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Sci Rep. 2016 Aug 8;6:30895. doi: 10.1038/srep30895.

The suppression of scale-free fMRI brain dynamics across three different sources of effort: aging, task novelty and task difficulty.

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Keenan Research Centre of the Li Ka Shing Knowledge Institute, St. Michael's Hospital, 30 Bond St, Toronto, ON, M5B 1W8, Canada.
Rotman Research Institute, Baycrest Hospital, 3560 Bathurst St, Toronto, ON, M6A 2E1, Canada.
Department of Medical Biophysics, University of Toronto, 101 College St, Suite 15-701, Toronto, ON, M5G 1L7, Canada.
School of Nursing, University of Michigan, 426 N Ingalls St, Ann Arbor, MI, 48104, USA.
Department of Psychology, University of Michigan, 400 N Ingalls St, Ann Arbor, MI, 48109, USA.
Department of Radiology, University of Washington, 4245 Roosevelt Way NE, Seattle, WA, 98105, USA.
College of Nursing, Chungnam National University, Jung-gu, Munhwa-ro 266, Daejeon, 301-747, South Korea.
Biomedical Engineering, University of Michigan, 2200 Bonisteel Blvd, Ann Arbor, MI, 48109, USA.
Institute of Medical Science, University of Toronto, 1 King's College Cir, Toronto, ON, M5S 1A8, Canada.
Department of Psychology, University of Chicago, 5848 S University Ave, Chicago, IL, 60637, USA.
Grossman Institute for Neuroscience, Quantitative Biology and Human Behavior, 5812 S Ellis Ave, Chicago, IL, 60637, USA.


There is growing evidence that fluctuations in brain activity may exhibit scale-free ("fractal") dynamics. Scale-free signals follow a spectral-power curve of the form P(f ) ∝ f(-β), where spectral power decreases in a power-law fashion with increasing frequency. In this study, we demonstrated that fractal scaling of BOLD fMRI signal is consistently suppressed for different sources of cognitive effort. Decreases in the Hurst exponent (H), which quantifies scale-free signal, was related to three different sources of cognitive effort/task engagement: 1) task difficulty, 2) task novelty, and 3) aging effects. These results were consistently observed across multiple datasets and task paradigms. We also demonstrated that estimates of H are robust across a range of time-window sizes. H was also compared to alternative metrics of BOLD variability (SDBOLD) and global connectivity (Gconn), with effort-related decreases in H producing similar decreases in SDBOLD and Gconn. These results indicate a potential global brain phenomenon that unites research from different fields and indicates that fractal scaling may be a highly sensitive metric for indexing cognitive effort/task engagement.

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