Transcranial direct current stimulation modulates primate brain dynamics across states of consciousness.

Background: Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation approach that has been reported to perturb task activity and to benefit patients with a variety of diseases. Nevertheless, the effects of tDCS on brain dynamics and transitions in brain patterns across states of consciousness remain poorly understood. Objective: Here, we investigated the modulatory effect of prefrontal cortex (PFC) tDCS on brain dynamics, both in the awake state and during anesthesia-induced loss of consciousness in non-human primates. Methods: We acquired functional magnetic resonance imaging (fMRI) data before, during, and after the application of high-density tDCS (HD-tDCS) utilizing a prefrontal-occipital montage with two electrodes. In the awake state, macaques received either anodal or cathodal PFC stimulation. Under anesthesia, macaques underwent two consecutive anodal PFC stimulations of increasing intensity. Dynamical functional connectivity was measured using fMRI, and the resulting connectivity matrices were clustered into distinct brain patterns. Results: We found that cathodal PFC HD-tDCS robustly disrupted the rich dynamic repertoire of brain patterns of the awake state. It increased the brain structure-function correlation, decreased Shannon entropy, and strongly favored Markov chain transition probabilities towards patterns closest to anatomy. Under anesthesia, 2 mA anodal PFC HD-tDCS significantly changed the distribution of brain patterns and reduced the structure-function correlation. Conclusion: Our findings offer compelling evidence that prefrontal tDCS induces a striking modification in the fMRI-based dynamic organization of the brain across varying states of consciousness. This contributes to an enhanced understanding of the neural mechanisms underlying tDCS neuromodulation.