Transcranial Alternating Current Stimulation (tACS) is a method for modulating neural activity by applying weak electric current to the scalp at the frequency of naturally occurring neural oscillations. Being able to selectively target specific frequencies in the brain would be a significant advance; it would enable us to determine if neural oscillations are the true cause of cognitive function, and if so provide a way to treat abnormal neural oscillations associated with schizophrenia and Alzheimer's disease, as well as a potential path to cognitive enhancement.
However, the exact mechanism by which tACS works is unclear and effects are mixed and sometimes difficult to replicate. So an important step in developing this method to its full potential is the ability to observe brain activity during stimulation to see how it is changing neural oscillations and if it is having any effect at all.
This was the subject of my PhD thesis.
Early attempts were published in 2016:
And more recently:
A common theme of my research into tACS is the exploration of different stimulation waveforms. The majority of tACS studies to date have used a sinusoidal waveform, however an alternating current does not have to be sinusoidal; it can be a square wave, triangular, pulsed or any arbitrary waveform.
In particular I have focused on sawtooth waves:
One cycle of a sine wave, positive ramp sawtooth and negative ramp sawtooth (from left to right). Positive and negative ramp sawtooth waves contain identical amounts of positive and negative charge, i.e., the area under the curve is the same.
Recently I have begun combining tACS with visual flicker (SSVEPs):
SSVEPs can be enhanced with frequency‐matched tACS. The effect of the tACS was only seen when the frequency of stimulation closely matched the flicker frequency, and the current intensity was sufficiently high. As this was a clear effect, visible at the individual subject level, it is a promising protocol for administering tACS in future studies.