Hypoxia, Leptin and Hippocampal Neurons Firing Figure 2. Network activity changes along with culture development. Raster plots of two representative MEAs at DIV 11 (top) and DIV 18 (bottom). Each line represents the signals detected by a single electrode of the MEA array, during 90 seconds recordings. Uniquely active channels are shown. Synchronization of the spontaneous activity is almost absent in younger cultures, while it is clearly visible at DIV 18 (rectangle). doi:10.1371/journal.pone.0041530.g002 Figure 3. Synchronization of the activity among the electrodes increases with time in culture. Probability vs time diagrams indicating the cross-correlation between one reference electrode and another representative channel, respectively at DIV 11 (A) and DIV 18 (B). The diagrams are plotted at two different time scales: 63.5 s (top) and 60.5 s (bottom). Inset shows the net increment of cross-correlation between the electrodes after 18 DIV (black bars) in comparison with younger cultures (light grey bars) (p,0.001). Data are expressed as means 6 S.E.M and statistical significance was calculated by using Student’s paired t-test. Values of p,0.05 were considered significant. C. Different synchronization on DIV 11 and DIV 18 is shown for two representative MEAs. doi:10.1371/journal.pone.0041530.g003 PLoS ONE | www.plosone.org 5 July 2012 | Volume 7 | Issue 7 | e41530