ICcc neurons develop more rapidly than those of the neurons of NL, requiring as little as a single stimulus presentation per ITD to show coherent ITD tuning. ICcc neurons also display a greater dynamic range, with a maximal difference in firing rates due to ITD cues approximately double that seen in NL. These results suggest that ICcc neurons sum across a population of similarly tuned NL neurons to produce an averaged response with greater single-unit reliability.