Contribution of TMS and TMS-EEG to the Understanding of Mechanisms Underlying Physiological Brain Aging

Exploring the potential of combining transcranial magnetic stimulation and electroencephalography to investigate mild cognitive impairment and Alzheimer's disease: a systematic review

Transcranial magnetic stimulation (TMS) and electroencephalography (EEG) are non-invasive techniques used for neuromodulation and recording brain electrical activity, respectively. The integration of TMS-EEG has emerged as a valuable tool for investigating the complex mechanisms involved in age-related disorders, such as mild cognitive impairment (MCI) and Alzheimer's disease (AD). By systematically synthesizing TMS-EEG studies, this review aims to shed light on the neurophysiological mechanisms underlying MCI and AD, while also exploring the practical applications of TMS-EEG in clinical settings. PubMed, ScienceDirect, and PsychInfo were selected as the databases for this review. The 22 eligible studies included a total of 592 individuals with MCI or AD as well as 301 cognitively normal adults. TMS-EEG assessments unveiled specific patterns of corticospinal excitability, plasticity, and brain connectivity that distinguished individuals on the AD spectrum from cognitively normal older adults. Moreover, the TMS-induced EEG features were observed to be correlated with cognitive performance and the presence of AD pathological biomarkers. The comprehensive examination of the existing studies demonstrates that the combination of TMS and EEG has yielded valuable insights into the neurophysiology of MCI and AD. This integration shows great potential for early detection, monitoring disease progression, and anticipating response to treatment. Future research is of paramount importance to delve into the potential utilization of TMS-EEG for treatment optimization in individuals with MCI and AD.

Modern neurophysiological techniques indexing normal or abnormal brain aging

Brain aging is associated with a decline in cognitive performance, motor function and sensory perception, even in the absence of neurodegeneration. The underlying pathophysiological mechanisms remain incompletely understood, though alterations in neurogenesis, neuronal senescence and synaptic plasticity are implicated. Recent years have seen advancements in neurophysiological techniques such as electroencephalography (EEG), magnetoencephalography (MEG), event-related potentials (ERP) and transcranial magnetic stimulation (TMS), offering insights into physiological and pathological brain aging. These methods provide real-time information on brain activity, connectivity and network dynamics. Integration of Artificial Intelligence (AI) techniques promise as a tool enhancing the diagnosis and prognosis of age-related cognitive decline. Our review highlights recent advances in these electrophysiological techniques (focusing on EEG, ERP, TMS and TMS-EEG methodologies) and their application in physiological and pathological brain aging. Physiological aging is characterized by changes in EEG spectral power and connectivity, ERP and TMS parameters, indicating alterations in neural activity and network function. Pathological aging, such as in Alzheimer's disease, is associated with further disruptions in EEG rhythms, ERP components and TMS measures, reflecting underlying neurodegenerative processes. Machine learning approaches show promise in classifying cognitive impairment and predicting disease progression. Standardization of neurophysiological methods and integration with other modalities are crucial for a comprehensive understanding of brain aging and neurodegenerative disorders. Advanced network analysis techniques and AI methods hold potential for enhancing diagnostic accuracy and deepening insights into age-related brain changes.

Changes in Cortical Activation by Transcranial Magnetic Stimulation Due to Coil Rotation Are Not Attributable to Cranial Muscle Activation

Transcranial magnetic stimulation coupled with electroencephalography (TMS-EEG) allows for the study of brain dynamics in health and disease. Cranial muscle activation can decrease the interpretability of TMS-EEG signals by masking genuine EEG responses and increasing the reliance on preprocessing methods but can be at least partly prevented by coil rotation coupled with the online monitoring of signals; however, the extent to which changing coil rotation may affect TMS-EEG signals is not fully understood. Our objective was to compare TMS-EEG data obtained with an optimal coil rotation to induce motor evoked potentials (M1standard) while rotating the coil to minimize cranial muscle activation (M1emg). TMS-evoked potentials (TEPs), TMS-related spectral perturbation (TRSP), and intertrial phase clustering (ITPC) were calculated in both conditions using two different preprocessing pipelines based on independent component analysis (ICA) or signal-space projection with source-informed reconstruction (SSP-SIR). Comparisons were performed with cluster-based correction. The concordance correlation coefficient was computed to measure the similarity between M1standard and M1emg TMS-EEG signals. TEPs, TRSP, and ITPC were significantly larger in M1standard than in M1emg conditions; a lower CCC than expected was also found. These results were similar across the preprocessing pipelines. While rotating the coil may be advantageous to reduce cranial muscle activation, it may result in changes in TMS-EEG signals; therefore, this solution should be tailored to the specific experimental context.

