Local and Distributed fMRI Changes Induced by 40 Hz Gamma tACS of the Bilateral Dorsolateral Prefrontal Cortex: A Pilot Study

Cognitive and Neuropsychiatric Effects of 40 Hz tACS Simultaneously with Cognitive Exercises for Dementia: A Randomized, Crossover, Double-Blind, Sham-Controlled Study

Background and Objectives: Transcranial alternating current stimulation (tACS) at 40 Hz has shown potential to enhance cognitive function. However, research on its combination with cognitive exercises, particularly its long-term effects in a dementia population, remains limited. This study investigated the effects of 40 Hz tACS paired with simultaneous cognitive exercises on cognition, neuropsychiatric symptoms, and the depression status of individuals with dementia in a sham-controlled, double-blind crossover design. Materials and Methods: A total of 42 participants with dementia were randomized into two groups: (1) the R1S2 group received 40 Hz real tACS with cognitive exercises, followed by a ≥8-week washout period, and then sham tACS with cognitive exercises; (2) the S1R2 group received the reversed sequence. tACS was applied at 1.5 mA peak-to-peak with electrodes over the left dorsolateral prefrontal cortex and contralateral supraorbital area. Participants received two 30 min stimulation sessions per day, 5 days per week, for 4 consecutive weeks, paired with cognitive exercises using the MindTriggers app (2.9.1). The primary outcome was the Alzheimer's Disease Assessment Scale-Cognitive Subscale (ADAS-Cog) and the secondary outcomes included the Montgomery-Åsberg Depression Rating Scale (MADRS) and the Neuropsychiatric Inventory Questionnaire (NPI-Q). All outcome measures were assessed before and after each treatment block. Results: Real tACS paired with cognitive exercises significantly improved ADAS-Cog scores post-treatment compared to pre-treatment (p-value = 0.019), whereas sham tACS did not. Furthermore, real tACS produced significant long-term improvements approximately 2-3 months post-treatment in ADAS-Cog scores compared to sham (p-value = 0.048). Both real (p-value = 0.003) and sham (p-value = 0.015) tACS significantly reduced NPI-Q scores post-treatment. MADRS scores significantly improved (p-value = 0.007) post-treatment for real tACS but not sham. Conclusions: The 40 Hz tACS paired with cognitive exercises improves cognition, neuropsychiatric symptoms, and depression post-treatment in dementia, with sustained cognitive effects. The findings highlight its potential as a non-invasive therapeutic intervention for dementia.

Modulating positive self-referential processing by 40 Hz tACS in individuals with subthreshold depression: A double-blind, sham-controlled study

BACKGROUND

A positive self-evaluation is essential to mental well-being. Despite of its importance, little is known how to modulate or enhance positive self-evaluation. Gamma-frequency (40 Hz) transcranial alternating current stimulation (tACS) has been shown to promote emotion regulation and memory, which may foster positive self-evaluations. Here, we investigated whether 40 Hz tACS over the medial prefrontal cortex (mPFC), a key brain region implicating self-referential processing, could enhance positive self-evaluation among individuals exhibiting subthreshold depression. We hypothesized that the 40 Hz stimulation would enhance self-evaluation.

METHODS

Participants with subthreshold depression were screened using the Center for Epidemiological Studies Depression Scale Revised-10 item (CES-D-10). In a double-blind, randomized, sham-controlled, between-subjects experiment, sixty participants were randomly assigned to two groups: in the active stimulation group, 31 participants received 20-min session of 40 Hz tACS over the mPFC via high-density tACS. In the sham group, 29 participants received the sham stimulation over the same region. Before and after the tACS, participants completed the Self-Referential Encoding Task (SRET), which they endorsed and recalled positive and negative personality traits.

RESULTS

We found a significant interaction among stimulation group, depressive symptoms, trait valence (positive or negative). Among participants who received the 40 Hz stimulation, higher levels of baseline depressive symptoms were associated with increased endorsement/recall of positive personality traits compared to the sham group (p < 0.05).

CONCLUSION

40 Hz gamma tACS over the mPFC enhanced positive self-referential processing among individuals with subthreshold depression, an effect particularly evident among those with higher depressive symptoms. This effect highlights the potential therapeutic benefits of gamma-frequency stimulation in promoting positive self-evaluation among individuals with depression.

Mystery of gamma wave stimulation in brain disorders

Neuronal oscillations refer to rhythmic and periodic fluctuations of electrical activity in the central nervous system that arise from the cellular properties of diverse neuronal populations and their interactions. Specifically, gamma oscillations play a crucial role in governing the connectivity between distinct brain regions, which are essential in perception, motor control, memory, and emotions. In this context, we recapitulate various current stimulation methods to induce gamma entrainment. These methods include sensory stimulation, optogenetic modulation, photobiomodulation, and transcranial electrical or magnetic stimulation. Simultaneously, we explore the association between abnormal gamma oscillations and central nervous system disorders such as Alzheimer's disease, Parkinson's disease, stroke, schizophrenia, and autism spectrum disorders. Evidence suggests that gamma entrainment-inducing stimulation methods offer notable neuroprotection, although somewhat controversial. This review comprehensively discusses the functional role of gamma oscillations in higher-order brain activities from both physiological and pathological perspectives, emphasizing gamma entrainment as a potential therapeutic approach for neuropsychiatric disorders. Additionally, we discuss future opportunities and challenges in implementing such strategies.

