Stroke-induced slow oscillations in dynamic brain organization

A focal ischemic lesion is thought to alter neuronal activity beyond the area of structural damage and thereby interfere with the whole network architecture. We used a combination of transcranial magnetic stimulation and electroencephalography (TMS-EEG) together with dynamic connectivity analyses and graph theory to study alterations and reorganization of cortical reactivity and connectivity in a cohort of 42 stroke patients longitudinally after stroke. We found a link between an increase of low-frequency coupling in the delta band and alterations in neural information processing in the first weeks after stroke and their relevance for motor recovery more than three months later. We demonstrate that stroke to the motor system leading to impaired hand motor function induce an enhancement of slow activity and delta coupling between frontocentral and parietal regions. In addition, we could show a loss of the physiological network architecture with a decrease of small-worldness and modularity in the delta frequency, implying that a focal ischemic lesion distant to the cerebral cortex interferes with both cortical information integration as well as functional segregation within the days and weeks after stroke. While we found a link between bifrontal coupling in the alpha spectrum and the degree of the motor deficit in the early post-acute phase, the amount of small worldness disruption early after stroke indicated the motor outcome three months later. In contrast, recovery of motor function and cortical reorganization after more than three months post-stroke were paralleled by a normalization of increased low-frequency coupling and a reinstatement of the complex network structure featuring a small-world and modular topology.