Age increases overall complexity of human spontaneous CBV fluctuations

Background and Purpose

Complexity of spontaneous CBV fluctuations - earlier demonstrated by us in young adults ¹ — can emerge from random, fractal ² or chaotic processes ³ . Our aims were to define the contribution of these patterns to the observed complexity and to evaluate the effect of age and gender on it.

Methods

Total hemoglobin content as the measure of CBV was monitored by near infrared spectroscopy (NIRS) on volunteers (male n=19, age=20-78 years; female n=23, age=21-79 years). Random and fractal pattern was distinguished by the spectral index (β)². Chaos was identified by the surrogate analysis of correlation dimension and largest Lyapunov exponent ⁴ .

Results

In spontaneous CBV fluctuations both fast random and slow fractal dynamics are present as demonstrated in the power spectral density of the CBV fluctuations (Figure 1), where they are separated by a cutoff frequency, f′. Below f′ the pattern is fractal (self-similar), in that power rises inversely with frequency as 1/f′. Above f′ the pattern is of white noise with ^highβ around 0.f′ decreases with age in both genders. Neither in pre- nor in postmanupause age groups (group 1 and 2, respectively) shows ^lowβ, the slope across the power estimates at the low frequencies, gender difference (female_group1:1.23±0.25 vs. male_group1:1.14±0.22, p=0.329, female_group2:1.39±0.26 vs. male_group2:1.17±0.20, p=0.089, respectively). The range of fluctuation amplitudes produced by the slow fractal dynamics is always larger than that of the fast random fluctuations, which difference significantly (p=0.013) decreases with age in women, only. Surrogate analysis demonstrated that CBV dynamics cannot be characterized on grounds of deterministic chaos.

Figure 1

Conclusions

Fractals are an ordered form of complexity due to their special autocorrelation structure. The complexity of random (white) noise is maximal in that it is without any correlation structure or order. Age affects the balance between the orderly and disorderly forms of CBV complexity rendering the overall pattern more complex.