Subthreshold outward currents enhance temporal integration in auditory neurons

Many auditory neurons possess low-threshold potassium currents (I_KLT) that enhance their responsiveness to rapid and coincident inputs. We present recordings from gerbil medial superior olivary (MSO) neurons in vitro and modeling results that illustrate how I_KLT improves the detection of brief signals, of weak signals in noise, and of the coincidence of signals (as needed for sound localization). We quantify the enhancing effect of I_KLT on temporal processing with several measures: signal-to-noise ratio (SNR), reverse correlation or spike-triggered averaging of input currents, and inter-aural time difference (ITD) tuning curves. To characterize how I_KLT, which activates below spike threshold, influences a neuron's voltage rise toward threshold, i.e., how it filters the inputs, we focus first on the response to weak and noisy signals. Cells and models were stimulated with a computer-generated steady barrage of random inputs, mimicking weak synaptic conductance transients (the "noise"), together with a larger but still subthreshold postsynaptic conductance, EPSG (the "signal"). Reduction of I_KLT decreased the SNR, mainly due to an increase in spontaneous firing (more "false positive"). The spike-triggered reverse correlation indicated that I_KLT shortened the integration time for spike generation. I_KLT also heightened the model's timing selectivity for coincidence detection of simulated binaural inputs. Further, ITD tuning is shifted in favor of a slope code rather than a place code by precise and rapid inhibition onto MSO cells (Brand et al. 2002). In several ways, low-threshold outward currents are seen to shape integration of weak and strong signals in auditory neurons.