Alpha power increase after transcranial alternating current stimulation at alpha frequency (α-tacs) reflects plastic changes rather than entrainment

Background: Periodic stimulation of occipital areas using transcranial alternating current stimulation (tACS) at alpha (α) frequency (8–12 Hz) enhances electroencephalographic (EEG) α-oscillation long after tACS-offset. Two mechanisms have been suggested to underlie these changes in oscillatory EEG activity: tACS-induced entrainment of brain oscillations and/or tACS-induced changes in oscillatory circuits by spike-timing dependent plasticity.<p></p> Objective: We tested to what extent plasticity can account for tACS-aftereffects when controlling for entrainment “echoes.” To this end, we used a novel, intermittent 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 endogenous α-frequency.<p></p> Methods: 12 healthy participants were stimulated at around individual α-frequency for 15–20 min in four sessions using intermittent tACS or sham. Successive tACS events were either phase-continuous or phase-discontinuous, and either 3 or 8 s long. EEG α-phase and power changes were compared after and between episodes of α-tACS across conditions and against sham.<p></p> Results: α-aftereffects were successfully replicated after intermittent stimulation using 8-s but not 3-s trains. These aftereffects did not reveal any of the characteristics of entrainment echoes in that they were independent of tACS phase-continuity and showed neither prolonged phase alignment nor frequency synchronization to the exact stimulation frequency.<p></p> Conclusion: Our results indicate that plasticity mechanisms are sufficient to explain α-aftereffects in response to α-tACS, and inform models of tACS-induced plasticity in oscillatory circuits. Modifying brain oscillations with tACS holds promise for clinical applications in disorders involving abnormal neural synchrony

The neurobiological mechanisms of transcranial direct current stimulation: insights from human neuroimaging and psychophysics

The research aimed to investigate the neurobiological basis of transcranial direct current stimulation (tDCS); a neuromodulation technique capable of inducing prolonged changes in behavioural performance. The past 15 years have seen a dramatic increase in tDCS-oriented studies, yet the underpinnings of the method are not completely understood. Consequently, this series of experiments was designed to investigate the mechanisms that contribute to the effects of the method. Focusing on neuroimaging, modulations of excitatory and inhibitory neurochemicals were assessed using Magnetic Resonance Spectroscopy (MRS); incorporating distinct spectral editing sequences to define the precise role of inhibitory neurotransmission. Additionally, concurrent DC stimulation and Magnetoencephalography (MEG) was developed, which permitted the novel investigation of excitatory and inhibitory processes via the influence of tDCS on electrophysiological responses in the motor and visual systems. This simultaneous tDCS-MEG investigation is one of only a few existing studies and was the first such endeavour by a group based in the United Kingdom. Finally, a unique psychophysical approach was adopted whereby variations of a vibrotactile adaptation task were utilised to assess the effects of tDCS on amplitude discrimination ability. The paradigms used were specifically chosen due to their physiological similarity to tDCS, thereby enabling inferences on the underpinnings of the method on the basis of changes in somatosensory task performance. These studies provided varying degrees of support for the neurobiological mechanisms proposed in the existing literature, most likely reflecting the influence of distinctions in stimulation protocols and the presence of individual difference factors thought to modify responses to stimulation. Consequently, in addition to the established insights regarding the underpinnings of tDCS, valuable perspectives on the optimisation of stimulation-based methodology were achieved by conducting the outlined investigations

Using TMS-EEG to assess the effects of neuromodulation techniques: a narrative review

Over the past decades, among all the non-invasive brain stimulation (NIBS) techniques, those aiming for neuromodulatory protocols have gained special attention. The traditional neurophysiological outcome to estimate the neuromodulatory effect is the motor evoked potential (MEP), the impact of NIBS techniques is commonly estimated as the change in MEP amplitude. This approach has several limitations: first, the use of MEP limits the evaluation of stimulation to the motor cortex excluding all the other brain areas. Second, MEP is an indirect measure of brain activity and is influenced by several factors. To overcome these limitations several studies have used new outcomes to measure brain changes after neuromodulation techniques with the concurrent use of transcranial magnetic stimulation (TMS) and electroencephalogram (EEG). In the present review, we examine studies that use TMS-EEG before and after a single session of neuromodulatory TMS. Then, we focused our literature research on the description of the different metrics derived from TMS-EEG to measure the effect of neuromodulation

