The Journal of Neuroscience, January 1, 2002, 22(1):284-293
From Spectrum to Space: The Contribution of Level Difference Cues
to Spatial Receptive Fields in the Barn Owl Inferior Colliculus
David R.
Euston and
Terry T.
Takahashi
Institute of Neuroscience, University of Oregon, Eugene, Oregon
97403
Space-specific neurons in the owl's inferior colliculus have
spatial receptive fields (RFs) computed from interaural time (ITD) and
level (ILD) differences. Because of the shape of the owl's
head, these cues vary with frequency in a manner specific for each
location. We sought to determine the contribution of ILD to spatial
selectivity. We measured the normal spatial receptive fields of
space-specific neurons using virtual sound sources (i.e., noises
filtered to simulate external sound sources, presented using
headphones). The virtual-source filters were then altered so that ITD
was fixed while frequency-specific ILDs varied according to location in
the usual manner. The resulting "ILD-alone" RF typically revealed a
horizontal band of excitation that included the normal RF. Above and
below, the neurons were inhibited. Interestingly, the maxima of
ILD-alone RFs were generally outside the normal RF, suggesting that
space-specific neurons are not optimally tuned to the ILD spectrum
occurring at the normal RF location. Congruously, frequency-specific
ILD tuning, assessed with tones, better matched the ILDs at the peak of
the ILD-alone RF than those at the peak of the normal RF. The firing
evoked from the normal RF may thus reflect the balance of excitatory
and inhibitory inputs needed to appropriately restrict the receptive field.
Frequency-specific ILD tuning curves were combined with measured
head-filtering characteristics to predict responses to the frequency-specific ILDs at each location. The predicted ILD-alone RFs,
which are based on a simple sum of frequency-specific inputs, accounted
for 56% of the variance in our measured ILD-alone RFs.
Key words:
sound localization; binaural; spectral integration; interaural intensity difference; head-related transfer function; virtual auditory space
Copyright © 2002 Society for Neuroscience 0270-6474/02/221284-10$05.00/0
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