Electroencephalographic Fractal Dimension in Healthy Ageing and Alzheimer’s Disease

Abstract

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, p < .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 (p = .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.

Complexity Measures for Quantifying Changes in Electroencephalogram in Alzheimer’s Disease

“…In this case, the results show that HFD values for AD patients are lower than those for normal subjects. This result is consistent with the finding in other studies [46,80]. As with the TsEn, the differences between HFD biomarkers for AD patients and normal subjects for the EEG frequency bands (i.e., delta, theta, and alpha bands) were Fp1 Fp2 F7 F3 FZ F4 F8 T3 C3 CZ C4 T4 T5 P3 PZ P4 T6 O1 O2 EEG channel Whole EEG …”

“…In addition, HFD provides a more accurate measure of the complexity of signals compared to other methods [75,78,79] and it has been shown to be an efficient method for discriminating between AD patients and normal subjects [31,80]. HFD of the EEG is potentially a good biomarker of AD diagnosis as it is significantly lower in AD patients than in normal subjects [46,80,81]. Smits et al found that HFD is sensitive to neural changes selectively related to AD patients and normal subjects.…”

Fractal Analysis of Brain Blood Oxygenation Level Dependent (BOLD) Signals from Children with Mild Traumatic Brain Injury (mTBI)

“…A recent study by Smits et al [29] showed calculations of FD in resting state EEG recordings from 67 AD patients and 41 healthy controls. They found the FD and signal complexity decreased with age in normal controls and that it further decreased in AD patients, especially in temporal-occipital regions [29]. This study supports the concept that brain signal FD correlates with changes in brain connectivity and complexity.…”

Healthy and Pathological Brain Aging: From the Perspective of Oscillations, Functional Connectivity, and Signal Complexity

“…The signal complexity of EEG, a reflection of the irregularity of the wave shape and dynamics, is reported to be influenced by healthy aging and associated with brain disorders such as AD [73, 74]. Studies using fractal dimension demonstrated that resting-state EEG signal complexity increases from youth to maturity, followed by a reduction in healthy elderly.…”

“…Studies using fractal dimension demonstrated that resting-state EEG signal complexity increases from youth to maturity, followed by a reduction in healthy elderly. These studies also provided evidence indicating that, in AD, the signal complexity is further reduced as cognitive function declines [73, 74]. …”