A method for assessing significance of peaks in cross-correlation histograms

Cross-correlation histograms have been widely used to analyze the interdependence of two simultaneously observed trains of neuronal spikes or muscle motor unit discharges. Here, a formula is presented for calculating a synchronization index from such a histogram to reliably detect subtle correlations such as short-term synchronization, even in the case of relatively sparse data, as well as allowing comparison of the degree of synchronization of grouped or correlated motor unit discharges. The index takes into account the number of counts in the histogram, number of bins, and width of the histogram peak. A table of critical values of the index, at several levels of statistical significance, is included.