On the difficulties of separating retinal from cortical origins of phosphenes when using transcranial alternating current stimulation (tACS)

A Phosphenotron Device for Sensoric Spatial Resolution of Phosphenes within the Visual Field Using Non-Invasive Transcranial Alternating Current Stimulation

This study presents phosphenotron, a device for enhancing the sensory spatial resolution of phosphenes in the visual field (VF). The phosphenotron employs a non-invasive transcranial alternating current stimulation (NITACS) to modulate brain activity by applying weak electrical currents to the scalp or face. NITACS's unique application induces phosphenes, a phenomenon where light is perceived without external stimuli. Unlike previous invasive methods, NITACS offers a non-invasive approach to create these effects. The study focused on assessing the spatial resolution of NITACS-induced phosphenes, crucial for advancements in visual aid technology and neuroscience. Eight participants were subjected to NITACS using a novel electrode arrangement around the eye orbits. Results showed that NITACS could generate spatially defined phosphene patterns in the VF, varying among individuals but consistently appearing within their VF and remaining stable through multiple stimulations. The study established optimal parameters for vibrant phosphene induction without discomfort and identified electrode positions that altered phosphene locations within different VF regions. Receiver Operating characteristics analysis indicated a specificity of 70.7%, sensitivity of 73.9%, and a control trial accuracy of 98.4%. These findings suggest that NITACS is a promising, reliable method for non-invasive visual perception modulation through phosphene generation.

Non-invasive transcranial alternating current stimulation of spatially resolved phosphenes

This study focused on the use of Non-Invasive Transcranial Alternating Current Stimulation (NITACS) to induce and map phosphenes (spark-like percepts in the visual field) in healthy individuals. The study found optimal stimulation parameters to induce reliable phosphenes without skin irritation or pain. The results suggest NITACS can be used as a tool to investigate the relationship between facial stimulation location and phosphene localization within the field of vision (FOV) and raise questions about the origin of phosphenes generated through NITACS. The outcomes of this study could serve as a source of inspiration for creating non-invasive visual aids in the future.

Contrast detection is enhanced by deterministic, high-frequency transcranial alternating current stimulation with triangle and sine waveform

Stochastic resonance (SR) describes a phenomenon where an additive noise (stochastic carrier-wave) enhances the signal transmission in a nonlinear system. In the nervous system, nonlinear properties are present from the level of single ion channels all the way to perception and appear to support the emergence of SR. For example, SR has been repeatedly demonstrated for visual detection tasks, also by adding noise directly to cortical areas via transcranial random noise stimulation (tRNS). When dealing with nonlinear physical systems, it has been suggested that resonance can be induced not only by adding stochastic signals (i.e., noise) but also by adding a large class of signals that are not stochastic in nature that cause "deterministic amplitude resonance" (DAR). Here, we mathematically show that high-frequency, deterministic, periodic signals can yield resonance-like effects with linear transfer and infinite signal-to-noise ratio at the output. We tested this prediction empirically and investigated whether nonrandom, high-frequency, transcranial alternating current stimulation (tACS) applied to the visual cortex could induce resonance-like effects and enhance the performance of a visual detection task. We demonstrated in 28 participants that applying 80-Hz triangular-waves or sine-waves with tACS reduced the visual contrast detection threshold for optimal brain stimulation intensities. The influence of tACS on contrast sensitivity was equally effective to tRNS-induced modulation, demonstrating that both tACS and tRNS can reduce contrast detection thresholds. Our findings suggest that a resonance-like mechanism can also emerge when deterministic electrical waveforms are applied via tACS.NEW & NOTEWORTHY Our findings extend our understanding of neuromodulation induced by noninvasive electrical stimulation. We provide the first evidence showing acute online benefits of transcranial alternating current stimulation (tACS)triangle and tACSsine targeting the primary visual cortex (V1) on visual contrast detection in accordance with the resonance-like phenomenon. The "deterministic" tACS and "stochastic" high-frequency-transcranial random noise stimulation (tRNS) are equally effective in enhancing visual contrast detection.

