Motor cortex plasticity following tACS
In recent years brain stimulation methods are more often used in aiding therapy of mental and neurological disorders (read more in the Brain Stimulation Therapy section). This is because studies have shown that brain stimulation can change the activity of the brain. Given the right circumstances, these effects stay for a while after stimulation. Although these so-called 'after-effects' have been demonstrated for tDCS and TMS, evidence for after-effects following tACS is more limited (read more on tACS in the Neuroscience Methods section).
We wanted to test potential after-effects of tACS in the motor cortex. Since tACS primarily effects rhytmic activity of the brain (oscillations), we opted to stimulate a brain rhythm that is clearly observed in the motor cortex and is related to movement: The beta rhythm.
After-effects in the brain are related to a mechanism called synaptic plasticity. Synaptic plasticity means that the connection between neurons is made stronger. So, here we tested whether tACS induces synaptic plasticity.
Methods
We measured the effects after tACS in two ways. First, we looked if the activity of the motor cortex was increased by looking at its output. That is, the motor cortex is connected to the muscles, so if muscle responses are larger, output of the motor cortex must be larger. Second, we looked if changes in beta rhythms were observed after stimulation.
To make sure that any changes are not due to a placebo effect, we needed a control condition in which the effects of tACS are blocked. That means, any effects on synaptic plasticity needed to be blocked. We achieved this by giving participants a medication called dextromethorphan. This medication blocks plasticity up to 7 hours. Participants feel a little drowsy, but it is not dangerous. Interestingly, dextromethorphan is the main element of coughing syrup, though in lower doses than we used here.
Results
As we predicted, tACS increased synaptic plasticity. That means, output to the muscles, as well as beta activity was increased. This effect lasted for at least 60 minutes. We did not measure any longer than this. Importantly, in the control condition, where participants received a medication blocking plasticity, tACS did not have any after effects.
Discussion
In this study we showed that a single session tACS can have after-effects that last for at least one hour. This is an important finding, because it means that tACS could be important in a clinical set up, to treat mental and neurological illnesses. Of course, our findings are limited to the motor cortex and we can conclude that our effects hold up in the entire brain.
Additional support for the clinical potential of tACS is related to the observation that tACS after-effects are related to synaptic plasticity. In synaptic plasticity the strength between neurons is increased. That means tACS seems to be able to induce actual physical and chemical changes in the brain. If so, multiple session may have even longer lasting effects, which is crucial in a therapeutic situation.
Sources
Wischnewski, Engelhardt, Salehinejad, Schutter, Kuo & Nitsche (2019). NMDA receptor-mediated motor cortex plasticity after 20 Hz transcranial alternating current stimulation. Cereb Cort, 29(7), 2914-2931. https://doi.org/10.1093/cercor/bhy160
Wischnewski, Schutter & Nitsche (2019). Effects of beta-tACS on corticospinal excitability: A meta-analysis. Brain Stimul, 12(6), 1382-1389. https://doi.org/10.1016/j.brs.2019.07.023