Spontaneous phase-coupling within cortico-cortical networks: How time counts for brain-state-dependent stimulation

The two primary motor cortices (M1) interact via functional connectivity during resting state and motor activity [1]. Focal transcranial magnetic stimulation (TMS) of one M1 results in inhibition of the TMS response of the contralateral M1. This short-interval interhemispheric inhibition (SIHI) is a measure of M1-M1 effective connectivity [2]. The “communication through coherence” theory [3] predicts that alignment of excitation/inhibition periods through synchronization of oscillatory phases in two nodes of a neural network will directly determine the level of effective connectivity between them. Accordingly, SIHI is strongest at instants of time when both M1 are synchronized in a high-excitability state (i.e., the negative peak of the ongoing sensorimotor μ-oscillation) [4]. Moreover, spike-timing-dependent plasticity models [5] predict that induction of plasticity of the M1-M1 pathway (or any other pathway) will be most effective if the two nodes are stimulated consistently whenever synchronized in the high-excitability state.