Efficient Coding and Sparse Coding in the Brain

It is widely thought that spikes (action potentials) are the main carrier of information in the brain.

But what is the neural code, that is, what aspects of the spike trains carry the information? The detailed temporal structure or maybe only the average firing rate?  And is there information in the correlation between spike trains in populations of similar neurons?  

The guest has thought about these and other coding questions throughout his career.