Aron T. Hill

Use of theta-burst stimulation in changing excitability of motor cortex: A systematic review and meta-analysis

Noninvasive brain stimulation has been demonstrated to modulate cortical activity in humans. In particular, theta burst stimulation (TBS) has gained notable attention due to its ability to induce lasting physiological changes after short stimulation durations. The present study aimed to provide a comprehensive meta-analytic review of the efficacy of two TBS paradigms; intermittent (iTBS) and continuous (cTBS), on corticospinal excitability in healthy individuals. Literature searches yielded a total of 87 studies adhering to the inclusion criteria. iTBS yielded moderately large MEP increases lasting up to 30 min with a pooled SMD of 0.71 (p<0.00001). cTBS produced a reduction in MEP amplitudes lasting up to 60 min, with the largest effect size seen at 5 min post stimulation (SMD=-0.9, P<0.00001). The collected studies were of heterogeneous nature, and a series of tests conducted indicated a degree of publication bias. No significant change in SICI and ICF was observed, with exception to decrease in SICI with cTBS at the early time point (SMD=0.42, P=0.00036). The results also highlight several factors contributing to TBS efficacy, including the number of pulses, frequency of stimulation and BDNF polymorphisms. Further research investigating optimal TBS stimulation parameters, particularly for iTBS, is needed in order for these paradigms to be successfully translated into clinical settings.

Effects of prefrontal bipolar and high-definition transcranial direct current stimulation on cortical reactivity and working memory in healthy adults

ABSTRACT Transcranial direct current stimulation (tDCS) is a well‐recognised neuromodulatory technology which has been shown to induce short‐lasting changes in motor‐cortical excitability. The recent and rapid expansion of tDCS into the cognitive domain, however, necessitates deeper mechanistic understanding of its neurophysiological effects over non‐motor brain regions. The present study utilised transcranial magnetic stimulation combined with electroencephalography (TMS‐EEG) to probe the immediate and longer‐term effects of both a bipolar (BP‐tDCS) and more focal 4×1 High‐Definition tDCS (HD‐tDCS) montage applied over the left DLPFC on TMS‐evoked potentials (TEPs) and oscillations in 19 healthy adult participants. 2‐back working memory (WM) performance was also assessed as a marker of cognitive function. Region of interest (ROI) analyses taken from the F1 electrode directly adjacent to the stimulation site revealed increased P60 TEP amplitudes at this location 5 min following BP‐tDCS and 30 min following HD‐tDCS. Further global cluster based analyses of all scalp electrodes revealed widespread neuromodulatory changes following HD‐tDCS, but not BP‐tDCS, both five and 30 min after stimulation, with reductions also detected in both beta and gamma oscillatory power over parieto‐occipital channels 30 min after stimulation. No significant changes in WM performance were observed following either HD‐tDCS or BP‐tDCS. This study highlights the capacity for single‐session prefrontal anodal tDCS montages to modulate neurophysiological processes, as assessed with TMS‐EEG. HighlightsHD‐tDCS and BP‐tDCS applied over the DLPFC.Neurophysiological effects examined with TMS‐EEG.Working memory performance examined with 2‐back task.HD‐tDCS produced robust and widespread changes in cortical reactivity.No significant changes in working memory performance were observed.

TMS-EEG: A window into the neurophysiological effects of transcranial electrical stimulation in non-motor brain regions.

Transcranial electrical stimulation (tES) techniques are able to induce changes in cortical excitability and plasticity through the administration of weak currents to the brain and are currently being used to manipulate a vast array of cognitive processes. Despite the widespread use of tES technologies within both research and remedial settings, their precise neurophysiological mechanisms of action are not well established outside of the motor cortex. The expanding use of tES within non-motor brain regions highlights the growing need for a more comprehensive understanding of the effects of stimulation across a diversity of cortical locations. The combination of transcranial magnetic stimulation with electroencephalography (TMS-EEG) provides a method of directly probing both local and widespread changes in brain neurophysiology, through the recording of TMS-evoked potentials and cortical oscillations. In this review we explore TMS-EEG as a tool for examining the impact of tES on cortical function and argue that multimodal approaches which combine tES with TMS-EEG could lead to a deeper understanding of the mechanisms which underlie tES-induced cognitive modulation.

Modulation of putative mirror neuron activity by both positively and negatively valenced affective stimuli: a TMS study.

