Abstract
We applied transcranial alternating current stimulation (tACS) to the primary motor cortex (M1) at different frequencies during an index–thumb pinch-grip observation task. To estimate changes in the corticospinal output, we used the size of motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation (TMS) of M1 using an online MRI-guided simultaneous TMS-tACS approach. The results of the beta-tACS confirm a non-selective increase in corticospinal excitability in subjects at rest; an increase was observed for both of the tested hand muscles, the first dorsal interosseous (FDI) and the abductor digiti minimi (ADM). However, during action observation of the pinch-grip movement, the increase of corticospinal excitability was only observed for the prime mover FDI muscle and took place during alpha-tACS, while gamma-tACS affected both the FDI and control muscle (ADM) responses. These phenomena likely reflect the hypothesis that the mu and gamma rhythms specifically index the downstream modulation of primary sensorimotor areas by engaging mirror neuron activity. The current neuromodulation approach confirms that tACS can be used to induce neurophysiologically detectable state-dependent enhancement effects, even in complex motor-cognitive tasks.
Highlights
We applied transcranial alternating current stimulation to the primary motor cortex (M1) at different frequencies during an index–thumb pinch-grip observation task
We investigated the selectivity of these effects at the cortical level through simultaneous recording of transcranial magnetic stimulation (TMS) responses during transcranial alternating current stimulation (tACS) from two hand muscles, the first dorsal interosseous (FDI) and the abductor digiti minimi (ADM), which share peripheral innervation and have a similar cortical representation[30,31]
We used TMS of the M1 to assess the level of corticospinal excitability by measuring changes in motor evoked potentials (MEPs) size from two hand muscles (FDI, ADM) during rhythmic tACS application at different frequencies and during different states of brain activity
Summary
We applied transcranial alternating current stimulation (tACS) to the primary motor cortex (M1) at different frequencies during an index–thumb pinch-grip observation task. Compelling evidence indicates that, in the human motor system, the application to the scalp of low-intensity transcranial alternating current stimulation (tACS) with a frequency matching the idling beta rhythm of the resting precentral areas causes an increase in the corticospinal output, measured by the size of the motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) of the primary motor cortex (M1)[1,2,3] These neurophysiological effects, which point to a less selective motoneuronal recruitment driven by beta activity[1], have an overt behavioural counterpart as tACS at beta range may slow down some kinematic aspects of voluntary movements[4,5,6,7]. While the former is fully engaged in the pinch-grip AO examined in this study, the latter is thought not to be involved in this task
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