Subthreshold outward currents enhance temporal integration in auditory neurons

Abstract

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 interaural time difference (ITD) tuning curves. To characterize how I KLT , which activates below spike threshold, influences a neurons 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 models 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.