Fractal Regulation in Temporal Activity Fluctuations: A Biomarker for Circadian Control and Beyond

Abstract

Motor activity in humans and other animals possesses fractal temporal fluctuations that co-exists with circadian or daily activity rhythms. The perturbations in fractal activity patterns are often accompanied by altered circadian/daily rhythms. The goal of this study is to test whether fractal regulation in motor activity provides physiological information independent from 24-h/circadian rhythmicity. To achieve the goal, we studied locomotor activity recordings of rats with the lesion of the suprachiasmatic nucleus (SCN) that are known to have diminished circadian/daily activity rhythms and perturbed fractal regulation. By restricting feeding time (i.e., food was only availability in the dark period of the 12h: 12h light-dark cycles), we found that mean activity levels in these animals displayed significant 24-h rhythms. In contrast, the restricted feeding had no influences on the perturbed fractal regulation in these SCN-lesioned animals, i.e., activity fluctuations in these animals remained random over a wide range of time scales from 2-20h. Our results indicate that 24-h rhythm of food availability can restore/improve circadian/daily rhythms in the SCN-lesioned animals but not necessarily improve the disrupted fractal activity regulation in these animals. This study provides clear and direct evidence that fractal activity patterns offer complementary information about motor activity regulation at multiple time scales that is beyond 24-h rhythm control.

Keywords: Circadian rhythm; Food restriction; Fractal activity regulation; Inter-daily stability (IS); Motor activity.

Figure 1

Two 7-day locomotor activity recordings of a rat with the lesioned SCN. (A) The animal had ad libitum food access. (B) The animal had restricted feeding during the 12-h dark phase of the light-dark cycles.

Figure 2

Group averages of mean activity levels during the light phase and during the dark phase. Restricted feeding caused a significant increase in the mean activity levels during the dark phase (indicated by **: p<0.01) but not during the light phase. Such effects of restricted feeding led to a significant light-dark rhythm (indicated by ##: p<0.01).

Figure 3

Effect of restricted feeding on inter-daily stability (IS) of 24-h activity rhythm. IS was significantly higher when food was only available during the 12-h dark phase (indicated by *: p<0.05).

Figure 4

Different temporal activity correlations at different time scales in SCN-lesioned rats. (A) Fluctuation functions of the activity recordings of an SCN-lesioned rat under ad libitum and restricted feeding conditions. Data are shown on log-log plots. Results were obtained from the signals shown in Figure 1 using the detrended fluctuation analysis. (B) Group averages of scaling exponents at time scales of 0.2–2h and at 4–20h. ### indicates the significant difference between the two scale regions (p<0.0001). The dashed black line indicates the average behavior of intact rats with a scaling exponent of 1.0. The dashed red line indicates the scaling exponent for white noise with random fluctuations.