Bilateral 5 Hz transcranial alternating current stimulation on fronto-temporal areas modulates resting-state EEG

Rhythmic non-invasive brain stimulations are promising tools to modulate brain activity by entraining neural oscillations in specific cortical networks. The aim of the study was to assess the possibility to influence the neural circuits of the wake-sleep transition in awake subjects via a bilateral transcranial alternating current stimulation at 5 Hz (theta-tACS) on fronto-temporal areas. 25 healthy volunteers participated in two within-subject sessions (theta-tACS and sham), one week apart and in counterbalanced order. We assessed the stimulation effects on cortical EEG activity (28 derivations) and self-reported sleepiness (Karolinska Sleepiness Scale). theta-tACS induced significant increases of the theta activity in temporo-parieto-occipital areas and centro-frontal increases in the alpha activity compared to sham but failed to induce any online effect on sleepiness. Since the total energy delivered in the sham condition was much less than in the active theta-tACS, the current data are unable to isolate the specific effect of entrained theta oscillatory activity per se on sleepiness scores. On this basis, we concluded that theta-tACS modulated theta and alpha EEG activity with a topography consistent with high sleep pressure conditions. However, no causal relation can be traced on the basis of the current results between these rhythms and changes on sleepines

Neurophysiological mechanisms of transcranial alternating current stimulation

Neuronal oscillations are the primary basis for precise temporal coordination of neuronal processing and are linked to different brain functions. Transcranial alternating current stimulation (tACS) has demonstrated promising potential in improving cognition by entraining neural oscillations. Despite positive findings in recent decades, the results obtained are sometimes rife with variance and replicability problems, and the findings translation to humans is quite challenging. A thorough understanding of the mechanisms underlying tACS is necessitated for accurate interpretation of experimental results. Animal models are useful for understanding tACS mechanisms, optimizing parameter administration, and improving rational design for broad horizons of tACS. Here, we review recent electrophysiological advances in tACS from animal models, as well as discuss some critical issues for results coordination and translation. We hope to provide an overview of neurophysiological mechanisms and recommendations for future consideration to improve its validity, specificity, and reproducibility

Can Transcranial Electrical Stimulation Localize Brain Function?

Transcranial electrical stimulation (TES) uses constant (TDCS) or alternating currents (TACS) to modulate brain activity. Most TES studies apply low-intensity currents through scalp electrodes (≤2 mA) using bipolar electrode arrangements, producing weak electrical fields in the brain (<1 V/m). Low-intensity TES has been employed in humans to induce changes in task performance during or after stimulation. In analogy to focal transcranial magnetic stimulation, TES-induced behavioral effects have often been taken as evidence for a causal involvement of the brain region underlying one of the two stimulation electrodes, often referred to as the active electrode. Here, we critically review the utility of bipolar low-intensity TES to localize human brain function. We summarize physiological substrates that constitute peripheral targets for TES and may mediate subliminal or overtly perceived peripheral stimulation during TES. We argue that peripheral co-stimulation may contribute to the behavioral effects of TES and should be controlled for by “sham” TES. We discuss biophysical properties of TES, which need to be considered, if one wishes to make realistic assumptions about which brain regions were preferentially targeted by TES. Using results from electric field calculations, we evaluate the validity of different strategies that have been used for selective spatial targeting. Finally, we comment on the challenge of adjusting the dose of TES considering dose–response relationships between the weak tissue currents and the physiological effects in targeted cortical areas. These considerations call for caution when attributing behavioral effects during or after low-intensity TES studies to a specific brain region and may facilitate the selection of best practices for future TES studies

Das Potenzial der frequenzspezifischen Neuromodulation für die Untersuchung und Verbesserung der Sprachverarbeitung im Gehirn

Das akustische Sprachsignal lässt sich nicht eindeutig in einzelne Wörter unterteilen und ähnelt eher einem kontinuierlichen Lautstrom. Dennoch gibt es zeitliche Regelmäßigkeiten, die möglicherweise bei der Wahrnehmung genutzt werden. Die Dauer von Sprachelementen wie Phonemen, Silben und prosodischen Phrasen spiegelt sich in der Hirnaktivität bei der Sprachwahrnehmung wider. Trotzdem ist nicht klar, ob tatsächlich ein kausaler Zusammenhang zwischen der neuronalen Antwort und der Qualität der Sprachwahrnehmung besteht. Der vorliegende Übersichtsartikel präsentiert einleitend die relevanten Bestandteile des akustischen Sprachsignals, beschreibt die neuronale Antwort auf Sprachreize im Gehirn und zeigt auf, welche neurophysiologischen Veränderungen mit Sprachfunktionsstörungen einhergehen. Es werden verschiedene Verfahren der nichtinvasiven Neuromodulation vorgestellt und erste Ergebnisse präsentiert, welche darauf hindeuten, dass sich durch gezielte Neuromodulation die Sprachfunktionen verbessern lassen können

