Binaural cross-correlation and auditory localization in the barn owl: a theoretical study

The barn owl is a nocturnal predator that is able to capture mice in comple te darkness using only sound to localize prey. Two binaural cues are used b y the barn owl to determine the spatial position of a sound source: differe nces in thr time of arrival of sounds at the two ears for the azimuth (inte raural time differences (ITDs)) and differences in their amplitude for the elevation (interaural level differences (ILDs)). Neurophysiological investi gations have revealed that two different neural pathways starting from the cochlea seem to be specialized for processing ITDs and ILDs. Much evidence suggests that in the barn owl the localization of the azimuth is based on a cross-correlation-like treatment of the auditory inputs at the two ears. I n particular, in the external nucleus of the inferior colliculus (ICx), whe re cells are activated by specific values of ITD, neural activation has bee n recently observed to be dependent on some measure of the level of cross-c orrelation between the input auditory signals. However, it has also been ob served that these neurons are less sensitive to noise than predicted by dir ect binaural cross-correlation. The mechanisms underlying such signal-to-no ise improvement are not known. In this paper, by focusing on a model of the barn owl's neural pathway to the optic tectum dedicated to the localizatio n of the azimuth, we study the mechanisms by which the ITD tuning of ICx un its is achieved. By means of analytical examinations and computer simulatio ns, we show that strong analogies exist between the process by which the ba rn owl evaluates the azimuth of a sound source and the generalized cross-co rrelation algorithm, one of the most robust methods for the estimate of tim e delays. (C) 1999 Elsevier Science Ltd. All rights reserved.