Neuromodulatory effects of transcranial electrical stimulation on emotion regulation in internalizing psychopathologies

OBJECTIVE

We hypothesize offset theta-tACS (transcranial alternating current stimulation) improves emotion regulation (ER) and psychopathology more than transcranial direct current stimulation (tDCS) in participants with internalizing psychopathologies (IPs).

METHODS

This pilot study utilized a double-blind, pseudo-counterbalanced, sham-controlled design with participants with IPs. Participants were assigned to receive tDCS or tACS, underwent four stimulation sessions (two sham), and completed an emotion regulation task (ERT) during or after stimulation. Participants completed the Beck Depression Inventory before/after the study, the Spielberger State and Trait Anxiety Index after each ERT, and rated their arousal, valence, and perceived reappraisal ability during the ERT.

RESULTS

Participants receiving either stimulation type showed a reduction in anxiety, depression, and valence and arousal ratings. We additionally discovered an effect demonstrating those who received sham stimulation first displayed little-to-no change in any score across the study, but tACS participants who received verum stimulation first showed significant improvements in each metric.

CONCLUSIONS

Improving ER capabilities via theta tACS has the potential to yield beneficial clinical effects.

SIGNIFICANCE

This study adds validity to the use of non-invasive neuromodulatory methods, especially tACS, to alleviate IPs. Additional research is needed to better understand the effects of sham stimulation. Careful consideration of sham incorporation should be made in future studies.

The influence of sensory potentials on transcranial magnetic stimulation - Electroencephalography recordings

OBJECTIVE

It remains unclear to what extent Transcranial Magnetic Stimulation-evoked potentials (TEPs) reflect sensory (auditory and somatosensory) potentials as opposed to cortical excitability. The present study aimed to determine; a) the extent to which sensory potentials contaminate TEPs using a spatially-matched sham condition, and b) whether sensory potentials reflect auditory or somatosensory potentials alone, or a combination of the two.

METHODS

Twenty healthy participants received active or sham stimulation, with the latter consisting a sham coil click combined with scalp electrical stimulation. Two additional conditions i) electrical stimulation and ii) auditory stimulation alone, were included in a subset of 13 participants.

RESULTS

Signals from active and sham stimulation were correlated in spatial and temporal domains > 55 ms post-stimulation. Relative to auditory or electrical stimulation alone, sham stimulation resulted in a) larger potentials, b) stronger correlations with active stimulation and c) a signal that was not a linear sum of electrical and auditory stimulation alone.

CONCLUSIONS

Sensory potentials can confound interpretations of TEPs at timepoints > 55 ms post-stimulation. Furthermore, TEP contamination cannot be explained by auditory or somatosensory potentials alone, but instead reflects a non-linear interaction between both.

SIGNIFICANCE

Future studies may benefit from controlling for sensory contamination using spatially-matched sham conditions, and which consist of combined auditory and somatosensory stimulation.

Recording brain responses to TMS of primary motor cortex by EEG - utility of an optimized sham procedure

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Optimized sham TMS-EEG is introduced and tested.

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Sham combined auditory and supramaximal electrical somatosensory stimulation.

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Subjects reported equal sensory perception during sham and real TMS.

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Subtraction revealed evoked EEG potentials and beta-band power specific to real TMS.

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The optimized sham procedure is relevant in research and therapeutic settings.

Introduction

Electroencephalography (EEG) is increasingly used to investigate brain responses to transcranial magnetic stimulation (TMS). A relevant issue is that TMS is associated with considerable auditory and somatosensory stimulation, causing peripherally evoked potentials (PEPs) in the EEG, which contaminate the direct cortical responses to TMS (TEPs). All previous attempts to control for PEPs suffer from significant limitations.

Objective/Hypothesis

To design an optimized sham procedure to control all sensory input generated by subthreshold real TMS targeting the hand area of the primary motor cortex (M1), enabling reliable separation of TEPs from PEPs.

Methods

In 23 healthy (16 female) subjects, we recorded EEG activity evoked by an optimized sham TMS condition which masks and matches auditory and somatosensory co-stimulation during the real TMS condition: auditory control was achieved by noise masking and by using a second TMS coil that was placed on top of the real TMS coil and produced a calibrated sound pressure level. Somatosensory control was obtained by electric stimulation (ES) of the scalp with intensities sufficient to saturate somatosensory input. ES was applied in both the sham and real TMS conditions. Perception of auditory and somatosensory inputs in the sham and real TMS conditions were compared by psychophysical testing. Transcranially evoked EEG signal changes were identified by subtraction of EEG activity in the sham condition from EEG activity in the real TMS condition.

Results

Perception of auditory and somatosensory inputs in the sham vs. real TMS conditions was comparable. Both sham and real TMS evoked a series of similar EEG signal deflections and induced broadband power increase in oscillatory activity. Notably, the present procedure revealed EEG potentials and a transient increase in beta band power at the site of stimulation that were only present in the real TMS condition.

