Neurophysiological mechanisms of transcranial alternating current stimulation

Dopamine-integrated all-hydrogel multi-electrode arrays for neural activity recording

Investigation of brain neural circuits is essential for deciphering the diagnostics and therapeutics of neurodegenerative diseases. The main concerns with traditional rigid metal electrodes include intrinsic mechanical mismatch between sensing electrodes and tissues, unavoidable foreign body responses, and inadequate spatiotemporal resolution, resulting in a deficiency of sensing performance. All-hydrogel neural electrodes with multi-electrode arrays (MEAs) suggest a viable way to modulate the trade-off between tissue-mechanical compliance and excellent spatiotemporal recording capacity, but still face the issues of insufficient conductivity and unstable interlayer bonding. Herein, we constructed a four-layer all-hydrogel neural electrode, by sandwiching a conductive hydrogel layer within two encapsulation hydrogel layers, with a shielding hydrogel layer located on top. We introduce a dual-strategy treatment to induce controllable phase separation in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hydrogel, which achieved ultra-high conductivity (up to 4176 S cm-1) comparable to that of metals and precise spatial resolution (∼15 μm) suitable for single neuron recording. In addition, the utilization of polyphenol chemistry mediated adaptive adhesion endowed this neural electrode with flexible and stable interlayer bonding among conductive-encapsulation-shielding layers and the tissue-electrode interface. Consequently, the all-hydrogel neural electrode exhibited a tenfold higher signal-to-noise ratio than a commercial silver electrode, realized the recording of weak neural activity signals within single and multiple neurons in epileptic rats, and applied man-made stimulation to the cerebral cortex of rats during seizures. This work provides a useful tool to understand the development, function and treatment of neurodegenerative diseases.

Parvalbumin interneuron cell-to-network plasticity: mechanisms and therapeutic avenues

Alzheimer's disease (AD) and schizophrenia (SCZ) represent two major neuropathological conditions with a high disease burden. Despite their distinct etiologies, patients suffering from AD or SCZ share a common burden of disrupted memory functions unattended by current therapies. Recent preclinical analyses highlight cell-type-specific contributions of parvalbumin interneurons (PVIs), particularly the plasticity of their cellular excitability, towards intact neuronal network function (cell-to-network plasticity) and memory performance. Here we argue that deficits of PVI cell-to-network plasticity may underlie memory deficits in AD and SCZ, and we explore two therapeutic avenues: the targeting of PVI-specific neuromodulation, including by neuropeptides, and the recruitment of network synchrony in the gamma frequency range (40 Hz) by external stimulation. We finally propose that these approaches be merged under consideration of recent insights into human brain physiology.

Impact of theta transcranial alternating current stimulation on language production in adult classic galactosemia patients

Patients with classic galactosemia (CG), an inborn error of galactose metabolism, suffer from impairments in cognition, including language processing. Potential causes are atypical brain oscillations. Recent electroencephalogram (EEG) showed differences in the P300 event-related-potential (ERP) and alterations in the alpha/theta-range during speech planning. This study investigated whether transcranial alternating current stimulation (tACS) at theta-frequency compared to sham can cause a normalization of the ERP post stimulation and improves language performance. Eleven CG patients and fourteen healthy controls participated in two tACS-sessions (theta 6.5 Hz/sham). They were engaged in an active language task, describing animated scenes at three moments, that is, pre/during/post stimulation. Pre and post stimulation, behavior (naming accuracy, voice-onset-times; VOT) and mean-amplitudes of ERP were compared, by means of a P300 time-window analysis and cluster-based-permutation testing during speech planning. The results showed that theta stimulation, not sham, significantly reduced naming error-percentage in patients, not in controls. Theta did not systematically speed up naming beyond a general learning effect, which was larger for the patients. The EEG analysis revealed a significant pre-post stimulation effect (P300/late positivity), in patients and during theta stimulation only. In conclusion, theta-tACS improved accuracy in language performance in CG patients compared to controls and altered the P300 and late positive ERP-amplitude, suggesting a lasting effect on neural oscillation and behavior.

Transcranial alternating current stimulation (tACS) at gamma frequency: an up-and-coming tool to modify the progression of Alzheimer's Disease

The last decades have witnessed huge efforts devoted to deciphering the pathological mechanisms underlying Alzheimer's Disease (AD) and to testing new drugs, with the recent FDA approval of two anti-amyloid monoclonal antibodies for AD treatment. Beyond these drug-based experimentations, a number of pre-clinical and clinical trials are exploring the benefits of alternative treatments, such as non-invasive stimulation techniques on AD neuropathology and symptoms. Among the different non-invasive brain stimulation approaches, transcranial alternating current stimulation (tACS) is gaining particular attention due to its ability to externally control gamma oscillations. Here, we outline the current knowledge concerning the clinical efficacy, safety, ease-of-use and cost-effectiveness of tACS on early and advanced AD, applied specifically at 40 Hz frequency, and also summarise pre-clinical results on validated models of AD and ongoing patient-centred trials.

Exploring the Prospects of Transcranial Electrical Stimulation (tES) as a Therapeutic Intervention for Post-Stroke Motor Recovery: A Narrative Review

INTRODUCTION

Stroke survivors often have motor impairments and related functional deficits. Transcranial Electrical Stimulation (tES) is a rapidly evolving field that offers a wide range of capabilities for modulating brain function, and it is safe and inexpensive. It has the potential for widespread use for post-stroke motor recovery. Transcranial Direct Current Stimulation (tDCS), Transcranial Alternating Current Stimulation (tACS), and Transcranial Random Noise Stimulation (tRNS) are three recognized tES techniques that have gained substantial attention in recent years but have different mechanisms of action. tDCS has been widely used in stroke motor rehabilitation, while applications of tACS and tRNS are very limited. The tDCS protocols could vary significantly, and outcomes are heterogeneous.

PURPOSE

the current review attempted to explore the mechanisms underlying commonly employed tES techniques and evaluate their prospective advantages and challenges for their applications in motor recovery after stroke.

CONCLUSION

tDCS could depolarize and hyperpolarize the potentials of cortical motor neurons, while tACS and tRNS could target specific brain rhythms and entrain neural networks. Despite the extensive use of tDCS, the complexity of neural networks calls for more sophisticated modifications like tACS and tRNS.

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.

Would frontal midline theta indicate cognitive changes induced by non-invasive brain stimulation? A mini review

To the best of our knowledge, neurophysiological markers indicating changes induced by non-invasive brain stimulation (NIBS) on cognitive performance, especially one of the most investigated under these procedures, working memory (WM), are little known. Here, we will briefly introduce frontal midline theta (FM-theta) oscillation (4-8 Hz) as a possible indicator for NIBS effects on WM processing. Electrophysiological recordings of FM-theta oscillation seem to originate in the medial frontal cortex and the anterior cingulate cortex, but they may be driven more subcortically. FM-theta has been acknowledged to occur during memory and emotion processing, and it has been related to WM and sustained attention. It mainly occurs in the frontal region during a delay period, in which specific information previously shown is no longer perceived and must be manipulated to allow a later (delayed) response and observed in posterior regions during information maintenance. Most NIBS studies investigating effects on cognitive performance have used n-back tasks that mix manipulation and maintenance processes. Thus, if considering FM-theta as a potential neurophysiological indicator for NIBS effects on different WM components, adequate cognitive tasks should be considered to better address the complexity of WM processing. Future research should also evaluate the potential use of FM-theta as an index of the therapeutic effects of NIBS intervention on neuropsychiatric disorders, especially those involving the ventral medial prefrontal cortex and cognitive dysfunctions.