Modulation of neural oscillations and associated behaviour by transcranial Alternating Current Stimulation (tACS)

Vossen, Alexandra Yvonne (2017) Modulation of neural oscillations and associated behaviour by transcranial Alternating Current Stimulation (tACS). PhD thesis, University of Glasgow.

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Transcranial alternating current stimulation (tACS) is a non-invasive brain stimulation method that involves the application of weak electric currents to the scalp. tACS has the potential to be an inexpensive, easily administrable, and well-tolerated multi-purpose tool for cognitive and clinical neuroscience as it could be applied to establish the functional role of rhythmic brain activity, and to treat neural disorders, in particular those where these rhythms have gone awry. However, the mechanisms by which tACS produces both "online" and "offline" effects (that is, those that manifest during stimulation and those that last beyond stimulation offset) are to date still poorly understood. If the potential of tACS is to be harnessed effectively to alter brain activity in a controlled manner, it is fundamental to have a good understanding of how tACS interacts with neuronal dynamics, and of the conditions that promote its effect. This thesis describes three experiments that were conducted to elucidate the mechanisms by which tACS interacts with underlying neural network activity.
Experiments 1 and 2 investigated the mechanism by which tACS at alpha frequencies (8 12 Hz, α-tACS) over occipital cortex induces the lasting aftereffects on posterior α power that were previously described in the literature. Two mechanisms have been suggested to underlie alpha power enhancement after α tACS: entrainment of endogenous brain oscillations and/or changes in oscillatory neural networks through spike timing-dependent plasticity (STDP). In Experiment 1, we tested to what extent plasticity can account for tACS-aftereffects when controlling for entrainment characteristics. To this end, we used a novel, intermittent α-tACS protocol and investigated the strength of the aftereffect as a function of phase continuity between successive tACS episodes, as well as the match between stimulation frequency and individual alpha frequency (IAF). Alpha aftereffects were successfully replicated with enhanced α power after intermittent stimulation compared to sham. These aftereffects did not exhibit any of the expected characteristics of prolonged entrainment in that they were independent of tACS phase-continuity and did not show stable phase alignment or synchronisation to the stimulation frequency. These results indicate that prolonged entrainment is insufficient to explain the aftereffects and suggest that the latter emerge through some form of network plasticity.
To clarify the nature of these plasticity mechanisms, we then aimed to assess whether STDP could explain the α power increase. We developed a conceptual STDP model that predicted bi-directional changes in α power depending on the relative mismatch between the tACS frequency and IAF. After observing in Experiment 1 that tACS at frequencies slightly lower than the IAF produced α enhancement, Experiment 2 used a similar intermittent protocol that manipulated tACS frequency to be either slightly lower or higher than IAF to respectively enhance or suppress α activity. In addition, a control condition with continuous stimulation aimed to replicate previous results from other groups. However, we did not observe a systematic α power change in any of the active conditions. The lack of consistency between the two experiments raises concerns regarding the reproducibility and effect size of tACS aftereffects.
The third experiment investigated the mechanism of online effects and tested predictions that were based on the assumption that entrainment is the underlying process mediating behavioural changes during tACS. We capitalised on two well-described phenomena: firstly, the association between α power lateralisation and visuospatial attention, and secondly, the fluctuation of perceptual performance with α phase. Specifically, the experiment tested whether event-related α-tACS applied over right parieto-occipital cortex can induce a visuospatial bias in a peripheral dot detection task that would reflect α power lateralisation, and whether detection performance depends on the phase of the tACS waveform. In control trials either no tACS or 40 Hz-tACS (gamma) was applied to make use of the putative opposing roles of alpha and gamma oscillations in visual processing. As expected from lateralised enhancement of alpha oscillations, visual detection accuracy was weakly impaired for targets presented in the left visual field, contralateral to tACS. However, this effect was neither frequency specific nor waveform phase-dependent. Therefore, it is unlikely that the negative effect of tACS on visuospatial performance reflects entrainment.
Overall, the results of these experiments only partially met our hypotheses. Experiment 1 produced the α enhancement that was expected based on the literature while the follow-up experiment failed to reproduce these results under similar conditions. This outcome demonstrates at best that tACS aftereffects on α activity are not robust, may vary widely across individuals, and might be extremely sensitive to small changes in experimental parameters and state variables. The results of the third experiment call into question the assumption of online entrainment as basis for the observed behavioural effect. These findings point to the need for improved methodology, for more systematic and exhaustive exploration of the relative effects of tACS across different parameter settings, tasks, and individuals; and for the replication of promising but thus far often anecdotal results. They also inspire guidelines for more informative experimental designs.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: transcranial alternating current stimulation, tACS, oscillations, entrainment, alpha, EEG, non-invasive brain stimulation, brain rhythms.
Subjects: B Philosophy. Psychology. Religion > BF Psychology
Q Science > QP Physiology
R Medicine > RC Internal medicine > RC0321 Neuroscience. Biological psychiatry. Neuropsychiatry
Colleges/Schools: College of Science and Engineering > School of Psychology
Supervisor's Name: Thut, Prof. Gregor
Date of Award: 2017
Depositing User: Dr Alexandra Y Vossen
Unique ID: glathesis:2017-7958
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 23 Feb 2017 09:30
Last Modified: 20 Mar 2017 09:06

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