Volume 16, Number 13,
Issue of July 1, 1996
pp. 4300-4309
Copyright ©1996 Society for Neuroscience
Binaural Cross-Correlation Predicts the Responses of Neurons in
the Owl's Auditory Space Map under Conditions Simulating Summing
Localization
Received Jan. 29, 1996; revised April 8, 1996; accepted April 11, 1996.
Clifford H. Keller and
Terry T. Takahashi
Institute of Neuroscience, University of Oregon, Eugene, Oregon
97403-1254
Summing localization describes the perceptions of human listeners
to two identical sounds from different locations presented with delays
of 0-1 msec. Usually a single source is perceived to be located
between the two actual source locations, biased toward the earlier
source. We studied neuronal responses within the space map of the barn
owl to sounds presented with this same paradigm. The owl's primary cue
for localization along the azimuth, interaural time difference (ITD),
is based on a cross-correlation-like treatment of the signals arriving
at each ear. The output of this cross-correlation is displayed as
neural activity across the auditory space map in the external nucleus
of the owl's inferior colliculus. Because the ear input signals
reflect the physical summing of the signals generated by each speaker,
we first recorded the sounds at each ear and computed their
cross-correlations at various interstimulus delays. The resulting
binaural cross-correlation surface strongly resembles the pattern of
activity across the space map inferred from recordings of single
space-specific neurons. Four peaks are observed in the
cross-correlation surface for any nonzero delay. One peak occurs at the
correlation delay equal to the ITD of each speaker. Two additional
peaks reflect ``phantom sources'' occurring at correlation delays
that match the signal of the left speaker in one ear with the signal of
the right speaker in the other ear. At zero delay, the two phantom
peaks coincide. The surface features are complicated further by the
interactions of the various correlation peaks.
Key words:
auditory scene analysis;
echo suppression;
inferior colliculus;
interaural time difference;
precedence effect;
sound localization