Discussion

The results validate the effectiveness of our optimized sham approach. Despite the presence of typical responses attributable to sensory input, the procedure provided evidence for direct cortical activation by subthreshold TMS of M1. The findings are relevant for future TMS-EEG experiments that aim at measuring regional brain target engagement controlled by an optimized sham procedure.

Efficacy of tRNS and 140 Hz tACS on motor cortex excitability seemingly dependent on sensitivity to sham stimulation

This study investigates the effect of corticospinal excitability during sham stimulation on the individual response to transcranial non-invasive brain stimulation (tNIBS). Thirty healthy young adults aged 24.2 ± 2.8 S.D. participated in the study. Sham, as well as 1 mA of tRNS and 140 Hz tACS stimulation were applied for 10 min each at different sessions. The effect of each stimulation type was quantified by recording TMS-induced, motor evoked potentials (MEPs) before (baseline) and at fixed time points after stimulation (T0, T30, T60 min.). According to the individual response to sham stimulation at T0 in comparison to baseline MEPs, subjects were regarded as responder or non-responder to sham. Following, MEPs at T0, T30 and T60 after verum or sham stimulation were assessed with a repeated measures ANOVA with the within-subject factor stimulation (sham, tRNS, 140 Hz tACS) and the between-subjects factor group (responder vs non-responder). We found that individuals who did not show immediately changes in excitability in sham stimulation sessions were the ones who responded to active stimulation conditions. On the other hand, individuals who responded to sham condition, by either increases or decreases in MEPS, did not respond to active verum stimulation. This result suggests that the presence or lack of responses to sham stimulation can provide a marker for how individuals will respond to tRNS/tACS and thus provide an explanation for the variability in interindividual response. The results of this study draw attention to the general reactivity of the brain, which can be taken into account when planning future studies using tNIBS.

The non-transcranial TMS-evoked potential is an inherent source of ambiguity in TMS-EEG studies

Transcranial Magnetic Stimulation (TMS) excites populations of neurons in the stimulated cortex, and the resulting activation may spread to connected brain regions. The distributed cortical response can be recorded with electroencephalography (EEG). Since TMS also stimulates peripheral sensory and motor axons and generates a loud "click" sound, the TMS-evoked EEG potentials (TEPs) reflect not only neural activity induced by transcranial neuronal excitation but also neural activity due to somatosensory and auditory processing. In 17 healthy young individuals, we systematically assessed the contribution of multisensory peripheral stimulation to TEPs using a TMS-compatible EEG system. Real TMS was delivered with a figure-of-eight coil over the left para-median posterior parietal cortex or superior frontal gyrus with the coil being oriented perpendicularly or in parallel to the target gyrus. We also recorded the EEG responses evoked by realistic sham stimulation over the posterior parietal and superior frontal cortex, mimicking the auditory and somatosensory sensations evoked by real TMS. We applied state-of-the-art procedures to attenuate somatosensory and auditory confounds during real TMS, including the placement of a foam layer underneath the coil and auditory noise masking. Despite these precautions, the temporal and spatial features of the cortical potentials evoked by real TMS at the prefrontal and parietal site closely resembled the cortical potentials evoked by realistic sham TMS, both for early and late TEP components. Our findings stress the need to include a peripheral multisensory control stimulation in the design of TMS-EEG studies to enable a dissociation between truly transcranial and non-transcranial components of TEPs.

Evaluation of sham transcranial direct current stimulation for randomized, placebo-controlled clinical trials

BACKGROUND

Transcranial direct current stimulation (tDCS) has been investigated as therapeutic intervention in various psychiatric and neurologic disorders. As placebo responses to technical interventions may be pronounced in many clinical conditions, it is important to thoroughly develop placebo conditions which meet the requirements for application in randomized double-blind controlled trials.

OBJECTIVE

The two-part experiment reported here aims at evaluating a new sham tDCS condition in healthy subjects and device operators. Sham or active tDCS is delivered after entering a number code to the device and allows blinding of the operator before and during tDCS. The sham mode has no short stimulation period.

METHODS

The experimental sequence was as follows: 1) Evaluation of successful blinding by comparing placebo to active stimulation at prefrontal sites based on the rating of subjects undergoing tDCS, 2) Evaluation of successful blinding by comparing placebo to active stimulation at prefrontal sites based on the operator/observer ratings.

RESULTS

Subjects were not able to distinguish between active and sham tDCS for prefrontal stimulation. Overall there was no relevant discomfort and tDCS was well tolerated. Operators/observers were able to identify sham stimulation based on skin reddening after active, but not after sham tDCS.

CONCLUSIONS

The tDCS sham condition investigated here may be suitable for placebo-controlled trials keeping subjects blind to treatment conditions. However, operators can easily be aware of the condition applied and they should not get involved in rating outcome measures during the course of high standard placebo-controlled trials.