Recurrence Plot Based Measures of Complexity and its Application to Heart Rate Variability Data

The knowledge of transitions between regular, laminar or chaotic behavior is essential to understand the underlying mechanisms behind complex systems. While several linear approaches are often insufficient to describe such processes, there are several nonlinear methods which however require rather long time observations. To overcome these difficulties, we propose measures of complexity based on vertical structures in recurrence plots and apply them to the logistic map as well as to heart rate variability data. For the logistic map these measures enable us not only to detect transitions between chaotic and periodic states, but also to identify laminar states, i.e. chaos-chaos transitions. The traditional recurrence quantification analysis fails to detect the latter transitions. Applying our new measures to the heart rate variability data, we are able to detect and quantify the laminar phases before a life-threatening cardiac arrhythmia occurs thereby facilitating a prediction of such an event. Our findings could be of importance for the therapy of malignant cardiac arrhythmias

Behavioral thermoregulation is highly repeatable and unaffected by digestive status in Agama atra

The precision and the extent of behavioral thermoregulation are likely to provide fitness benefits to ectotherms. Yet the factors driving variation in selected or preferred body temperature (Tset) and its usefulness as a proxy for optimal physiological temperature (Topt) are still debated. Although Tset is often conserved among closely related species, substantial variation at the individual, population and species level has also been reported. However, the repeatability (calculated as the intra-class correlation coefficient) of Tset is generally low. One factor that influences Tset is feeding status, with fed reptiles typically showing higher Tset, a process thought to aid meal digestion. Here, using experiments simulating realistic feeding and fasting regimes in Agama atra, a heliothermic lizard from southern Africa, we test if Tset and its repeatability under these 2 states significantly differ. Daily Tset ranged from 33.7 to 38.4 °C, with a mean (± SE) of 36.7 ± 0.1 °C for fed and 36.6 ± 0.1 °C for unfed individuals. Comparisons of repeatability showed that females tend to be more consistent in the selection of body temperature than males, but not significantly so regardless of feeding status. We report some of the highest repeatability estimates of Tset to date (full range: 0.229–0.642), and that the weak positive effects of feeding status on Tset did not increase its repeatability. In conclusion, one of the major prerequisites for natural selection, consistent among-individual variation, is present, making the adaptive significance of Tset considerably more plausible

Self and cross-velocity correlation functions and diffusion coefficients in liquids: a molecular dynamics study of binary mixtures of soft spheres

Molecular dynamics simulation is applied to the study of the diffusion properties in binary liquid mixtures made up of soft-sphere particles with different sizes and masses. Self- and distinct velocity correlation functions and related diffusion coefficients have been calculated. Special attention has been paid to the dynamic cross correlations which have been computed through recently introduced relative mean molecular velocity correlation functions which are independent on the reference frame. The differences between the distinct velocity correlations and diffusion coefficients in different reference frames (mass-fixed, number-fixed, and solvent-fixed) are discussed