Offline 20 Hz transcranial alternating current stimulation over the right inferior frontal gyrus increases theta activity during a motor response inhibition task

OBJECTIVES

Previous studies have shown that the right inferior frontal gyrus (rIFG) and the pre-supplementary motor area (preSMA) play an important role in motor inhibitory control. The aim of the study was to use theta frequency transcranial alternating current stimulation (tACS) to modulate brain activity in the rIFG and preSMA and to test the effects of stimulation using a motor response inhibition task.

METHODS

In four sessions, 20 healthy participants received tACS at 6 Hz over preSMA or rIFG, or 20 Hz over rIFG (to test frequency specificity), or sham stimulation before task processing. After each type of stimulation, the participants performed the Go/NoGo task with simultaneous electroencephalogram (EEG) recording.

RESULTS

By stimulating rIFG and preSMA with 6 Hz tACS, we were not able to modulate either behavioral performance nor the EEG correlate. Interestingly, 20 Hz tACS over the rIFG significantly increased theta activity, however without behavioral effects. This increased theta activity did not coincide with the stimulation area and was localized in the fronto-central and centro-parietal areas.

CONCLUSIONS

The inclusion of a control frequency is crucial to test for frequency specificity. Our findings are in accordance with previous studies showing that after effects of tACS are not restricted to the stimulation frequency but can also occur in other frequency bands.

Amplitude modulating frequency overrides carrier frequency in tACS-induced phosphene percept

The amplitude modulated (AM) neural oscillation is an essential feature of neural dynamics to coordinate distant brain areas. The AM transcranial alternating current stimulation (tACS) has recently been adopted to examine various cognitive functions, but its neural mechanism remains unclear. The current study utilized the phosphene phenomenon to investigate whether, in an AM-tACS, the AM frequency could modulate or even override the carrier frequency in phosphene percept. We measured the phosphene threshold and the perceived flash rate/pattern from 12 human subjects (four females, aged from 20-44 years old) under tACS that paired carrier waves (10, 14, 18, 22 Hz) with different envelope conditions (0, 2, 4 Hz) over the mid-occipital and left facial areas. We also examined the phosphene source by adopting a high-density stimulation montage. Our results revealed that (1) phosphene threshold was higher for AM-tACS than sinusoidal tACS and demonstrated different carrier frequency functions in two stimulation montages. (2) AM-tACS slowed down the phosphene flashing and abolished the relation between the carrier frequency and flash percept in sinusoidal tACS. This effect was independent of the intensity change of the stimulation. (3) Left facial stimulation elicited phosphene in the upper-left visual field, while occipital stimulation elicited equally distributed phosphene. (4) The near-eye electrodermal activity (EDA) measured under the threshold-level occipital tACS was greater than the lowest power sufficient to elicit retinal phosphene. Our results show that AM frequency may override the carrier frequency and determine the perceived flashing frequency of AM-tACS-induced phosphene.

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

Over the past few years, the possibility of modulating fast brain oscillatory activity in the gamma (γ) band through transcranial alternating current stimulation (tACS) has been discussed in the context of both cognitive enhancement and therapeutic scenarios. However, the effects of tACS targeting regions outside the motor cortex, as well as its spatial specificity, are still unclear. Here, we present a concurrent tACS-fMRI block design study to characterize the impact of 40 Hz tACS applied over the left and right dorsolateral prefrontal cortex (DLPFC) in healthy subjects. Results suggest an increase in blood oxygenation level-dependent (BOLD) activity in the targeted bilateral DLPFCs, as well as in surrounding brain areas affected by stimulation according to biophysical modeling, i.e., the premotor cortex and anterior cingulate cortex (ACC). However, off-target effects were also observed, primarily involving the visual cortices, with further effects on the supplementary motor areas (SMA), left subgenual cingulate, and right superior temporal gyrus. The specificity of 40 Hz tACS over bilateral DLPFC and the possibility for network-level effects should be considered in future studies, especially in the context of recently promoted gamma-induction therapeutic protocols for neurodegenerative disorders.

