Entropy and Information in Neural Spike Trains
  The nervous system represents time-dependent signals in sequences of discrete action potentials or spikes, all spikes are identical so that information is carried only in the spike arrival times. We show how to quantify this information, in bits, free from any assumptions about which features of the spike train or input waveform are most important. We apply this approach to the analysis of experiments on a motion-sensitive neuron in the fly visual system. 
S. P. Strong, Roland Koberle, Rob R. de Ruyter van Steveninck and   William Bialek