Bits and brains: Information flow in the nervous system

Until recently there have been no convincing quantitative measurements on the rates of information transmission in real neurons. Here we review the theoretical basis for making such measurements, together with the data which demonstrate remarkably high information rates in a variety of systems. In fact these rates are within a factor of two of the absolute physical limits set by the entropy of neural spike trains. Theses observations lead to sharp theoretical questions about the structure of the code and the strategy for adapting the code to different ensembles of input signals.