We present a time-dependent level-crossing theory for linear dynamical systems perturbed by colored Gaussian noise. We apply these results to approximate the firing statistics of conductancebased integrate-and-fire neurons receiving excitatory and inhibitory Poissonian inputs. Analytical expressions are obtained for three key quantities characterizing the neuronal response to time-varying inputs: the mean firing rate, the linear response to sinusoidally-modulated inputs, and the pairwise spike-correlation for neurons receiving correlated inputs. The theory yields tractable results that are shown to accurately match numerical simulations, and provides useful tools for the analysis of interconnected neuronal populations.PACS numbers: 05.10. Gg,87.19.lj,87.19.ll,87.19.lm Understanding the dynamics of interconnected networks of neurons is of fundamental importance in theoretical neuroscience. An essential step in solving this problem is to determine the input-output relationship of individual neurons given an underlying biophysical model. For white-noise-driven integrate-and-fire (IF) neurons this problem is generally tractable (e.g., [1][2][3][4]), but efforts to integrate key aspects of neuronal signaling into the IF formalism have added to its complexity. First, synaptic input consists of discrete action potentials, which results in non-Gaussian voltage distributions and affects firing statistics [5][6][7][8]. Second, synaptic communication is mediated by two separate (excitatory and inhibitory) systems with distinct kinetics. The majority of theoretical studies include only one synaptic type and assume either fast (e.g., [9-11]) or slow [11,12] kinetics compared with the membrane integration time, but experimental evidence suggests that these timescales are often comparable [13]. Finally, neurons sharing presynaptic partners exhibit correlations in their synaptic input. While network models typically assume sparse connectivity for which correlations are negligible, recent reports suggest important functional roles for this type of "noise" correlation [14,15]. For IF neurons, obtaining the input-output relationship essentially involves computing moments of the first-passage-time (FPT) to threshold, but analytical solutions are rarely possible. Here, we show that for membrane-to-synaptic time constant ratios of the order of unity, level-crossing statistics provide good approximations to the FPT while retaining sufficient tractability to incorporate additional biological detail. Analytical expressions are given for the mean firing rate, pairwise spike-correlation and linear response to sinusoidally-modulated inputs, which compare favorably with numerical simulations of conductance-based IF neurons. The method thus provides a complete set of input- * Present address: Laboratory for Circuit Mechanisms of Sensory Perception, RIKEN Brain Science Institute, Wako, Saitama, Japan output properties needed for an analysis at the network level. Although the focus of the present paper is on neuroscience applications, the ...