Journal of Neuroscience, Vol 6, 962-973, Copyright © 1986 by Society for Neuroscience
Binaural response organization within a frequency-band representation of the inferior colliculus: implications for sound localization
JJ Wenstrup, LS Ross and GD Pollak
The auditory system of the mustache bat (Pteronotus parnellii) contains a
disproportionately large representation of a narrow frequency band,
corresponding to the dominant, 60 kHz component of its echolocation signal.
In the inferior colliculus (IC), the 60 kHz representation comprises an
architectonically distinct region called the dorsoposterior division (DPD),
which is accessible for detailed physiological study. We examined the
topographic distribution of binaural responses within this one
frequency-band representation of the inferior colliculus. We describe two
primary results. First, neurons with different binaural response properties
are spatially segregated into one of four binaural response-specific
regions of the DPD: a large region of monaural (EO) responses; two regions
containing neurons excited by sound from both ears (EE); and a region
containing neurons excited by one ear and inhibited by the other (EI).
Regions dominated by 60 kHz EI responses are also found in the lateral
extremity of the IC, probably within the external nucleus. These results
demonstrate functionally defined subdivisions in a single frequency-band
representation of the IC. Moreover, they suggest that brain stem auditory
projections to the DPD and/or intrinsic connections within the DPD are
highly organized. Second, within the EI region of the DPD, there is a
systematic shift in the sensitivity of EI multiunit responses to interaural
intensity disparities (IIDs). Dorsally, EI neurons are suppressed only by
relatively loud ipsilateral sounds, and there is a systematic decrease in
the relative ipsilateral intensity required for suppression at more ventral
recording sites. This result demonstrates that neurons sensitive to a sound
localization cue are systematically organized within a frequency-band
representation of the inferior colliculus. It has implications for the
manner in which the location of a sound source is encoded within the
primary auditory pathway.
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