Correlated firing and the information represented by neurons in short epochs

Is the information transmitted by an ensemble of neurons determined solely by the number of spikes fired by each cell, or do correlations in the emission of action potentials also play a significant role? We derive a simple formula which enables this question to be answered rigorously for short timescales. Using a power series expansion, the mutual information the ensemble of neurons conveys about external stimuli is broken down into firing rate and correlation components. This analysis leads to a new quantification procedure which is directly applicable to simultaneous multiple neuron recordings. It also shows rigorously that even when noise correlations are not stimulus modulated, synergistic effects can arise if signal and noise correlation are of opposite sign.