Brain oscillations exhibit long-range temporal correlations (LRTCs), which
reflect the regularity of their fluctuations: low values representing more
random (decorrelated) while high values more persistent (correlated) dynamics.
LRTCs constitute supporting evidence that the brain operates near criticality,
a state where neuronal activities are balanced between order and randomness.
Here, healthy adults used closed-loop brain training (neurofeedback, NFB) to
reduce the amplitude of alpha oscillations, producing a significant increase in
spontaneous LRTCs post-training. This effect was reproduced in patients with
post-traumatic stress disorder, where abnormally random dynamics were reversed
by NFB, correlating with significant improvements in hyperarousal. Notably,
regions manifesting abnormally low LRTCs (i.e., excessive randomness)
normalized toward healthy population levels, consistent with theoretical
predictions about self-organized criticality. Hence, when exposed to
appropriate training, spontaneous cortical activity reveals a residual capacity
for "self-tuning" its own temporal complexity, despite manifesting the abnormal
dynamics seen in individuals with psychiatric disorder. Lastly, we observed an
inverse-U relationship between strength of LRTC and oscillation amplitude,
suggesting a breakdown of long-range dependence at high/low synchronization
extremes, in line with recent computational models. Together, our findings
offer a broader mechanistic framework for motivating research and clinical
applications of NFB, encompassing disorders with perturbed LRTCs.