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The Journal of Neuroscience, August 15, 1998, 18(16):6608-6622
Sensitivity to Interaural Time Differences in the Medial Superior
Olive of a Small Mammal, the Mexican Free-Tailed Bat
Benedikt
Grothe1 and
Thomas J.
Park2
1 Zoologisches Institut, Universität
München, D-80333 München, Germany, and
2 Neurobiology Group, Department of Biological Sciences,
University of Illinois at Chicago, Chicago, Illinois 60607-7060
Neurons in the medial superior olive (MSO) are thought to encode
interaural time differences (ITDs), the main binaural cues used for
localizing low-frequency sounds in the horizontal plane. The underlying
mechanism is supposed to rely on a coincidence of excitatory inputs
from the two ears that are phase-locked to either the stimulus
frequency or the stimulus envelope. Extracellular recordings from MSO
neurons in several mammals conform with this theory. However, there are
two aspects that remain puzzling. The first concerns the role of the
MSO in small mammals that have relatively poor low-frequency hearing
and whose heads generate only very small ITDs. The second puzzling
aspect of the scenario concerns the role of the prominent binaural
inhibitory inputs to MSO neurons.
We examined these two unresolved issues by recording from MSO cells in
the Mexican free-tailed bat. Using sinusoidally amplitude-modulated tones, we found that the ITD sensitivities of many MSO cells in the bat
were remarkably similar to those reported for larger mammals. Our data
also indicate an important role for inhibition in sharpening ITD
sensitivity and increasing the dynamic range of ITD functions. A simple
model of ITD coding based on the timing of multiple inputs is proposed.
Additionally, our data suggest that ITD coding is a by-product of a
neuronal circuit that processes the temporal structure of sounds.
Because of the free-tailed bat's small head size, ITD coding is most
likely not the major function of the MSO in this small mammal and
probably other small mammals.
Key words:
medial superior olive; interaural time disparities; coincidence detection; inhibition; bat; amplitude modulation; mammalian
auditory brainstem
Copyright © 1998 Society for Neuroscience 0270-6474/98/18166608-15$05.00/0
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