Therapeutic potential of gamma entrainment using sensory stimulation for cognitive symptoms associated with schizophrenia

Schizophrenia is a complex neuropsychiatric disorder with significant morbidity. Treatment options that address the spectrum of symptoms are limited, highlighting the need for innovative therapeutic approaches. Gamma Entrainment Using Sensory Stimulation (GENUS) is an emerging treatment for neuropsychiatric disorders that uses sensory stimulation to entrain impaired oscillatory network activity and restore brain function. Aberrant oscillatory activity often underlies the symptoms experienced by patients with schizophrenia. We propose that GENUS has therapeutic potential for schizophrenia. This paper reviews the current status of schizophrenia treatment and explores the use of sensory stimulation as an adjunctive treatment, specifically through gamma entrainment. Impaired gamma frequency entrainment is observed in patients, particularly in response to auditory and visual stimuli. Thus, sensory stimulation, such as music listening, may have therapeutic potential for individuals with schizophrenia. GENUS holds novel therapeutic potential to improve the lives of individuals with schizophrenia, but further research is required to determine the efficacy of GENUS, optimize its delivery and therapeutic window, and develop strategies for its implementation in specific patient populations.

Transcranial alternating current stimulation improves quality of life in Parkinson's disease: study protocol for a randomized, double-blind, controlled trial

BACKGROUND

The neural cells in the brains of patients with Parkinson's disease (PWP) display aberrant synchronized oscillatory activity within the beta frequency range. Additionally, enhanced gamma oscillations may serve as a compensatory mechanism for motor inhibition mediated by beta activity and also reinstate plasticity in the primary motor cortex affected by Parkinson's disease. Transcranial alternating current stimulation (tACS) can synchronize endogenous oscillations with exogenous rhythms, thereby modulating cortical activity. The objective of this study is to investigate whether the addition of tACS to multidisciplinary intensive rehabilitation treatment (MIRT) can improve symptoms of PWP so as to enhance the quality of life in individuals with Parkinson's disease based on the central-peripheral-central theory.

METHODS

The present study was a randomized, double-blind trial that enrolled 60 individuals with Parkinson's disease aged between 45 and 70 years, who had Hoehn-Yahr scale scores ranging from 1 to 3. Participants were randomly assigned in a 1:1 ratio to either the tACS + MIRT group or the sham-tACS + MIRT group. The trial consisted of a two-week double-blind treatment period followed by a 24-week follow-up period, resulting in a total duration of twenty-six weeks. The primary outcome measured the change in PDQ-39 scores from baseline (T0) to 4 weeks (T2), 12 weeks (T3), and 24 weeks (T4) after completion of the intervention. The secondary outcome assessed changes in MDS-UPDRS III scores at T0, the end of intervention (T1), T2, T3, and T4. Additional clinical assessments and mechanistic studies were conducted as tertiary outcomes.

DISCUSSION

The objective of this study is to demonstrate that tACS can enhance overall functionality and improve quality of life in PWP, based on the framework of MIRT. Additionally, it seeks to establish a potential correlation between these therapeutic effects and neuroplasticity alterations in relevant brain regions. The efficacy of tACS will be assessed during the follow-up period in order to optimize neuroplasticity and enhance its potential impact on rehabilitation efficiency for PWP.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR2300071969. Registered on 30 May 2023.

A scientometric review of the growing trends in transcranial alternating current stimulation (tACS)

Objective

The aim of the current study was to provide a comprehensive picture of tACS-related research in the last decade through a bibliometric approach in order to systematically analyze the current status and cutting-edge trends in this field.

Methods

Articles and review articles related to tACS from 2013 to 2022 were searched on the Web of Science platform. A bibliometric analysis of authors, journals, countries, institutions, references, and keywords was performed using CiteSpace (6.2.R2), VOSviewer (1.6.19), Scimago Graphica (1.0.30), and Bibliometrix (4.2.2).

Results

A total of 602 papers were included. There was an overall increase in annual relevant publications in the last decade. The most contributing author was Christoph S. Herrmann. Brain Stimulation was the most prolific journal. The most prolific countries and institutions were Germany and Harvard University, respectively.

Conclusion

The findings reveal the development prospects and future directions of tACS and provide valuable references for researchers in the field. In recent years, the keywords "gamma," "transcranial direct current simulation," and "Alzheimer's disease" that have erupted, as well as many references cited in the outbreak, have provided certain clues for the mining of research prefaces. This will act as a guide for future researchers in determining the path of tACS research.

Modulation of resting-state networks following repetitive transcranial alternating current stimulation of the dorsolateral prefrontal cortex

Transcranial alternating current stimulation (tACS) offers a unique method to temporarily manipulate the activity of the stimulated brain region in a frequency-dependent manner. However, it is not clear if repetitive modulation of ongoing oscillatory activity with tACS over multiple days can induce changes in grey matter resting-state functional connectivity and white matter structural integrity. The current study addresses this question by applying multiple-session theta band stimulation on the left dorsolateral prefrontal cortex (L-DLPFC) during arithmetic training. Fifty healthy participants (25 males and 25 females) were randomly assigned to the experimental and sham groups, half of the participants received individually adjusted theta band tACS, and half received sham stimulation. Resting-state functional magnetic resonance (rs-fMRI) and diffusion-weighted imaging (DWI) data were collected before and after 3 days of tACS-supported procedural learning training. Resting-state network analysis showed a significant increase in connectivity for the frontoparietal network (FPN) with the precuneus cortex. Seed-based analysis with a seed defined at the primary stimulation site showed an increase in connectivity with the precuneus cortex, posterior cingulate cortex (PCC), and lateral occipital cortex. There were no effects on the structural integrity of white matter tracts as measured by fractional anisotropy, and on behavioral measures. In conclusion, the study suggests that multi-session task-associated tACS can produce significant changes in resting-state functional connectivity; however, changes in functional connectivity do not necessarily translate to changes in white matter structure or behavioral performance.