BINAURAL INTERACTION IN THE LATERAL SUPERIOR OLIVE - TIME DIFFERENCE SENSITIVITY STUDIED IN MOUSE-BRAIN SLICE

1. The sensitivity of lateral superior olive (ISO) neurons to interaural time differences was examined in an in vitro brain slice preparation. Brain slices, 400-500 mum, were taken through the superior olivary complex of C57 BL/6J mice and were maintained in an oxygenated saline solution for single-unit recording. Both extracellular and intracellular recordings were made with glass pipettes filled with 4 M potassium acetate. Responses were elicited by applying current pulses to the trapezoid body through bipolar stimulating electrodes located ipsilateral or contralateral to the olivary complex. Binaural interactions were studied by manipulating the timing and intensity of paired ipsilateral and contralateral pulses. 2. In extracellular recordings, stimulation of the ipsilateral trapezoid body usually elicited a single action potential, whereas stimulation of the contralateral trapezoid body failed to produce a spike response. Bilateral stimulation resulted in the complete suppression of the evoked spike, indicating the presence of a contralateral inhibitory effect. The degree of inhibition depended on the inter-pulse interval between ipsilateral and contralateral stimulation. With sufficiently large ipsilateral lead times, the probability of eliciting an extracellular spike was 1.0. As the interpulse interval war, gradually shifted to reduce the ipsilateral lead time, the response probability precipitously dropped to 0.0. Most neurons could be completely suppressed by simultaneous stimulation. The dynamic range, defined as the range of interpulse intervals over which response probability changed from 0.9 to 0. 1, was between 125 and 225 mus for most cells tested. 3. with increasing contralateral lead times, the extracellularly recorded spike was eventually released from inhibition. The duration of contralateral suppression was reflected in the length of the interpulse interval associated with restoration of the spike. Different patterns of inhibition were described in terms of the probability of eliciting a spike as the interpulse interval was increased. There was considerable variability among neurons in the time course of contralateral inhibition. In some cells the inhibition lasted only 1-2 ms and disappeared rapidly as contralateral lead time was increased. In others, the inhibition lasted as long as 5-6 ms and disappeared much more gradually. Examples were found of neurons with inhibitory periods that fell between these extremes. 4. Intracellular recordings showed that ipsilateral stimulation produced excitatory postsynaptic potentials (EPSPs) and contralateral stimulation produced inhibitory postsynaptic potentials (IPSPs) in the same LSO neuron. Increasing ipsilateral current strength resulted in a graded depolarization, and increasing contralateral current resulted in a graded hyperpolarization of the cell membrane. Contralateral IPSPs were usually longer in duration than ipsilateral EPSPs, but the duration varied from cell to cell. The probability of generating an action potential in LSO was determined by the interplay of ipsilateral and contralateral postsynaptic potentials. 5. The effect of interpulse interval on intracellular potentials was examined for single neurons in LSO. With sufficiently large ipsilateral lead times, an EPSP dominated the postsynaptic response. As interpulse interval was shifted in favor of the contralateral side, the EPSP was at first cancelled and finally replaced by an IPSP. The probability of generating a spike was determined by the relative dominance of excitatory and inhibitory postsynaptic potentials. The time course of spike suppression was directly related to the duration of the IPSP. 6. Both intracellular and extracellular data indicate that LSO neurons are inherently sensitive to small differences in bilateral timing of afferent impulses and, therefore, may play a role in coding binaural time differences that serve as peripheral cues for sound localization.