Transcranial Magnetic Stimulation Across the Lifespan: Impact of Developmental and Degenerative Processes

Transcranial magnetic stimulation (TMS) has emerged as a pivotal noninvasive technique for investigating cortical excitability and plasticity across the lifespan, offering valuable insights into neurodevelopmental and neurodegenerative processes. In this review, we explore the impact of TMS applications on our understanding of normal development, healthy aging, neurodevelopmental disorders, and adult-onset neurodegenerative diseases. By presenting key developmental milestones and age-related changes in TMS measures, we provide a foundation for understanding the maturation of neurotransmitter systems and the trajectory of cognitive functions throughout the lifespan. Building on this foundation, the paper delves into the pathophysiology of neurodevelopmental disorders, including autism spectrum disorder, attention-deficit/hyperactivity disorder, Tourette syndrome, and adolescent depression. Highlighting recent findings on altered neurotransmitter circuits and dysfunctional cortical plasticity, we underscore the potential of TMS as a valuable tool for unraveling underlying mechanisms and informing future therapeutic interventions. We also review the emerging role of TMS in investigating and treating the most common adult-onset neurodegenerative disorders and late-onset depression. By outlining the therapeutic applications of noninvasive brain stimulation techniques in these disorders, we discuss the growing body of evidence supporting their use as therapeutic tools for symptom management and potentially slowing disease progression. The insights gained from TMS studies have advanced our understanding of the underlying mechanisms in both healthy and disease states, ultimately informing the development of more targeted diagnostic and therapeutic strategies for a wide range of neuropsychiatric conditions.

Non-Invasive Neuromodulation Methods to Alleviate Symptoms of Huntington's Disease: A Systematic Review of the Literature

Huntington's disease (HD) is a progressive and debilitating neurodegenerative disease. There is growing evidence for non-invasive neuromodulation tools as therapeutic strategies in neurodegenerative diseases. This systematic review aims to investigate the effectiveness of noninvasive neuromodulation in HD-associated motor, cognitive, and behavioral symptoms. A comprehensive literature search was conducted in Ovid MEDLINE, Cochrane Central Register of Clinical Trials, Embase, and PsycINFO from inception to 13 July 2021. Case reports, case series, and clinical trials were included while screening/diagnostic tests involving non-invasive neuromodulation, review papers, experimental studies on animal models, other systematic reviews, and meta-analyses were excluded. We have identified 19 studies in the literature investigating the use of ECT, TMS, and tDCS in the treatment of HD. Quality assessments were performed using Joanna Briggs Institute's (JBI's) critical appraisal tools. Eighteen studies showed improvement of HD symptoms, but the results were very heterogeneous considering different intervention techniques and protocols, and domains of symptoms. The most noticeable improvement involved depression and psychosis after ECT protocols. The impact on cognitive and motor symptoms is more controversial. Further investigations are required to determine the therapeutic role of distinct neuromodulation techniques for HD-related symptoms.

Cognitive deficit in post-acute COVID-19: an opportunity for EEG evaluation?

BACKGROUND AND PURPOSE

Among the most common post-COVID symptoms, many patients experienced subjective cognitive deficit, commonly named "brain fog," that might be present also in those individuals without severe acute COVID-19 respiratory involvement. Some studies have investigated some of the mechanisms that might be associated with the brain fog with objective techniques including transcranial magnetic stimulation and neuroimaging.

METHODS

The aim of this study was to investigate the presence of electroencephalographic (EEG) alterations in people with post-COVID self-reported cognitive deficit.

RESULTS

Out of the 90 patients attending the post-COVID neurology ambulatory service, twenty patients presenting brain fog at least 4 weeks after acute non-severe COVID-19 infection, and without previous history of epilepsy, were investigated with 19-channel EEG, Montreal Cognitive Assessment (MoCA), and magnetic resonance imaging (MRI). EEG was found altered in 65% of the sample, among which 69% presented a slowing activity and 31% were characterized by epileptic discharges principally in the frontal areas. None of the patients showed DWI MRI lesions.

CONCLUSIONS

These findings highlight the usefulness of EEG analysis to objectively describe possible neurophysiological abnormalities in post-COVID patients presenting subjective cognitive deficit.

