Localization and identification of concurrent sounds in the owl’s auditory space map

In nature, sounds from multiple sources sum at the eardrums, generating complex cues for sound localization and identification. In this clutter, the auditory system must determine “what is where.” We examined this process in the auditory space map of the barn owl’s inferior colliculus using two spatially separated sources simultaneously emitting uncorrelated noises, which were uniquely identified by different frequencies of sinusoidal amplitude modulation. The neurons responded when either source was placed within the receptive field, suggesting two clearly segregated foci of activity at appropriate loci on the space map. The spike trains were locked strongly to the AM of the source within the receptive field, while the other source had minimal influence. Two sources AMed at the same rate were successfully resolved, suggesting that source separation is based on differences of fine structure. The spike rate and synchrony were stronger for whichever source had the stronger average binaural level. A computational model showed that neuronal activity was largely proportional to the degree of matching between the momentary binaural cues and the neuron’s preferred values. Individual neurons respond to and synchronize with sources in their receptive field if there are frequencies having an average binaural level-advantage over a second source. [NIH-DC003925.]