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

Abstract

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.

Core and Matrix Thalamic Sub-Populations Relate to Spatio-Temporal Cortical Connectivity Gradients

“…The window range avoids zero residuals across the shortest intervals and lowfrequency confounds identified in the power-spectral density across the long intervals. Consistent with (Churchill et al, 2016;He, 2011), we observed a crossover between two fractal ranges, consisting of an initial large increase of ( ) and large H for small window lengths due to serial autocorrelations which decay rapidly transitioning to smaller H values.…”

“…autocorrelation (Murray et al, 2014;Watanabe et al, 2019) and fractal based (He, 2011;Churchill et al, 2016) techniques. While both measures identify temporal selfsimilarity, autocorrelation techniques typically focus on linear correlations with past values of a particular region, whereas in contrast, fractal analysis is used as a measure of complexity (Dong et al, 2018) and can be used to discern the significance of differing time-scale frequencies.…”

“…While both measures identify temporal selfsimilarity, autocorrelation techniques typically focus on linear correlations with past values of a particular region, whereas in contrast, fractal analysis is used as a measure of complexity (Dong et al, 2018) and can be used to discern the significance of differing time-scale frequencies. For instance, a purely fractal signal suggests all frequencies contain information (Churchill et al, 2016).…”

Dynamic Functional Connectivity between Order and Randomness and its Evolution across the Human Adult Lifespan

“…Our observation of multifractal scaling in dFC is not completely unexpected. Quite on the contrary, it is well known that fMRI time-series at rest and during tasks display very characteristic multifractal spectral properties, relating to both behavioural performance and pathological alterations and modulated by task difficulty or aging (Maxim et al, 2005;Ciuciu et al, 2012;He, 2014;Churchill et al, 2016;Dong et al, 2018). Tools way more sophisticated than the ones we adopt here have been used to characterize and confirm multifractality in fMRI signals (Ciuciu et al, 2017;La Rocca et al, 2018).…”

“…To this heterogeneity of regional involvement, may correspond a parallel heterogeneity of dFC properties, which we fully ignore in this first study. It is known that variance of FC links in time (Chen et al, 2017) or even the fractal scaling properties of fMRI signals (Maxim et al, 2005;Churchill et al, 2016;Dong et al, 2018) are affected heterogeneously across brain regions. In a companion study (Lombardo et al, submitted jointly) we make a further step forward, probing variations of cognitive performance in selective attention and other cognitive functions, before and after sleep deprivation.…”