Abstract :
[en] Background: Automatic motor inhibition is an important and adaptive process through which an activated motor plan is suppressed if the movement is not intended to be executed. Neuronal networks are characterized by oscillatory activity. In the brain, a large variety of rhythms have been described that differ in their frequency, origin and reactivity to changes in task demands. Recent studies have demonstrated that active cortical networks are susceptible to weak sinusoidal perturbations of exogenous electric fields.
Objective/Hypothesis: The aim of this study was to investigate the frequency-specific effect of
transcranial alternate current stimulation (tACS) over the automatic control of movement in healthy volunteers. We hypothesized that applying two different tACS frequencies during a visuo-motor
task would result in different behavioural effects and in diverse modulation of cortical excitability.
Methods: In this study we used tACS to interact non-invasively with the ongoing task-related
oscillatory activity. Stimulation was delivered at alpha (10 Hz) and beta (20 Hz) frequency over the
supplementary motor area and the primary motor cortex (SMA-M1) connections, which are part of
the BG-cortical motor loop, during the execution of the subliminal masked prime task. We measured the effects on task performance and on motor cortex corticospinal excitability by means of motor evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS).
Results: Results indicate a specific effect of 10 and 20-Hz tACS on functional inhibition in the
SMA-M1 circuit. Behaviorally there is an interference in task-related automatic inhibition: 10 Hz tACS reduced the automatic inhibition. In contrast 20 Hz tACS increased the automatic inhibition.
At a neurophysiological level there is a modulation in excitability of M1: 20 Hz tACS reduced MEP amplitudes, whereas there was no change after 10 Hz tACS.
Conclusion(s): The current study provides novel evidence that automatic mechanisms of motor
behaviour can be modulated by imposing synchronized electrical oscillatory activity upon motor
cortical regions. Our study has important implications for cognitive neuroscience studies
suggesting that the use tACS might offer the possibility to demonstrate a causal link between
endogenous brain oscillations, specific exogenous alternate current frequencies and specific
cognitive processes.
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