Approximate entropy of motoneuron firing patterns during a motor preparation task.

The aim of this study was to test whether approximate entropy (ApEn) analysis provides a suitable method of detecting differences induced by a motor preparation task in time-ordered inter-spike intervals (ISIs) recorded in tonically firing motoneurons. Unlike classical methods of analyzing neuronal discharge variability, in which serial order is no taken into account, the approximate entropy (ApEn) was proposed by Pincus [Pincus SM. Approximate entropy as a measure of system complexity. Proc Natl Acad Sci USA 1991;88:2297–301] to analyze ordered series. ApEn statistic is a number assigned to an ordered series, where higher values correspond to greater serial irregularity. In the present study, the activity of 31 single motor units (SMUs) was recorded in human extensor carpi radialis muscles and the ISI durations were analyzed during the performance of a pre-cueing reaction time motor task involving a 3-s preparatory period. ApEn values were computed for each SMU during three steps of the preparatory period and during the preceding control period. Lower ApEn values, were found during preparatory period. The decrease in ApEn values, i.e., the increase in serial regularity, was monotonic from the control to the end of the preparatory period. These results show that ApEn model-independent statistics are a relevant means of detecting changes related to motor preparation in the regularity of time-ordered inter-spike intervals (ISIs).