Correlation between uncoupled conductance-based integrate-and-fire neuronsdue to common and synchronous presynaptic firing

We investigate the firing characteristics of conductance-based integrate-an d-fire neurons and the correlation of firing for uncoupled pairs of neurons as a result of common input and synchronous firing of multiple synaptic in puts. Analytical approximations are derived for the moments of the steady s tate potential and the effective time constant. We show that postsynaptic f iring barely depends on the correlation between inhibitory inputs; only the inhibitory firing rate matters. In contrast, both the degree of synchrony and the firing rate of excitatory inputs are relevant. A coefficient of var iation CV > 1 can be attained with low inhibitory firing rates and (Poisson -modulated) synchronized excitatory synaptic input, where both the number o f presynaptic neurons in synchronous firing assemblies and the synchronous firing rate should be sufficiently large. The correlation in firing of a pa ir of uncoupled neurons due to common excitatory input is initially increas ed for increasing firing rates of independent inhibitory inputs but decreas es for large inhibitory firing rates. Common inhibitory input to a pair of uncoupled neurons barely induces correlated firing, but amplifies the effec t of common excitation. Synchronous firing assemblies in the common input f urther enhance the correlation and are essential to attain experimentally o bserved correlation values. Since uncorrelated common input (i.e., common i nput by neurons, which do not fire in synchrony) cannot induce sufficient p ostsynaptic correlation, we conclude that lateral couplings are essential t o establish clusters of synchronously firing neurons.