Role of correlations in population coding

Correlations among spikes, both on the same neuron and across neurons, are ubiquitous in the brain. For example cross-correlograms can have large peaks, at least in the periphery, and smaller -- but still non-negligible -- ones in cortex, and auto-correlograms almost always exhibit non-trivial temporal structure at a range of timescales. Although this has been known for over forty years, it's still not clear what role these correlations play in the brain -- and, indeed, whether they play any role at all. The goal of this chapter is to shed light on this issue by reviewing some of the work on this subject.