Synchronization analysis using joint peri-stimulus time histograms for human motor units

The concurrence of action potentials from two motor units (MUs) happens more frequently in human than what one would expect by chance. It can be visualized as the central peak of the Cross-Correlogram of spike trains from two MUs. It is widely believed that a common pre-synaptic input to motoneurons from the cortical tract generates the concurrence. A number of synchronization indices have been proposed in order to evaluate the quantity of the common input, however, these indices are dependent on the firing rates of the MUs. To resolve this issue, we introduce a new synchronization index beta using a joint peri-stimulus time histograms (JPSTH). In this paper, we discuss the dependence of synchronization indices on firing rates using a spike model, and the application of JPSTH and the index beta to experimental data for the quantitative analysis of MU synchronization. We confirm that the novel index beta represents the synchronization state independently from firing rates, and that it also accurately traces event-related temporal modulation of synchronization. In conclusion, we consider the index beta to be a valuable index for the analysis of MU synchronization.