An inherent difficulty in determining the size of a neuronal assembly is that without an explicit goal, it is not possible to quantitatively define which neurons belong to the primary assembly and which represent feedback activation of assembly members or newly recruited assemblies, serving other goals. Although many neurons can contribute to a cell assembly, the contribution of individual members is most often strongly skewed, as is the case for musical orchestras. For example, activity of just a few strongly firing hippocampal place cells can be much more informative about the rat’s position than several dozens of simultaneously recorded other neurons from the same volume and with the same total number of spikes (Wilson and McNaughton, 1993). Similarly, neurons that can predict the future choice of the animal in the hippocampus and prefrontal cortex represent only 1 to 10 percent of the recorded active cells yet they are more informative about the behavioral outcome than the entire remaining population (Ferbinteanu and Shapiro, 2003; Frank et al., 2000; Fujisawa et al., 2008; Pastalkova et al., 2008; Quian Quirroga et al., 2005;
