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, 174 (2), 292-300

Endogenous Multifractal Brain Dynamics Are Modulated by Age, Cholinergic Blockade and Cognitive Performance

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Endogenous Multifractal Brain Dynamics Are Modulated by Age, Cholinergic Blockade and Cognitive Performance

John Suckling et al. J Neurosci Methods.

Abstract

The intuitive notion that a healthy organism is characterised by regular, homeostatic function has been challenged by observations that a loss of complexity is, in fact, indicative of ill-health. Monofractals succinctly describe complex processes and are controlled by a single time-invariant scaling exponent, H, simply related to the fractal dimension. Previous analyses of resting fMRI time-series demonstrated that ageing and scopolamine administration were both associated with increases in H and that faster response in a prior encoding task was also associated with increased H. We revisit this experiment with a novel, multifractal approach in which fractal dynamics are assumed to be non-stationary and defined by a spectrum of local singularity exponents. Parameterisation of this spectrum was capable of refracting the effects of age, scopolamine and task performance as well as a refining a description of the associated signal changes. Using the same imaging data, we also explored turbulence as a possible mechanism underlying multifractal dynamics. Evidence is provided that Carstaing's model of turbulent information flow from high to low scales has only limited validity, and that scale invariance of energy dissipation is better explained by critical-phase phenomena, supporting the proposition that the brain maintains a state of self-organised criticality.

Figures

Figure 1
Figure 1
Probability density of function. P(Δν(r)) of the difference in BOLD signal measurements, Δν(r), separated in time by r. Data points represents values of (top to bottom) r = 2, 4, 8, 16, 32 and 64 time-points (vertically offset for clarity) and the solid line is the regressed fit of the turbulence model.
Figure 2
Figure 2
Coronal (y = +22 mm), saggital (x = -42 mm) and axial (z = -10 mm) views of the MNI template with, (a) top row: significant negative correlation between mean reaction during the fame decision/facial encoding task and estimates of the Hurst exponent from subsequent resting time series; (b) bottom row: activation elicited by the contrast of all faces against cross-hair fixation during the fame decision/facial encoding task. Yellow corresponds to activated regions, blue to deactivated regions. Left of the image corresponds to the right-hand of the brain. R = right and L = left hand-sides of the brain.
Figure 3
Figure 3
Regions of significant main effects of (a) age and (b) drug on the Hurst exponent of resting fMRI time series. For all maps, the right side of the brain is represented by the right side of the image; the origin of the x and y dimensions of Talairach space is indicated by the cross-hair; the z coordinate of each section, i.e., mm below the intercommissural plane, is indicated by numbers adjacent to each section. Reproduced in part (with permission) from Wink et al (2006).
Figure 4
Figure 4
Singularity spectra extracted from regions demonstrating: (a) main effect of age in right hemisphere; (b) main effect of age in left hemisphere; (c) main effect of drug, based on a univariate voxelwise test of H. In each plot the hashed line is the spectrum extracted from the participant with the lowest value of H, and the solid line for the highest value of H in the labelled groups (drug or age).
Figure 4
Figure 4
Singularity spectra extracted from regions demonstrating: (a) main effect of age in right hemisphere; (b) main effect of age in left hemisphere; (c) main effect of drug, based on a univariate voxelwise test of H. In each plot the hashed line is the spectrum extracted from the participant with the lowest value of H, and the solid line for the highest value of H in the labelled groups (drug or age).
Figure 4
Figure 4
Singularity spectra extracted from regions demonstrating: (a) main effect of age in right hemisphere; (b) main effect of age in left hemisphere; (c) main effect of drug, based on a univariate voxelwise test of H. In each plot the hashed line is the spectrum extracted from the participant with the lowest value of H, and the solid line for the highest value of H in the labelled groups (drug or age).
Figure 5
Figure 5
Boxplots of regional mean parameters hmax, W+and Wextracted from regions demonstrating significant: (a) main effect of age in right hemisphere; (b) main effect of age in left hemisphere; (c) main effect of drug, based on a univariate, voxelwise test of H.
Figure 6
Figure 6
Regional relationship between lnλ2 and r for each of the regions identified in the univariate analyses of H. (a) main effect of age (left and right regions); (b) main effect of drug; (c) correlation with mean reaction time of fame decision/facial encoding task

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