Correlation-based estimate of the fraction of spikes corresponding to neuronal activity for a low detection threshold. Hippocampal culture, 15 min recording. A: Rasterplot of the activity. Dark colors correspond to highly correlated spikes. B: Fractions classified as neuronal spikes. C: Results after shuffling the channel identities of spikes to preserve nonstationarities and different firing rates. D: Result for a 30 min recording of an empty MEA.