Electrical neurostimulation in glaucoma with progressive vision loss

Background

The retrospective study provides real-world evidence for long-term clinical efficacy of electrical optic nerve stimulation (ONS) in glaucoma with progressive vision loss.

Methods

Seventy glaucoma patients (45 to 86 y) with progressive vision loss despite therapeutic reduction of intraocular pressure (IOP) underwent electrical ONS. Closed eyes were separately stimulated by bipolar rectangular pulses with stimulus intensities up to 1.2 mA sufficient to provoke phosphenes. Ten daily stimulation sessions within 2 weeks lasted about 80 min each. Right before ONS at baseline (PRE), vision loss was documented by static threshold perimetry and compared to the same assessment approximately 1 year afterwards (POST). Mean defect (MD) was defined as primary outcome parameter. Perimetries with a reliability factor (RF) of max. 20% were considered.

Results

Perimetry follow-up of 101 eyes in 70 patients fulfilled the criterion of a max. 20% RF. Follow-up was performed on average 362.2 days after ONS. MD significantly decreased from PRE 14.0 dB (median) to POST 13.4 dB (p < 0.01). 64 eyes in 49 patients showed constant or reduced MD as compared to baseline (PRE 13.4 dB vs. POST 11.2 dB). In 37 eyes of 30 patients, MD increased from PRE 14.9 dB to POST 15.6 dB.

Conclusions

Innovative treatments that preserve visual function through mechanisms other than lowering IOP are required for glaucoma with progressive vision loss. The present long-term data document progression halt in more than 63% of affected eyes after ONS and, thus, extend existing evidence from clinical trials.

Central nervous system physiology

This is the second chapter of the series on the use of clinical neurophysiology for the study of movement disorders. It focusses on methods that can be used to probe neural circuits in brain and spinal cord. These include use of spinal and supraspinal reflexes to probe the integrity of transmission in specific pathways; transcranial methods of brain stimulation such as transcranial magnetic stimulation and transcranial direct current stimulation, which activate or modulate (respectively) the activity of populations of central neurones; EEG methods, both in conjunction with brain stimulation or with behavioural measures that record the activity of populations of central neurones; and pure behavioural measures that allow us to build conceptual models of motor control. The methods are discussed mainly in relation to work on healthy individuals. Later chapters will focus specifically on changes caused by pathology.

Amplitude modulated transcranial alternating current stimulation (AM-TACS) efficacy evaluation via phosphene induction

Amplitude modulated transcranial alternating current stimulation (AM-tACS) is a novel method of electrostimulation which enables the recording of electrophysiological signals during stimulation, thanks to an easier removable stimulation artefact compared to classical electrostimulation methods. To gauge the neuromodulatory potential of AM-tACS, we tested its capacity to induce phosphenes as an indicator of stimulation efficacy. AM-tACS was applied via a two-electrode setup, attached on FpZ and below the right eye. AM-tACS waveforms comprised of different carrier (50 Hz, 200 Hz, 1000 Hz) and modulation frequencies (8 Hz, 16 Hz, 28 Hz) were administered with at maximum 2 mA peak-to-peak stimulation strength. TACS conditions in the same frequencies were used as a benchmark for phosphene induction. AM-tACS conditions using a 50 Hz carrier frequency were able to induce phosphenes, but with no difference in phosphene thresholds between modulation frequencies. AM-tACS using a 200 Hz or 1000 Hz carrier frequency did not induce phosphenes. TACS conditions induced phosphenes in line with previous studies. Stimulation effects of AM-tACS conditions were independent of amplitude modulation and instead relied solely on the carrier frequency. A possible explanation may be that AM-tACS needs higher stimulation intensities for its amplitude modulation to have a neuromodulatory effect.