Research indicates that mirror neurons are important for social cognition, including emotion processing. Emerging evidence, however, also reveals that emotional stimuli might be capable of modulating human mirror neuron system (MNS) activity. The current study used transcranial magnetic stimulation (TMS) to assess putative mirror neuron function following emotionally evocative images in twenty healthy adults. Participants observed videos of either a transitive hand action or a static hand while undergoing TMS of the primary motor cortex. In order to examine the effect of emotion on the MNS, each video was preceded by an image of either a positive, negative or neutral valence. MNS activity was found to be augmented by both the positive and negative (relative to neutral) stimuli, thus providing empirical support for a bi-directional link between emotion and the MNS, whereby both positively and negatively valenced stimuli are capable of facilitating mirror neuron activity. The potential adaptive significance of this finding is discussed.

Effects of Anodal Transcranial Direct Current Stimulation on Working and Recognition Memory: A Systematic Review and Meta-Analysis of Findings from Healthy and Neuropsychiatric Populations

Systematic literature searches yielded a total of 21 studies satisfying inclusion criteria. The pooled results from these experiments demonstrated small improvements in working memory accuracy (SMD ¼ 0.18, CI ¼ 0.06, 0.30, p ¼ 0.004) and reaction time (SMD ¼ -0.16, 95% CI ¼ -0.28, -0.04, p ¼ 0.007). Subgroup analyses indicated that these results were significant for healthy cohorts (Accuracy: SMD ¼ 0.16, CI ¼ 0.02, 0.30, p ¼ 0.02; Reaction time: SMD ¼ -0.17, 95% CI ¼ -0.30, -0.03, p ¼ 0.01), with neuropsychiatric cohorts showing a trend towards significance for accuracy only (SMD ¼ 0.22, CI ¼ -0.01, 0.46, p ¼ 0.07). Additionally, significant small-to-moderate improvements in recognition memory were seen overall (SMD ¼ 0.44, CI ¼ 0.19, 0.68, p ¼ 0.0005) and at the subgroup level for both healthy (SMD ¼ 0.36, CI ¼ 0.06, 0.66, p ¼ 0.02) and neuropsychiatric (SMD ¼ 0.58, CI ¼ 0.16, 1.01, p ¼ 0.007) cohorts. Finally, there was some evidence to suggest that higher current densities and longer stimulation durations were favourable for enhancing working memory performance. Overall, this meta-analytical review demonstrates a varying degree of efficacy for the use of a-tDCS for enhancing memory function. Research is now needed to optimise the proficiency of this technology.

Psychiatry versus general physicians: who is better at differentiating epileptic from psychogenic non-epileptic seizures?

Objective: To determine how accurately psychiatry and general medical doctors can differentiate epileptic and psychogenic non-epileptic seizures based on videotaped events (closest proxy to witnessed events). This study aims to establish how confidently this distinction can be made, the reasons why a particular diagnosis is reached, and inter-rater agreement. Methods: 18 videos of patients demonstrating a heterogeneous mixture of epileptic and psychogenic non-epileptic seizures were collected and ordered in a random mix. These videos were shown to groups of general physicians, medical registrars and residents (n=19) as well as to psychiatrists and psychiatry registrars (n=8) who were provided with a questionnaire. Results: A total of 27 doctors participated in the study. The overall percentage of correct diagnoses was 55.4%. There were no significant differences in correct diagnosis rates between psychiatry and general medical doctors. There was poor inter-rater agreement (Kappa = 0.159). Neither group was particularly confident in reaching a diagnosis, and diverse reasons underpinned the diagnoses given. Conclusion: Among the participants, merely observing an epileptic or non-epileptic event is insufficient to establish a definitive diagnosis. The results indicate poor diagnostic accuracy and agreement among psychiatry and general medical doctors. This may have important implications for both education and clinical practice.

Simultaneous recording of EEG and electromyographic polygraphy increases the diagnostic yield of video-EEG monitoring.

Purpose: To investigate the usefulness of adjunctive electromyographic (EMG) polygraphy in the diagnosis of clinical events captured during long-term video-EEG monitoring. Methods: A total of 40 patients (21 women, 19 men) aged between 19 and 72 years (mean 43) investigated using video-EEG monitoring were studied. Electromyographic activity was simultaneously recorded with EEG in four patients selected on clinical grounds. In these patients, surface EMG electrodes were placed over muscles suspected to be activated during a typical clinical event. Results: Of the 40 patients investigated, 24 (60%) were given a diagnosis, whereas 16 (40%) remained undiagnosed. All four patients receiving adjunctive EMG polygraphy obtained a diagnosis, with three of these diagnoses being exclusively reliant on the EMG recordings. Specifically, one patient was diagnosed with propriospinal myoclonus, another patient was diagnosed with facio-mandibular myoclonus, and a third patient was found to have bruxism and periodic leg movements of sleep. Conclusions: The information obtained from surface EMG recordings aided the diagnosis of clinical events captured during video-EEG monitoring in 7.5% of the total cohort. This study suggests that EEG-EMG polygraphy may be used as a technique of improving the diagnostic yield of video-EEG monitoring in selected cases.