Descriptive statistics.

<p>Descriptive statistics.</p

Whole-brain average HFD in dependence on <i>k</i>_<i>max</i>.

<p>Dashed curves represent the standard error of the mean. Dotted lines at <i>k</i>_<i>max</i> = 35 and <i>k</i>_<i>max</i> = 65 represent, respectively, the starting point of HFD stability with respect to the difference between groups, and the selected <i>k</i>_<i>max</i> in our study.</p

Electroencephalographic Fractal Dimension in Healthy Ageing and Alzheimer’s Disease

<div><p>Brain activity is complex; a reflection of its structural and functional organization. Among other measures of complexity, the fractal dimension is emerging as being sensitive to neuronal damage secondary to neurological and psychiatric diseases. Here, we calculated Higuchi’s fractal dimension (HFD) in resting-state eyes-closed electroencephalography (EEG) recordings from 41 healthy controls (age: 20–89 years) and 67 Alzheimer’s Disease (AD) patients (age: 50–88 years), to investigate whether HFD is sensitive to brain activity changes typical in healthy aging and in AD. Additionally, we considered whether AD-accelerating effects of the copper fraction not bound to ceruloplasmin (also called “free” copper) are reflected in HFD fluctuations. The HFD measure showed an inverted U-shaped relationship with age in healthy people (R² = .575, <i>p</i> < .001). Onset of HFD decline appeared around the age of 60, and was most evident in central-parietal regions. In this region, HFD decreased with aging stronger in the right than in the left hemisphere (<i>p</i> = .006). AD patients demonstrated reduced HFD compared to age- and education-matched healthy controls, especially in temporal-occipital regions. This was associated with decreasing cognitive status as assessed by mini-mental state examination, and with higher levels of non-ceruloplasmin copper. Taken together, our findings show that resting-state EEG complexity increases from youth to maturity and declines in healthy, aging individuals. In AD, brain activity complexity is further reduced in correlation with cognitive impairment. In addition, elevated levels of non-ceruloplasmin copper appear to accelerate the reduction of neural activity complexity. Overall, HDF appears to be a proper indicator for monitoring EEG-derived brain activity complexity in healthy and pathological aging.</p></div

Experimental setup.

<p>A) 19-channel EEG cap. B) Regional HFD and HFD Homologous areas symmetry (HArS) estimates (explicative values). HFD is calculated over the whole time series measured at each derivation (windows of 2-seconds are displayed). HFD values are averaged across multiple derivations that overlay the same brain region (frontal and parietal). HArS estimates left vs. right normalized HFD difference (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0149587#sec006" target="_blank">methods</a>).</p

Electroencephalographic Fractal Dimension in Healthy Ageing and Alzheimer’s Disease - Fig 3

<p><b>Scatter plot of average whole-brain HFD values</b> of A) Healthy subjects in relationship with age (years), with the best fitting curve (R² = .575, <i>p</i> < .001); B) AD patients with MMSE scores. We considered that MMSE values do not distribute as a Gaussian, reporting the Spearman’s correlation in the results. Nevertheless, for graphical representation we reported the line corresponding to the Pearson’s correlation (also below significance threshold r = .264, p = .031). C) AD patients with NCC levels, with best fitting line (<i>r</i> = -.274, <i>p</i> = .025).</p