Abstract
Background: Temporal fractals are characterized by prominent scale-invariance and self-similarity across time scales. Monofractal analysis quantifies this scaling behavior in a single parameter, the Hurst exponent (H). Higher H reflects greater correlation in the signal structure, which is taken as being more fractal. Previous fMRI studies have observed lower H during conventional tasks relative to resting state conditions, and shown that H is negatively correlated with task difficulty and novelty. To date, no study has investigated the fractal dynamics of BOLD signal during naturalistic conditions.Methods: We performed fractal analysis on Human Connectome Project 7T fMRI data (n = 72, 41 females, mean age 29.46 ± 3.76 years) to compare H across movie-watching and rest.Results: In contrast to previous work using conventional tasks, we found higher H values for movie relative to rest (mean difference = 0.014; p = 5.279 × 10−7; 95% CI [0.009, 0.019]). H was significantly higher in movie than rest in the visual, somatomotor and dorsal attention networks, but was significantly lower during movie in the frontoparietal and default networks. We found no cross-condition differences in test-retest reliability of H. Finally, we found that H of movie-derived stimulus properties (e.g., luminance changes) were fractal whereas H of head motion estimates were non-fractal.Conclusions: Overall, our findings suggest that movie-watching induces fractal signal dynamics. In line with recent work characterizing connectivity-based brain state dynamics during movie-watching, we speculate that these fractal dynamics reflect the configuring and reconfiguring of brain states that occurs during naturalistic processing, and are markedly different than dynamics observed during conventional tasks.
Highlights
Fractals emerge when the repetition of a simple process creates a recursive structure (Eke et al, 2000), such as the growth of a tree from one branch to two
Overall, our findings suggest that movie-watching induces fractal signal dynamics
In line with recent work characterizing connectivity-based brain state dynamics during movie-watching, we speculate that these fractal dynamics reflect the configuring and reconfiguring of brain states that occurs during naturalistic processing, and are markedly different than dynamics observed during conventional tasks
Summary
Fractals emerge when the repetition of a simple process creates a recursive structure (Eke et al, 2000), such as the growth of a tree from one branch to two. They are defined by scaleinvariance, meaning that the pattern that emerges when the process is repeated remains the same regardless of the scale at which it is viewed. Temporal fractals are characterized by self-affinity and power-law behavior (Eke et al, 2002) The former refers to the notion that when a smaller element of the signal is magnified along the time axis it resembles a larger element of the whole. No study has investigated the fractal dynamics of BOLD signal during naturalistic conditions
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