Neural Population Coding is Optimized by Discrete Tuning Curves
  The sigmoidal tuning curve that maximizes the mutual information for a Poisson neuron, or population of Poisson neurons, is obtained. The optimal tuning curve is found to have a discrete structure that results in a quantization of the input signal. The number of quantization levels undergoes a hierarchy of phase transitions as the length of the coding window is varied. We postulate, using the mammalian auditory system as an example, that the presence of a subpopulation structure within a neural population is consistent with an optimal neural code. 
Alexander P. Nikitin, Nigel G. Stocks, Robert P. Morse and Mark D.   McDonnell