A Model for Interaural Time Difference Sensitivity in the Medial Superior Olive: Interaction of Excitatory and Inhibitory Synaptic Inputs, Channel Dynamics, and Cellular Morphology

Abstract

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 approximately 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.