During the stationary portion of neuron's spiking response to a stimulus, the stimulus could be coded in the average rate and, more elaborately, in the statistics of the sequence of interspike intervals. We use information processing theory to explicitly define when interval coding occurs and quantify the coding gain beyond rate coding provided by the interval code. We explicitly find the interval distribution commensurate with average rate coding. When we analyzed optomotor neural responses recorded from the crayfish eye, we found little interval coding occuring despite stimulus-induced changes from a unimodal to a bimodal interval distribution.