Fano factor of single neurons and of populations.

A-C: Mean Fano factor of individual neurons for different values of P_ext: 0.01 (A), 0.2 (B), 1 (C). The strength of the recurrent synapses (a_rec) is color-coded. The mean Fano factor increases with the strength of the recurrent synapses. D-F: Same as in A-C but for the Fano factor of the population activity. The points represent the population Fano factor obtained from the simulation, and the lines are a weighted 7th-degree polynomial, used only as a visual aid. For P_ext = 0.01, the increase in Fano factor of individual neurons (A) can have a stronger effect on the population Fano factor than decreasing the pairwise correlations, resulting in an increase of the population Fano factor with high values of a_rec (D). For higher values of P_ext, the pairwise correlations greatly increase the population Fano factor, which then decreases with increasing a_rec.