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Signal generation in the lateral superior olive : the rate code of interaural disparities of sound

University of British Columbia

The azimuthal location of a sound source imposes interaural intensity disparities (IIDs) on binaural sound, which are encoded by principal neurons of the lateral superior olive (LSO). These disparities are thought to be represented in the chopper discharge pattern, characterized by regular repetitive firing at a precisely timed onset. Presumably, the integration of ipsilateral excitation and contralateral inhibition determines the chopper rate code of IID. It was hypothesized that this rate code must remain stable during prolonged stimulation to provide an effective localization code. Extracellular recordings of LSO chopper units during dichotic tonal stimulation revealed not only that short-term adaptation occurs in both the ipsilateral and contralateral inputs, but that binaural responses are stable only when the IID is zero. Therefore, the chopper rate code of IID cannot provide a sufficient or reliable index of azimuthal location. The LSO chopper pattern was hypothesized to arise partly from the interplay of intrinsic membrane properties of the principal neuron. Whole-cell recordings of LSO neurons in brainstem slices during direct current injection revealed that membrane properties involving early peak polarizations emphasized and accelerated response onset, and contributed to chopper response generation. A transient depolarizing potential was sensitive to 600 nM tetrodotoxin, 50 µM nickel, and 3 mM cesium, indicating contributions from persistent sodium, transient low-threshold calcium, and hyperpolarization-activated cation conductances. Potassium conductances sensitive to 4- aminopyridihe shaped the decay of the transient potential. Two inward (anomalous) rectifiers sensitive to 0.2 mM barium and 3 mM cesium, respectively, accounted for rectification in the hyperpolarized range. Interestingly, this repertoire of conductances is similar to that proposed to support the preservation of temporal information in phaselocking neurons. This implies that chopper responses may encode temporal aspects of the auditory stimulus. The chopper pattern may not provide a simple rate code of IID, since it alone does not distinguish a moving sound source from a prolonged stationary one. Further, a specialized repertoire of intrinsic membrane conductances may allow the chopper neuron to encode intensity/IID dynamics of the complex stimulus. Future research endeavours employing more complex stimulation paradigms will certainly illuminate the exciting issue of sound coding in the LSO.

Thesis/Dissertation

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2009-07-21

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http://hdl.handle.net/2429/11086

Neuroscience

University of British Columbia

UBCV

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