Visually Evoked 40 Hz Gamma Activity Enhanced by Transcranial Electrical Stimulation

Abstract Background Non-invasive Visual Stimulation (VS) and Transcranial Electrical Stimulation (TES) can modulate neuronal oscillations, including gamma activity at 40 Hz, which is relevant for cognition and disrupted in dementia. Combining both techniques may increase effects, but simultaneously recording Electroencephalography (EEG) activity poses several challenges, so this approach is untested. Objectives We predicted that combined TES and VS in the lower gamma band would increase Steady-State Visually Evoked Potential (SSVEP) amplitude during and after stimulation, but only when targeting visual areas and at closely matching stimulation frequencies. Methods We administered combined VS and TES and simultaneously measured effects on EEG gamma activity in healthy participants. In experiment 1 (N=25), VS and TES frequencies were closely matched at ∼40 Hz, and TES sites varied between occipito-central, centro-occipital (reversed polarity), and centro-frontal. In experiment 2 (N=25), occipito-central TES was applied at ∼40 Hz, and VS frequency varied between 35, 40, and 45 Hz. Every 5-minute VS+TES trial was preceded and followed by a VS-only baseline trial. Electrical artifacts were removed using adaptive template subtraction. Results TES enhanced gamma SSVEP amplitudes most when applied to occipital and central sites, compared to frontal. Enhancement only occurred when TES and VS frequencies closely matched at ∼40 Hz, not when VS was slower (35 Hz) or faster (45 Hz) than TES. The effect was present during, not after, TES. Conclusion Multimodal visual and electrical stimulation evokes stronger oscillatory gamma activity than visual alone. Non-invasive gamma stimulation against cognitive decline in dementia may benefit from this optimised approach.