A comparative study of the quantification of REM occurrence patterns via REM density and REM time-connectivity

In a previous study, we have shown that the use of a Markovian model and its associated state-transition probability matrices (STPMs) is a viable non-invasive approach to quantify the dynamics of REM occurrence patterns. This paper describes an attempt to compress the information conveyed by the STPMs into one single scalar parameter, defined as the time-connectivity of inter-REM intervals. This parameter was studied in conjunction with REM density, another global index of phasic REM activity during sleep, and the two were compared in terms of their variability with respect to subject, night of sleep recording and REM period rank in six healthy young subjects. The density index was found to be invariant to night of recording and REM period rank, but not with respect to subject. The time-connectivity parameter index was found to be invariant will all three variables, yielding the same results as the use of STPMs. The invariance characteristics of both indices and the usefulness of using time-connectivity as a global descriptor of a possible underlying organizational structure in the REM generation process are discussed.