Neurosensory effects of transcranial alternating current stimulation

Electrical brain stimulation can elicit neurosensory side effects that are unrelated to the intended stimulation effects. This presents a challenge when designing studies with blinded control conditions.

The aim of this research was to investigate the role of different transcranial alternating current stimulation (tACS) parameters, i.e. intensity, frequency, and electrode montage, on the probability, duration and intensity of elicited neurosensory side effects.

In a first study, we examined the influence of tACS on sensations of phosphenes, dizziness, pressure, and skin sensation in fifteen healthy subjects, during 8 s of stimulation with different amplitudes (1500 μA, 1000 μA, 500 μA, 250 μA), frequencies (2 Hz, 4 Hz, 8 Hz, 16 Hz, 32 Hz, 64 Hz), and montages (F3/F4, F3/C4, F3/P4, P3/F4, P3/C4, P3/P4). In a second study, ten healthy subjects were exposed to 60 s of tACS (1000 μA, 2 Hz versus 16 Hz, F3/F4 versus P3/P4) and were asked to rate the intensity of sensations every 12 s.

The first study showed that all stimulation parameters had an influence on the probability and intensity of sensations. Phosphenes were most likely and strongest for frontal montages and higher frequencies. Dizziness was most likely and strongest for parietal montages and at stimulation frequency of 4 Hz. Skin sensations and pressure was more likely when stimulation was performed across central regions and at posterior montages, respectively. The second study also revealed that the probability and the intensity of sensations were neither modified during more extended periods of stimulation nor affected by carry-over effects.

We demonstrated that the strength and the likelihood of sensations elicited by tACS were specifically modulated by the stimulation parameters. The present work may therefore be instrumental in establishing effective blinding conditions for studies with tACS.