The hidden state-dependency of transcranial alternating current stimulation (tACS)

Kasten, Florian H.; Herrmann, Christoph S.

doi:10.1101/2020.12.23.423984

Cited by 5 publications

(4 citation statements)

References 44 publications

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“…Studies often find that tACS produces inconsistent effects, between and within participants (e.g., [ 10 – 12 ]), even with stimulation frequencies that are known to be linked to the specific behaviors under study. Although individual differences in neuroanatomy may explain some of these inconsistencies [ 11 ], variability in the participants’ ongoing brain activity also appears to shape the effects of tACS [ 13 – 17 ]. For example, asking participants to close their eyes—which increases the amplitude of endogenous alpha oscillations—reduces the subsequent effects of tACS [ 16 , 17 ].…”

Section: Introductionmentioning

confidence: 99%

Brain stimulation competes with ongoing oscillations for control of spike timing in the primate brain

et al. 2022

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Transcranial alternating current stimulation (tACS) is a popular method for modulating brain activity noninvasively. In particular, tACS is often used as a targeted intervention that enhances a neural oscillation at a specific frequency to affect a particular behavior. However, these interventions often yield highly variable results. Here, we provide a potential explanation for this variability: tACS competes with the brain’s ongoing oscillations. Using neural recordings from alert nonhuman primates, we find that when neural firing is independent of ongoing brain oscillations, tACS readily entrains spiking activity, but when neurons are strongly entrained to ongoing oscillations, tACS often causes a decrease in entrainment instead. Consequently, tACS can yield categorically different results on neural activity, even when the stimulation protocol is fixed. Mathematical analysis suggests that this competition is likely to occur under many experimental conditions. Attempting to impose an external rhythm on the brain may therefore often yield precisely the opposite effect.

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Section: Introductionmentioning

confidence: 99%

Brain stimulation competes with ongoing oscillations for control of spike timing in the primate brain

et al. 2022

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“…Statistical models that may be used to infer different states in a system may help to better characterize network effects and interindividual variability in response to different NIBS protocols. First applications suggest that only few spontaneous brain states may be susceptible to tACS-induced modulation (Kasten and Herrmann 2020). Identifying such brain states may increase the current understanding of stimulation effects and may, in the long run, contribute to increasing the stimulation efficiency.…”

Section: Discussionmentioning

confidence: 99%

Noninvasive Brain Stimulation: Multiple Effects on Cognition

Hartwigsen

Silvanto

2022

Neuroscientist

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Noninvasive brain stimulation (NIBS) techniques are widely used tools for the study and rehabilitation of cognitive functions. Different NIBS approaches aim to enhance or impair different cognitive processes. The methodological focus for achieving this has been on stimulation protocols that are considered either inhibitory or facilitatory. However, despite more than three decades of use, their application is based on incomplete and overly simplistic conceptualizations of mechanisms of action. Such misconception limits the usefulness of these approaches in the basic science and clinical domains. In this review, we challenge this view by arguing that stimulation protocols themselves are neither inhibitory nor facilitatory. Instead, we suggest that all induced effects reflect complex interactions of internal and external factors. Given these considerations, we present a novel model in which we conceptualize NIBS effects as an interaction between brain activity and the characteristics of the external stimulus. This interactive model can explain various phenomena in the brain stimulation literature that have been considered unexpected or paradoxical. We argue that these effects no longer seem paradoxical when considered from the viewpoint of state dependency.

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“…There are several subject-specific factors, for example anatomical and physiological factors, age, gender, brain state, hormonal levels, and pre-existing regional excitability, which could confound the effects of tACS (see e.g., Feurra et al, 2013 ; Neuling et al, 2013 ; Krause and Cohen Kadosh, 2014 ; Benwell et al, 2015 ; Alagapan et al, 2016 ; Kasten and Herrmann, 2020 ). These factors can at least partly explain the variable effectiveness of tACS between subjects within a study, but also the various outcomes between studies.…”

Section: Introductionmentioning

confidence: 99%

Detection of Transcranial Alternating Current Stimulation Aftereffects Is Improved by Considering the Individual Electric Field Strength and Self-Rated Sleepiness

et al. 2022

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Non-invasive electrical stimulation methods, such as transcranial alternating current stimulation (tACS), are increasingly used in human neuroscience research and offer potential new avenues to treat neurological and psychiatric disorders. However, their often variable effects have also raised concerns in the scientific and clinical communities. This study aims to investigate the influence of subject-specific factors on the alpha tACS-induced aftereffect on the alpha amplitude (measured with electroencephalography, EEG) as well as on the connectivity strength between nodes of the default mode network (DMN) [measured with functional magnetic resonance imaging (fMRI)]. As subject-specific factors we considered the individual electrical field (EFIELD) strength at target regions in the brain, the frequency mismatch between applied stimulation and individual alpha frequency (IAF) and as a covariate, subject’s changes in mental state, i.e., sleepiness. Eighteen subjects participated in a tACS and a sham session conducted on different days. Each session consisted of three runs (pre/stimulation/). tACS was applied during the second run at each subject’s individual alpha frequency (IAF), applying 1 mA peak-to-peak intensity for 7 min, using an occipital bihemispheric montage. In every run, subjects watched a video designed to increase in-scanner compliance. To investigate the aftereffect of tACS on EEG alpha amplitude and on DMN connectivity strength, EEG data were recorded simultaneously with fMRI data. Self-rated sleepiness was documented using a questionnaire. Conventional statistics (ANOVA) did not show a significant aftereffect of tACS on the alpha amplitude compared to sham stimulation. Including individual EFIELD strengths and self-rated sleepiness scores in a multiple linear regression model, significant tACS-induced aftereffects were observed. However, the subject-wise mismatch between tACS frequency and IAF had no contribution to our model. Neither standard nor extended statistical methods confirmed a tACS-induced aftereffect on DMN functional connectivity. Our results show that it is possible and necessary to disentangle alpha amplitude changes due to intrinsic mechanisms and to external manipulation using tACS on the alpha amplitude that might otherwise be overlooked. Our results suggest that EFIELD is really the most significant factor that explains the alpha amplitude modulation during a tACS session. This knowledge helps to understand the variability of the tACS-induced aftereffects.

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The hidden state-dependency of transcranial alternating current stimulation (tACS)

Cited by 5 publications

References 44 publications

Brain stimulation competes with ongoing oscillations for control of spike timing in the primate brain

Brain stimulation competes with ongoing oscillations for control of spike timing in the primate brain

Noninvasive Brain Stimulation: Multiple Effects on Cognition

Detection of Transcranial Alternating Current Stimulation Aftereffects Is Improved by Considering the Individual Electric Field Strength and Self-Rated Sleepiness

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