Alpha power increase after transcranial alternating current stimulation at alpha frequency (α-tACS) reflects plastic changes rather than entrainment

Vossen, Alexandra, Joachim Gross, and Gregor Thut. 2015. 'Alpha power increase after transcranial alternating current stimulation at alpha frequency (α-tACS) reflects plastic changes rather than entrainment', Brain stimulation, 8: 499-508.
 
Summary:        The mechanism of action for transcranial alternating current stimulation (tACS) is theorized to be via entrainment of neural activity. Vossen et al. set out to causally test the role of phase entrainment using a unique protocol of delivering intermittent periods of tACS that were either all phase aligned or randomly phase shifted (90-degree increments). The amplitude of alpha oscillations from tACS was increased for both in-phase and random-phase relative to sham, but the two conditions were not significantly different from each other. Thus, the authors conclude that the aftereffects of tACS are dependent on neural plasticity in the stimulated region and do not reflect a persistent entrainment effect.
 
Pros:

• Novel stimulation approach to address an important mechanistic question
• Individual alpha frequency was used for tACS

 
Open questions:

• What is the mechanism of neuroplasticity that explains the aftereffect of tACS?
• In-phase stimulation was numerically greater than random-phase stimulation. Would this difference be meaningful with a larger sample size?

 
Contributed by: Justin Riddle, PhD