Impact of repetitive theta burst stimulation on motor cortex excitability

Theta burst stimulation (TBS) alters cortical excitability in inhibitory or facilitatory directions depending on the pattern of stimulation used. Although continuous TBS (cTBS) decreases motor cortex excitability, intermittent TBS (iTBS) increases excitability by introducing an 8-second stimulation interval after 2 seconds of TBS. The after-effects induced by TBS last from 30 minutes up to 1 hour. Optimization of TBS techniques might be possible through manipulation of a variety of parameters such as number of pulses, stimulus intensity, duration of stimulation, and repetitive stimulation. The aim of this study was to assess the after-effects induced by introducing an interval between two TBS interventions to identify more efficient protocols. The study was divided in two groups, iTBS protocols and cTBS protocols, each of them with four sessions: classical TBS, TBS - 2 minutes - TBS, TBS - 5 minutes - TBS, TBS - 20 minutes - TBS. Our results show that cTBS - 20 minutes - cTBS and iTBS - 2 minutes - iTBS resulted in similar after-effects as those accomplished by a single TBS session, whereas a suppression of after-effects was observed in the other break durations. Repeated TBS with short break durations does not seem to be suitable to prolong the duration of excitability changes accomplished by single TBS. These results might be relevant for clinical applications of TBS, when long-lasting excitability alterations are needed.     

Close to threshold transcranial electrical stimulation preferentially activates inhibitory networks before switching to excitation with higher intensities

BackgroundRecently we have shown that transcranial random noise (tRNS) and 140 Hz transcranial alternating current stimulations (tACS), applied over the primary motor cortex (M1) and using 10 min stimulation duration and 1 mA intensity, significantly increases cortical excitability as measured by motor evoked potentials at rest before and after stimulation.Objective/hypothesisHere, by decreasing the stimulation intensity in 0.2 mA steps from 1.0 mA, we investigate to what extent intensity depends on the induced after-effects.MethodsAll twenty-five subjects participated in two different experimental sessions each. They received tACS using 140 Hz frequency and full spectrum tRNS at five different intensities on separate days. Sham stimulation was used as a control.ResultsInstead of receiving a simple threshold, unexpectedly, in these two independent data sets at threshold intensities of 0.4 mA we found a switch of the already known excitation achieved with an intensity of 1 mA to inhibition. The intermediate intensity ranges of 0.6 and 0.8 mA had no effect at all. Interestingly, the inhibition produced by 140 Hz tACS was stronger than that induced by tRNS.ConclusionsIn summary, we have shown here the possibility of selectively controlling the enhancement or reduction of M1 excitability by applying different intensities of high frequency transcranial electrical stimulation.