Long-interval intracortical inhibition in primary motor cortex related to working memory in middle-aged adults

Introduction

Excitability of the primary motor cortex measured with TMS has been associated with cognitive dysfunctions in patient populations. However, only a few studies have explored this relationship in healthy adults, and even fewer have considered the role of biological sex.

Methods

Ninety-seven healthy middle-aged adults (53 male) completed a TMS protocol and a neuropsychological assessment. Resting Motor Threshold (RMT) and Long-Interval Intracortical Inhibition (LICI) were assessed in the left motor cortex and related to attention, episodic memory, working memory, reasoning, and global cognition composite scores to evaluate the relationship between cortical excitability and cognitive functioning.

Results

In the whole sample, there was a significant association between LICI and cognition; specifically, higher motor inhibition was related to better working memory performance. When the sample was broken down by biological sex, LICI was only associated with working memory, reasoning, and global cognition in men. No associations were found between RMT and cognitive functions.

Conclusion

Greater intracortical inhibition, measured by LICI, could be a possible marker of working memory in healthy middle-aged adults, and biological sex plays a critical role in this association.

EEG, ERPs, and EROs in patients with neurodegenerative dementing disorders: A window into the cortical neurophysiology of cognition and behavior

In the human brain, physiological aging is characterized by progressive neuronal loss, leading to disruption of synapses and to a degree of failure in neurotransmission and information flow. However, there is increasing evidence to support the notion that the aged brain has a remarkable level of resilience (i.s. ability to reorganize itself), with the aim of preserving its physiological activity. It is therefore of paramount interest to develop objective markers able to characterize the biological processes underlying brain aging in the intact human, and to distinguish them from brain degeneration associated to age-related neurological progressive diseases like Alzheimer's disease. EEG, alone and combined with transcranial magnetic stimulation (TMS-EEG), is particularly suited to this aim, due to the functional nature of the information provided, and thanks to the ease with which it can be integrated in ecological scenarios including behavioral tasks. In this review, we aimed to provide the reader with updated information about the role of modern methods of EEG and TMS-EEG analysis in the investigation of physiological brain aging and Alzheimer's disease. In particular, we focused on data about cortical connectivity obtained by using readouts such graph theory network brain organization and architecture, and transcranial evoked potentials (TEPs) during TMS-EEG. Overall, findings in the literature support an important potential contribution of such neurophysiological techniques to the understanding of the mechanisms underlying normal brain aging and the early (prodromal/pre-symptomatic) stages of dementia.

Barriers to Enrollment in Post-Stroke Brain Stimulation in a Racially and Ethnically Diverse Population

BACKGROUND AND PURPOSE

Brain stimulation is an adjuvant strategy to promote rehabilitation after stroke. Here, we evaluated the influence of inclusion/exclusion criteria on enrollment in a transcranial direct current stimulation (tDCS) trial in the context of a racially/ethnically diverse acute stroke service at University of Texas Southwestern (UTSW).

METHODS

3124 (59.7 ± 14.5 years) racially/ethnically diverse (38.4% non-Hispanic white, (W), Hispanic (H) 22%, African American (AA) 33.5%, Asian (A) 2.3%) patients were screened in the acute stroke service at UTSW. Demographics, stroke characteristics, and reasons for exclusion were recorded prospectively.

RESULTS

2327 (74.5%) patients had a verified stroke. Only 44 of them (1.9%) were eligible. Causes for exclusion included in order of importance: (1) magnitude of upper extremity (UE) motor impairment, (2) prior strokes (s), (3) hemorrhagic stroke, (4) psychiatric condition or inability to follow instructions, and (5) old age, of these (2) and (4) were more common in AA patients but not in other minorities. 31 of the 44 eligible individuals were enrolled (W 1.68%, H 1.37%, AA .77%, A 3.774%). 90.5% of verified stroke patients did not exhibit contraindications for stimulation.

CONCLUSIONS

3 main conclusions emerged: (a) The main limitations for inclusion in brain stimulation trials of motor recovery were magnitude of UE motor impairments and stroke lesion characteristics, (b) most stroke patients could be stimulated with tDCS without safety concerns and (c) carefully tailored inclusion criteria could increase diversity in enrollment.Clinical Trial Registration-URL: http://clinicaltrials.gov. Unique identifier: NCT01007136.

