Transcranial Alternating Current Stimulation at Alpha Frequency Reduces Pain When the Intensity of Pain is Uncertain

Alpha activity directly before pain onset has been implicated in pain experience with higher pre-stimulus alpha associated with lower reported pain. However, expectations about pain intensity also seem to affect pre-stimulus alpha activity. To date, evidence for a relationship between alpha activity and pain experience has been largely correlational. Transcranial alternating current stimulation at alpha frequency (alpha tACS) permits direct manipulation of alpha activity and therefore an examination of the potential causal relationship between alpha activity and pain. We investigated whether somatosensory alpha tACS could reduce pain experience and whether this was influenced by uncertainty about pain intensity. In a within-subjects design, perceived pain intensity and unpleasantness were assessed in 23 participants during alpha tACS and sham stimulation. Visual cues preceding the pain stimulus were used to manipulate uncertainty. A significant tACS * uncertainty * stimulus intensity interaction was found for reported pain intensity (F₂,₄₄ = 4.50; p = .017; Partial Eta² = .17) and unpleasantness (F₁,₂₂ = 4.78; p = .040; Partial Eta² = .18). Pain experience during the application of somatosensory alpha tACS was significantly lowered compared to sham stimulation, but only when the intensity of an upcoming stimulus was uncertain.