Stimulating vision: measuring and modelling transcranial direct current stimulation of the visual cortex

Transcranial direct current stimulation (tDCS) has enjoyed something of a renaissance in neuroscientific research, however, this has not been accompanied by a commensurate increase in our understanding of its neurobiological mechanisms. At present, there remains a large explanatory gap between the stimulation effects on cells in in vivo or in vitro studies and the wide variety of behavioural findings in human studies. Consequently, tDCS research is currently confronted with a wide variety of conceptual and methodological challenges that have hampered the development of mature rationales for its use in healthy and clinical populations. This thesis aimed to address some of these challenges by combining data from behavioural and neuroimaging experiments with findings from individualised models of tDCS-induced electric fields. Experiments focused on the visual system, using relatively simple paradigms based on pattern-reversing checkerboards and the detection of achromatic dot stimuli to investigate stimulation effects on visual processing, The role of inter-individual variability – both in baseline sensory performance and in head anatomy – received particular attention in the design of studies. In the second chapter of the thesis, the question of suitable current waveforms for doubleblind, sham-controlled tDCS studies is discussed. The third chapter investigates the role of electrode montage in eliciting tDCS effects on contrast detection at central and peripheral visual field locations. In Chapters 4 and 5, inter-individual differences in anatomy are quantified using computational modelling of electric fields and neuroimaging methods. Work presented in Chapter 6 explores the feasibility of acquiring concurrent tDCS-NIRS-MEG data. Together, results from these studies suggest that the large parameter space for designing and interpreting human tDCS experiments calls for a broad range of methodological advances in future tDCS research

Neuronal Network Oscillations in the Control of Human Movement

The overarching aim of this thesis was to use neuroimaging and neuromodulation techniques to further understand the relationship between cortical oscillatory activity and the control of human movement. Modulations in motor cortical beta and alpha activity have been consistently implicated in the preparation, execution, and termination of movement. Here, I describe the outcome of four studies designed to further elucidate these motor-related changes in oscillatory activity. In Chapter 3, I report the findings of a study that used an established behavioural paradigm to vary the degree of uncertainty during the preparation of movement. I demonstrate that preparatory alpha and beta desynchronisation reflect a process of disengagement from the existing network to enable the creation of functional assemblies required for movement. Importantly, I also demonstrate a novel neural signature of transient alpha synchrony, that occurs after preparatory desynchronisation, that underlies the recruitment of functional assemblies required for directional control. The study described in Chapter 4 was designed to further investigate the functional role of preparatory alpha and beta desynchronisation by entraining oscillatory activity in the primary motor cortex (M1) using frequency-specific transcranial alternating current stimulation. No significant effects of stimulation were found on participant response times. However, no clear conclusion could be drawn due to limitations of the stimulation parameters that were used. In Chapter 5, I explored the inverse relationship between M1 beta power and cortical excitability using single-pulse transcranial magnetic stimulation to elicit motor-evoked potentials (MEPs). The amplitude of MEPs collected during a period of beta desynchronisation was significantly greater than during a resting baseline. Conversely, the amplitude of MEPs collected during the post-movement beta rebound that follows the termination of a movement was significantly reduced compared to baseline. This finding confirms the inverse relationship between M1 beta power and cortical excitability. The study in Chapter 6 explored the effect of experimental context on M1 beta power. When the participant was cued to expect an upcoming motor task, resting beta power was significantly increased, then when the likelihood of an upcoming motor requirement decreased, there was a significant concurrent decrease in resting beta power. This reflects increased coherence and functional connectivity within M1 and other motor areas, to ‘recalibrate’ the motor system in preparation for a synchronous input signal to more readily recruit the required functional assembly