define a 'spike code', as opposed to a 'firing rate code', as an encoding in which the ISIs (inter-spike intervals) are significant.
Idea for testing for the presence of such a code in the presence of knowledge about what is being encoded (the stimulus):
show different stimuli while recording spikes, then take the information about the stimulus in the isi, conditioned upon firing rate bins
note: might be better not to assume the stimulus. So, look for multimodal distribution in the ISI statistics.