Transcranial electrical stimulation has been used for many years in the treatment of neurological and psychiatric disorders, such as depression, epilepsy or stroke. However, the exact mechanisms underlying stimulation effects are largely unknown. Stimulation artifacts impeded exact assessment of neuromagnetic activity, particularly when the applied currents alternated their polarity. Scientists in Tübingen, Germany, have now introduced a novel stimulation method during whole-head magnetoencephalography (MEG) that allows millimeter precise mapping of entrained brain oscillations during transcranial alternating current stimulation (tACS). The new method promises to elucidate the underlying mechanisms of tACS and to improve stimulation strategies in the context of clinical applications.
The impact of electric currents on the human brain has been known for centuries and is increasingly used in the treatment of various diseases, such as severe depression, stroke, epilepsy, Parkinson’s disease or chronic pain. Particularly the application of weak electric currents through two or more scalp electrodes known as transcranial DC or AC stimulation (tDCS/tACS) was increasingly investigated in their clinical efficacy and applicability.
However, the exact underlying mechanisms of tDCS and tACS are largely unknown as stimulation artifacts impeded assessment of physiological brain activity. Only in 2013, scientists at the University of Tübingen, Germany, managed in collaboration with the National Institutes of Health (NIH), USA, to assess millisecond-to-millisecond neuromagnetic activity while the brain of a human subjects underwent transcranial DC stimulation (Soekadar et al. 2013, Nature Communications). Despite this success that allows for investigating the immediate effects of tDCS on brain oscillations (Garcia-Cossio et al. 2015, NeuroImage), artifact-free reconstruction of brain activity during AC stimulation remained unfeasible. It is assumed that tACS exerts its effect by synchronizing the phase of brain oscillations to the stimulation signal. During such tACS-induced entrainment of brain oscillations, stimulation artifacts could not be reliably differentiated from physiological neuromagnetic brain activity.
In their most recent study published today in NeuroImage (Witkowski et al. 2015), the same group has now succeeded to precisely map tACS-entrained brain oscillations using of a novel stimulation method allowing for reliable differentiation of neuromagnetic brain rhythms and tACS-related stimulation artifacts. The authors report that amplitude modulation of AC currents applied at frequencies beyond the physiological range of brain oscillations (>150Hz) avoided contamination of physiological frequency bands while such stimulation exerted a distinct entrainment effect at the modulation frequency.
Using this method enabled the scientists to precisely identify brain areas influenced by the stimulation including areas immediately underneath and in proximity of the stimulation electrodes. The researchers hope that the new approach will now help to elucidate the underlying mechanisms of tACS and other stimulation paradigms and improve its clinical efficacy.
Particularly „to adapt the stimulation to the individual anatomy and specific neurophysiological effects“ are important perspectives of the new method according to Dr. Surjo R. Soekadar, head of the working group Applied Neurotechnology at the University Hospital Tübingen. „As a next step, it is conceivable that the stimulation will be adapted to the individual brain ac-tivity in real time. Such closed-loop stimulation promises to provide better control of the stimulation effects than classical stimulation protocols“, adds Matthias Witkowski, lead author of the study.
Surjo R. Soekadar, MD
University of Tübingen
Department for Psychiatry and Psychotherapy & Institute of Medical Psy-chology and Behavioral Neurobiology
Applied Neurotechnology Lab
phone: +49 7071 29-82624
Witkowski M, Cossio EG, Chander BS, Braun C, Birbaumer N, Robinson SE, Soekadar SR. Mapping entrained brain oscillations during transcra-nial alternating current stimulation (tACS). Neuroimage. 2015 (in press). pii: S1053-8119(15)00934-9. doi: 10.1016/j.neuroimage.2015.10.024.
Garcia-Cossio E, Witkowski M, Robinson SE, Cohen LG, Birbaumer N, Soekadar SR. Simultaneous transcranial direct current stimulation (tDCS) and whole-head magnetoencephalography (MEG): assessing the impact of tDCS on slow cortical magnetic fields. Neuroimage. 2015 (in press). pii: S1053-8119(15)00891-5. doi: 10.1016/j.neuroimage.2015.09.068.
Soekadar SR, Witkowski M, Cossio EG, Birbaumer N, Robinson SE, Cohen LG. In vivo assessment of human brain oscillations during application of transcranial electric currents. Nat Commun. 2013;4:2032. doi: 10.1038/ncomms3032.