Binaural and cochlear disparities
- *Laboratory of Auditory Neurophysiology, University of Leuven, Campus Gasthuisberg, B-3000 Leuven, Belgium; and
- †Department of Physiology, University of Wisconsin Medical School, 1300 University Avenue, Madison, WI 53706
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Edited by Eric I. Knudsen, Stanford University School of Medicine, Stanford, CA, and approved June 27, 2006 (received for review February 18, 2006)
Abstract
Binaural auditory neurons exhibit “best delays” (BDs): They are maximally activated at certain acoustic delays between sounds at the two ears and thereby signal spatial sound location. BDs arise from delays internal to the auditory system, but their source is controversial. According to the classic Jeffress model, they reflect pure time delays generated by differences in axonal length between the inputs from the two ears to binaural neurons. However, a relationship has been reported between BDs and the frequency to which binaural neurons are most sensitive (the characteristic frequency), and this relationship is not predicted by the Jeffress model. An alternative hypothesis proposes that binaural neurons derive their input from slightly different places along the two cochleas, which induces BDs by virtue of the slowness of the cochlear traveling wave. To test this hypothesis, we performed a coincidence analysis on spiketrains of pairs of auditory nerve fibers originating from different cochlear locations. In effect, this analysis mimics the processing of phase-locked inputs from each ear by binaural neurons. We find that auditory nerve fibers that innervate different cochlear sites show a maximum number of coincidences when they are delayed relative to each other, and that the optimum delays decrease with characteristic frequency as in binaural neurons. These findings suggest that cochlear disparities make an important contribution to the internal delays observed in binaural neurons.
Footnotes
- ‡To whom correspondence should be addressed at: Laboratory of Auditory Neurophysiology, University of Leuven, Campus Gasthuisberg, O&N2 Bus 1021 Herestraat 49, B-3000 Leuven, Belgium. E-mail: philip.joris{at}med.kuleuven.be
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Author contributions: P.X.J. designed research; P.X.J., D.H.L., and M.v.d.H. performed research; P.X.J., B.V.d.S., D.H.L., and M.v.d.H. contributed new reagents/analytic tools; P.X.J., B.V.d.S., and M.v.d.H. analyzed data; and P.X.J. and M.v.d.H. wrote the paper.
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Conflict of interest statement: No conflicts declared.
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This paper was submitted directly (Track II) to the PNAS office.
- Abbreviations:
- BD,
- best delay;
- CF,
- characteristic frequency;
- DF,
- dominant frequency;
- IC,
- inferior colliculus;
- ITD,
- interaural time difference;
- MSO,
- medial superior olive.
- © 2006 by The National Academy of Sciences of the USA
