# Collective synchronized activation in small clusters.

(a) and (b) show the IBI and the NB widths distributions, respectively, in a typical isolated cluster. The IBI distribution is characterized by an asymmetric distribution with non vanishing probability for the occurrences of long intervals, while the width distribution is characterized by a typical time scale. The average NB rate (1/IBI) and width (orange line in (a) and (b), respectively) are calculated for every cluster separately and are presented as a function of the cluster's size (in log scale) in (c) and (d), respectively. Ninety-seven clusters from six different cultures (three CNT and three PDL-coated electrodes) were analyzed. Four additional cultures of uniform networks were analyzed for comparison (black dots). The average NB rate and width were calculated for consecutive and equal time bins on a logarithmic scale in (e) and (f), respectively. The error bars represent the standard deviation of the average value in every bin. The high variability in the activity properties observed in clusters is also observed in uniform networks of 10^{6} cells (black dots). The red area (corresponding to clusters of 2500–5000 µm^{2}) marks the estimated transition to spontaneously bursting clusters (its upper limit corresponds to approximately 40 cells).