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The Journal of Neuroscience, March 23, 2005, 25(12):3046-3058; doi:10.1523/JNEUROSCI.3064-04.2005
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Behavioral/Systems/Cognitive
A Model for Interaural Time Difference Sensitivity in the Medial Superior Olive: Interaction of Excitatory and Inhibitory Synaptic Inputs, Channel Dynamics, and Cellular Morphology
Yi Zhou,1
Laurel H. Carney,2 and
H. Steven Colburn1
1Department of Biomedical Engineering, Center for
Hearing Research, Boston University, Boston, Massachusetts 02215, and2
Department of Bioengineering and Neuroscience,
Institute for Sensory Research, Syracuse University, Syracuse, New York
13244-5290
This study reports simulations of recent physiological results from the
gerbil medial superior olive (MSO) that reveal that blocking glycinergic
inhibition can shift the tuning for the interaural time difference (ITD) of
the cell (Brand et al., 2002).
Our simulations indicate that the model proposed in the study by Brand et al.
(2002) requires precisely
timed, short-duration inhibition with temporal accuracy exceeding that
described in the auditory system. An alternative model is proposed that
incorporates two anatomic observations in the MSO: (1) the axon arises from
the dendrite that receives ipsilateral inputs; and (2) inhibitory synapses are
located primarily on the soma in adult animals. When the inhibitory currents
are activated or blocked, the model cell successfully simulates experimentally
observed shifts in the best ITD. The asymmetrical cell structure allows an
imbalance between the ipsilateral and contralateral excitatory inputs and
shifts the ITD curve such that the best ITD is not at zero. Fine adjustment of
the best ITD is achieved by the interplay of somatic sodium currents and
synaptic inhibitory currents. The shift of the best ITD in the model is
limited to 0.2 ms, which is behaviorally significant with respect to ITDs
encountered in perceptual tasks. The model suggests a mechanism for
dynamically "fine-tuning" the ITD sensitivity of MSO cells by the
opponency between depolarizing sodium currents and hyperpolarizing inhibitory
currents.
Key words: interaural time differences; medial superior olive; inhibition; asymmetrical cell morphology; binaural hearing; neural modeling
Received July 27, 2004;
revised January 26, 2005;
accepted January 26, 2005.
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