Abstract
Low frequency oscillations such as alpha (8–12 Hz) are hypothesized to rhythmically gate sensory processing, reflected by 40–100 Hz gamma band activity, via the mechanism of pulsed inhibition. We applied transcranial alternating current stimulation (TACS) at individual alpha frequency (IAF) and flanking frequencies (IAF-4 Hz, IAF+4 Hz) to the occipital cortex of healthy human volunteers during concurrent magnetoencephalography (MEG), while participants performed a visual detection task inducing strong gamma-band responses. Occipital (but not retinal) TACS phasically suppressed stimulus-induced gamma oscillations in the visual cortex and impaired target detection, with stronger phase-to-amplitude coupling predicting behavioral impairments. Retinal control TACS ruled out retino-thalamo-cortical entrainment resulting from (subthreshold) retinal stimulation. All TACS frequencies tested were effective, suggesting that visual gamma-band responses can be modulated by a range of low frequency oscillations. We propose that TACS-induced membrane potential modulations mimic the rhythmic change in cortical excitability by which spontaneous low frequency oscillations may eventually exert their impact when gating sensory processing via pulsed inhibition.
Highlights
Cortical oscillations and their cross-frequency interaction constitute important mechanisms for the organization of neuronal processing
The aim of this study was to test a core prediction of the pulsed inhibition hypothesis (Klimesch et al, 2007; Jensen and Mazaheri, 2010), namely that the phase of slower oscillations, in the alpha band, modulates the bottom-up processing of sensory information, which is reflected by stimulus-induced gamma-band oscillations (Bastos et al, 2015; Fries, 2015), and thereby affects perceptual performance
In line with this notion, we found that transcranial alternating current stimulation (TACS) to the occipital cortex
Summary
Cortical oscillations and their cross-frequency interaction constitute important mechanisms for the organization of neuronal processing. We applied transcranial alternating current stimulation (TACS) at individual alpha frequency (IAF) to the visual cortex (Oz-Cz montage) in human volunteers performing a visual detection task to mimic the impact of alpha phase-related cortical excitability fluctuations on endogenous gamma activity during visual stimulus processing. While simultaneous TACS-EEG recordings (Helfrich et al., 2014a; Helfrich et al, 2016) are limited by the spatial interference of stimulation and recording electrodes, both affixed to the scalp, the combination of TDCS/TACS and MEG (Soekadar et al, 2013; Neuling et al., 2015; Witkowski et al, 2015; Marshall et al, 2016) allowed us to transcranially impose oscillating currents on the visual cortex, while assessing stimulus-induced gamma power modulation in the visual cortex directly underlying the TACS electrodes (Oz-Cz montage).
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