P177 Transcranial alternating current stimulation with sawtooth waves: simultaneous stimulation and EEG recording. Issue 3 (March 2017) - Dowsett, J.
Abstract : Question: Transcranial alternating current stimulation (tACS) has until now mostly been administered as a sinusoidal wave. As tACS is a relatively new technique there is a huge parameter space of unexplored possibilities which may prove superior or complimentary to the traditional sinusoidal waveform. Evidence from animal models suggests that the gradient and direction of an electric current are important factors for the subsequent neural firing rate; we compared positive and negative ramp sawtooth waves to test this using a novel method for simultaneous EEG and tACS. Methods: An advantage of sawtooth waves is that the resulting artefact in the EEG recording is significantly simpler to remove than a sine wave as we can clearly see whether a residual artefact remains by the distinctive harmonics which can be seen when the cleaned data is viewed in the frequency domain. As such we can be sure the artefact has been successfully removed and are able to both observe alpha oscillations and recover event related potentials (ERPs) during stimulation. Results: We have found that positive ramp sawtooth, but not negative ramp, significantly enhances alpha power during stimulation, demonstrating that current direction and gradient are important factors in tACS. In addition, we are able to gradually ramp up the intensity of the current and observe the point at which the tACS starts to modulate visually evoked ERPs. Conclusions: The ability to observe on-going EEG activity is likely to significantly increase the usefulness of tACS in future research. A current limitation for all transcranial electrical stimulation is the inter-subject variation in effects due to differences in skull thickness, cerebrospinal fluid etc. Modulation of ERPs during tACS is an objective measure of the effect of the stimulation online, much like the motor evoked potential used for TMS research, which can be used to determine individualised current intensities and to test the effect of different electrode montages. This also opens up the possibility of real time feedback systems which can modulate neural oscillations. In addition, the fact that the effect of positive and negative ramp sawtooth waves is different, but the frequency, current density and skin sensations are identical, means this could serve as a better control condition in future studies.