How Often Should Noninvasive Brain Stimulation Be Repeated?
One single session of rTMS or tDCS can induce temporary alterations in cortical excitability and sometimes a transient improvement in performance has been demonstrated. However, for many simple tasks or motor functions, performance is unaltered in stroke patients and healthy participants. In contrast, repeated NIBS sessions with one or two NIBS sessions daily may prolong the aftereffects of NIBS even days after the end of the session (e.g.). This explains why NIBS is usually given repeatedly over several days or weeks in therapeutic trials to produce cumulative therapeutic effects. However, there is little knowledge about how often the NIBS sessions should be repeated in stroke patients to obtain the best possible clinical effect. Here, it would be very helpful to establish and validate neurophysiological or neuroimaging biomarkers that indicate after how many NIBS sessions the therapeutic effect has peaked and no longer needs further enhancement.
The effects of consecutive rTMS or tDCS sessions might interact with each other. By changing the initial state of the motor cortex (using a preconditioning stimulation session), it is possible to reverse the conditioning effects of 1-Hz rTMS in healthy individuals. These preconditioning effects suggest the existence of a homeostatic mechanism in the human motor cortex. In the context of repeated NIBS interventions, homeostatic metaplasticity-like effects might render the patients less sensitive to further manipulations in order to prevent destabilization of neuronal networks. In fact, it has been suggested that homeostatic-like phenomena might be one of the main factors that limit the duration or effect size of post-TMS effects (e.g.). Yet, it is unknown whether homeostatic counter-regulation still applies when a night of sleep separates two consecutive NIBS sessions. It is possible that a night of sleep re-sensitizes the brain networks to NIBS. In conclusion, the relevance of homeostatic-like plasticity for producing a sustained and stable clinical response after repeated NIBS sessions needs to be systematically addressed in therapeutic settings.