Older adults' episodic memory is related to a neurophysiological marker of brain cholinergic activity

Episodic memory is vulnerable to aging and may be influenced by age-related decline in the neurotransmitter acetylcholine. We probed this relation using a novel, minimally invasive transcranial magnetic stimulation marker of brain acetylcholine: short-latency afferent inhibition (SAI). We used neuropsychological testing to construct a composite score of episodic memory in N = 19 community-dwelling older adults, and stratified older adults into Higher- (N = 9) versus Lower-memory (N = 10) groups before SAI. The Higher-memory group showed significantly stronger SAI than the Lower-memory group, indicating an association between higher brain acetylcholine levels and better episodic memory. The two memory groups were equivalent in the potential confounds of age, education, mood, subjective sleep quality, and executive function. These data converge with others to suggest that episodic memory is related to acetylcholine in older adults. This relation should be further investigated, especially with pharmacology and neuroimaging.

Identification and verification of a 'true' TMS evoked potential in TMS-EEG

The concurrent combination of transcranial magnetic stimulation and electroencephalography (TMS-EEG) can unveil functional neural mechanisms with applications in basic and clinical research. In particular, TMS-evoked potentials (TEPs) potentially allow studying excitability and connectivity of the cortex in a causal manner that is not easily or non-invasively attainable with other neuroimaging techniques. The TEP waveform is obtained by isolating the EEG responses phase-locked to the time of TMS application. The intended component in a TEP waveform is the cortical activation by the TMS-induced electric current, free of instrumental and physiological artifact sources. This artifact-free cortical activation can be referred to as 'true' TEP. However, due to many unwanted auxiliary effects of TMS, the interpretation of 'true' TEPs has not been free of controversy. This paper reviews the most recent understandings of 'true' TEPs and their application. In the first part of the paper, TEP components are defined according to recommended methodologies. In the second part, the verification of 'true' TEP is discussed along with its sensitivity to brain-state, age, and disease. The various proposed origins of TEP components are then presented in the context of existing literature. Throughout the paper, lessons learned from the past TMS-EEG studies are highlighted to guide the identification and interpretation of 'true' TEPs in future studies.

Transcranial magnetic stimulation (TMS) for geriatric depression

BACKGROUND

The prevalence of treatment-resistant geriatric depression (GD) highlights the need for treatments that preserve cognitive functions and recognize polypharmacy in elderly, yet effectively reduce symptom burden. Transcranial magnetic stimulation (TMS) is a proven intervention for treatment-resistant depression in younger adults but the efficacy of TMS to treat depressed older adults is still unclear. This review provides an updated view on the efficacy of TMS treatment for GD, discusses methodological differences between trials in TMS application, and explores avenues for optimization of TMS treatment in the context of the ageing brain.

METHODS

A systematic review was conducted to identify published literature on the antidepressant efficacy of TMS for GD. Databases PubMed, Embase, and PsycINFO were searched for English language articles in peer-reviewed journals in March 2021.

RESULTS

Seven randomized controlled trials (RCTs) (total n = 260, active n = 148, control n = 112) and seven uncontrolled trials (total n = 160) were included. Overall, we found substantial variability in the clinical response, ranging from 6.7% to 54.3%.

CONCLUSIONS

The reviewed literature highlights large heterogeneity among studies both in terms of the employed TMS dosage and the observed clinical efficacy. This highlights the need for optimizing TMS dosage by recognizing the unique clinical features of GD. We showcase a set of novel approaches for the optimization of the TMS protocol for depression and discuss the possibility for a standardized TMS protocol tailored for the treatment of GD.

TMS-EEG responses across the lifespan: Measurement, methods for characterisation and identified responses

The combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) allows probing of the neurophysiology of any neocortical brain area in vivo with millisecond accuracy. TMS-EEG is particularly unique compared with other available neurophysiological methods, as it can measure the state and dynamics of excitatory and inhibitory systems separately. Because of these capabilities, TMS-EEG responses are sensitive to the brain state, and the responses are influenced by brain maturation and ageing, making TMS-EEG a suitable method to study age-specific pathophysiology. In this review, we outline the TMS-EEG measurement procedure, the existing methods used for characterising TMS-EEG responses and the challenges associated with identifying the responses. We also summarise the findings thus far on how TMS-EEG responses change across the lifespan and the TMS-EEG features that separate typical and atypical brain maturation and ageing. Finally, we give an overview of the gaps in current knowledge to provide directions for future studies.