Representation of multiple sound sources in the owl's auditory space map

The barn owl's inferior colliculus contains a retina-like map of space on which a sound generates a focus of activity whose position corresponds to the location of the sound source. When there is more than one source of sound, the sound waves sum and may generate spurious binaural cues that degrade the auditory image. We investigated the signal conditions under which neurons in the owl's auditory space map are able to resolve two simultaneously active sound sources. We recorded from space map neurons responding to sounds from a pair of speakers separated in azimuth by 45 degrees and mounted on a rotatable arm. Stimuli consisted of a sum of sinusoids or pseudorandom noise bursts emitted simultaneously and at equal overall levels. The characteristics of the sounds in each speaker were varied, and the neuron's response was plotted as a function of the speaker pair's position. When the speakers emitted different sets of summed sinusoids, the cells responded to each speaker separately; that is, the cells were able to resolve two separate targets. However, when the speakers emitted identical summed sinusoids generating binaural cues that were identical to those of a single phantom source between the two speakers, the neurons responded when the speakers were on either side of their receptive fields. By manipulating the amplitude at which each speaker emitted the various frequencies, we could control the position, number, and size of the phantom sources detected by the cell. The cells also resolved two separate sources when they emitted noise bursts that were statistically independent or temporally reversed versions of one another. Since the overall spectra of such waveforms are identical, we suggest that the space map relies on differences between noise bursts that exist over brief time spans.