Transcranial alternating current stimulation (TACS) and the effects on cognition
TACS is a tool that can modulate brain activity by applying alternating currents to the head. These currents alter oscillating brain activity. TACS could be a powerful tool in the treatment of various brain-related disorders, including mental disorders and neurological disorders. However, it is not a guarantee that tACS successfully alters brain activity. In a meta-analysis and systematic review we analyzed the minimum effective dose and other factors for successful application of tACS
Minimum effective dose
With tACS we apply relatively weak electric currents. This is great as it means that tACS is non-invasive with very little side effects. However, how low can we go with the current? If the current is too low, the electric field in the brain is too weak to change brain activity.
In a meta-analysis we investigated the minimum effective dose. It was found that in primates (which offer the best comparable model to humans) an induced electric field of approximately 0.3 mV/mm is sufficient to alter neural activity with a probability of 80%. This can typically be achieved with an applied current of 2 mA.
When attempting to modulate cognitive performance, it is important to understand the investigated function. The effect of tACS may differ accordingly.
1. Some cognitive functions underly a trade-off between functions (e.g. speed vs accuracy), modulating one will also affect the other.
2. Other cognitive functions underly nested oscillations (high-frequency waves riding on low-frequency waves). Cross-frequency tACS is useful in such situations.
3. Cognitive functions can be driven be networks (fronto-parietal, fronto-temporal, etc). Modulating synchrony using phase-specific tACS may be helpful
4. TACS may also affect metabolic activity, which in turn affects cognition.
Wischnewski*, Alekseichuk* & Opitz (2022). Neurocognitive, physiological, and biophysical effects of transcranial alternating current stimulation. Trends Cog Sci, In press. *shared first authors. https://doi.org/10.1016/j.tics.2022.11.013
Alekseichuk, Wischnewski & Opitz (2022). A minimum effective dose for (transcranial) alternating current stimulation. Brain Stimulation, 15(5), 1221-1222. https://doi.org/10.1016/j.brs.2022.08.018