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Temporal precision in coincidence detecting auditory neurons

John Rinzel, G. Svirskis, V. Kotak, and D. Sanes

NYU

Localization of low frequency sounds involves precise computation of interaural time differences (sub-ms range). The first neurons that receive binaural input and participate in this computation are, for mammals, in the medial superior olive (MSO) and they have distinct biophysical properties. They spike in response to transient but not to slowly varying stimuli and phase-lock well to periodic input. A low-threshold potassium current I_K-lt contributes to the MSO neuron's temporal processing qualities partly because the membrane's "time constant" is shortened by this extra conductance but the dynamic aspects of I_K-lt activation also play a role. Transient signals must outrace the activation of I_K-lt if they are to cause the cell to spike. We characterize the role of I_K-lt through in vitro experiments (gerbil MSO) and with computational models, of the Hodgkin-Huxley and enhanced integrate-and-fire types. We focus particularly on what makes these coincidence-detecting cells fire, i.e. on how they integrate subthreshold signals in the presence of a noisy synaptic background, typical of the peripheral auditory system. Our results show that partial block of I_K-lt reduces: (1) the signal-to-noise ratio, (2) the probability to fire in response to closely-timed inputs, (3) the quality (vector strength) of phase-locking, and (4) the temporal sharpness of transient currents that cause spiking (as seen with reverse correlation analysis).