Summary Despite decades of research, there is still a lack of understanding of the role and generating mechanisms of the ubiquitous fluctuations and oscillations found in recordings of brain dynamics. Here, we used a strength-dependent perturbative framework to provide a causal mechanistic description of how human brain function is perched at the delicate balance between fluctuation and oscillation. Applying local strength-dependent perturbations and subsequently measuring the perturbative complexity index clearly demonstrates that the overall balance of brain dynamics is shifted towards fluctuations for providing much needed flexibility. Importantly, stimulation in the fluctuation regime modulates specific resting state network, thus providing a mechanistic explanation of experimentally reported brain dynamics. Furthermore, this framework generates specific, testable empirical predictions for human stimulation studies using strength-dependent rather than constant perturbation. Overall, the strength-dependent perturbative framework demonstrates how the human brain is poised on the edge of criticality, between fluctuations to oscillations, allowing for maximal flexibility.

On the edge of criticality: strength-dependent perturbation unveils delicate balance between fluctuation and oscillation in brain dynamics