Online closed-loop tACS-fMRI for brain modulation

Recent studies suggest that transcranial electrical stimulation (tES) can be performed during functional magnetic resonance imaging (fMRI). The novel approach of using concurrent tES-fMRI to modulate and measure targeted brain activity/connectivity may provide unique insights into the causal interactions between the brain's neural responses and psychiatric/neurologic signs and symptoms, and importantly, guide the development of new treatments. However, tES stimulation parameters to optimally influence the underlying brain activity may vary with respect to the phase difference, frequency, intensity, and electrode montage among individuals. The dissertation proposes a protocol for closed-loop tES-fMRI to optimize the frequency and phase difference of alternating current stimulation (tACS) for two nodes (frontal and parietal regions or called as frontoparietal regions) in individual participants. It is carefully considered the challenges in an online optimization of tES parameters with concurrent fMRI, specifically in its safety, artifact in fMRI image quality, online evaluation of the tES effect, and parameter optimization method, and the protocol is designed to run an effective study to enhance frontoparietal connectivity and working memory performance with the optimized tACS using closed-loop tES-fMRI. The dissertation provides technical details of the protocol, including electrode types, electrolytes, electrode montages, concurrent tES-fMRI hardware, online fMRI processing pipelines, and the optimization algorithm. Result analyses confirmed the implementation of this protocol worked successfully to improve frontoparietal connectivity compared to the control group. However, it did not give a significant difference in working memory improvement compared to the control group. Therefore, through literature study, in the future work Chapter, a better protocol is proposed to enhance the stimulation effect by including electrode montage and electric current optimization

Non-invasive brain stimulation for Parkinson's disease: Clinical evidence, latest concepts and future goals: A systematic review.

Parkinson's disease (PD) is becoming a major public-health issue in an aging population. Available approaches to treat advanced PD still have limitations; new therapies are needed. The non-invasive brain stimulation (NIBS) may offer a complementary approach to treat advanced PD by personalized stimulation. Although NIBS is not as effective as the gold-standard levodopa, recent randomized controlled trials show promising outcomes in the treatment of PD symptoms. Nevertheless, only a few NIBS-stimulation paradigms have shown to improve PD's symptoms. Current clinical recommendations based on the level of evidence are reported in Table 1 through Table 3. Furthermore, novel technological advances hold promise and may soon enable the non-invasive stimulation of deeper brain structures for longer periods

Investigating the cortico-hippocampal dynamics involved in human episodic memory with neural stimulation

The human episodic memory system depends on specific interactions between the hippocampus and neocortex. The three studies performed as part of this doctoral thesis each sought to improve our understanding of the cortico-hippocampal system in the context of episodic memory. Each study used a different approach to directly manipulate neural activity with the aim of revealing causal relationships between certain patterns of neural activity and behaviour. In the first study the cortico-hippocampal network was investigated by using occipital transcranial alternating current stimulation (tACS) and auditory sensory stimulation with the aim of altering memory performance during an audio-visual association task. The electrical stimulation was hypothesized to interact with the auditory sensory stimulation after propagating from the neocortex to the hippocampus. This study was unsuccessful in modulating behaviour through stimulation. In the second study, the left Dorsolateral Prefrontal Cortex (DLPFC) was targeted using 1 Hz repetitive transcranial magnetic stimulation (rTMS) over the course of two experiments, during a set of list learning tasks. This study found a beneficial effect on memory performance when stimulation occurred over the left DLPFC compared to stimulation over the vertex (control site). This behavioural effect was further characterized by a beta-power decrease over parietal sensors as measured by electroencephalography (EEG). The third study probed the cortico-hippocampal network by directly stimulating the hippocampus and the neocortex, by applying direct electrical stimulation through implanted electrodes in human subjects. This study used measures of population activity as well as single neuron activity to monitor how the brain responds to direct stimulation. This study found that direct stimulation throughout the network produces a neural response that is characterized by short, intense excitation and prolonged follow-up inhibition which has the potential to travel throughout the brain. The ability of the response to travel between the neocortex and hippocampus was leveraged to measure a transduction delay of ~140 ms between the two regions. Together these findings have advanced our understanding on how different stimulation methods can be used to manipulate neural activity and consequently affect the episodic memory system. Through these methods we might one day be able to aid persons suffering from cognitive impairments or related pathologies