Journal of Neuroscience, Vol 14, 4780-4793, Copyright © 1994 by Society for Neuroscience
Representation of multiple sound sources in the owl's auditory space map
TT Takahashi and CH Keller
Institute of Neuroscience, University of Oregon, Eugene 97403.
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.
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