Corticospinal excitability enhancement with simultaneous transcranial near-infrared stimulation and anodal direct current stimulation of motor cortex

OBJECTIVES

Non-invasive brain stimulation (NIBS) is beneficial to many neurological and psychiatric disorders by modulating neuroplasticity and cortical excitability. However, recent studies evidence that single type of NIBS such as transcranial direct current stimulation (tDCS) does not have meaningful clinical therapeutic responses due to their small effect size. Transcranial near-infrared stimulation (tNIRS) is a novel form of NIBS. Both tNIRS and tDCS implement its therapeutic effects by modulating cortical excitability but with different mechanisms. We hypothesized that simultaneous tNIRS and tDCS is superior to single stimulation, leading to a greater cortical excitability.

METHODS

Sixteen healthy subjects participated in a double-blind, sham-controlled, cross-over designed study. Motor evoked potentials (MEPs) were used to measure motor cortex excitability. The changes of MEP were calculated and compared in the sham condition, tDCS stimulation condition, tNIRS condition and the simultaneous tNIRS and anodal tDCS condition.

RESULTS

tDCS alone and tNIRS alone both elicited higher MEP after stimulation, while the MEP amplitude in the simultaneous tNIRS and tDCS condition was significantly higher than either tNIRS alone or tDCS alone. The enhancement lasted up to at least 30 minutes after stimulation, indicating simultaneous 820 nm tNIRS with 2 mA anodal tDCS have a synergistic effect on cortical plasticity.

CONCLUSIONS

Simultaneous application of tNIRS with tDCS produces a stronger cortical excitability effect.

SIGNIFICANCE

The simultaneous tNIRS and tDCS is a promising technology with exciting potential as a means of treatment, neuro-enhancement, or neuro-protection.

Retinal and Cortical Contributions to Phosphenes During Transcranial Electrical Current Stimulation

It is generally believed that the phosphenes induced by transcranial electric current stimulation (tECS) are a product of retinal activation, even when electrode placement is directly over the primary visual cortex. However, the origins of these tECS-induced phosphenes have not yet been conclusively determined. In this study, phosphene detection thresholds using an FPz-Oz montage were compared with those from (i) an Oz-Cz montage to determine whether prefrontal regions, such as the retina, contribute to phosphenes and (ii) an FPz-Cz montage to determine whether the visual cortex in the occipital lobe contributes to phosphenes. Twenty-two participants received transcranial current stimulation with each of these montages (as well as a T3-T4 montage included for exploratory purposes) at 6, 10, 16, 20, 24, 28, and 32 Hz. To estimate differences in current density at the retina and occipital lobe across montages, modeling of current density at phosphene thresholds was measured across 20 head models. Consistent with the proposal that tECS-induced phosphenes are generated in the retina, increasing current density near the retina (FPz-Oz relative to Oz-Cz montage) reduced phosphene thresholds. However, increasing current density near the occipital cortex (FPz-Oz relative to FPz-Cz montage) also reduced phosphene thresholds while also requiring less current density at the retina according to the modeling estimates. This suggests that tECS of this occipital cortex also contributed to phosphene perception. © 2020 Bioelectromagnetics Society.

Impact of β-range-induced oscillatory activity on human input-output relationship of the corticospinal pathway

Objective: The aim of the study was to show that short-lasting (90 s) transcranial alternating current stimulation (tACS) at 20 Hz delivered over the left primary motor cortex (M1) is able to change the shape of recruitment curve of the corticospinal pathway.Methods: The corticospinal pathway was studied during tACS by means of the relationship between the intensity of transcranial magnetic stimulation (TMS) delivered over the left M1 and corresponding motor evoked potentials (MEPs) recorded from the right first dorsal interosseus muscle (FDI), in nine healthy subjects. In order to extract characteristics of the input-output relationship that have particular physiological relevance, data were fitted to the Boltzmann sigmoidal function by the Levenberg-Marquardt nonlinear, least mean squares algorithm.Results: The β-rhythm tACS influenced the shape and parameters of the input-output relation, so that the initial segment of the conditioned curve (from threshold to 30% of maximum muscle size) diverged, while the subsequent segment converged to overlap the unconditioned control curve.Discussion: β-rhythm tACS conditions only a definite subset of corticospinal elements influencing less than 30% of the entire motoneuronal pool. The fact that β-rhythm tACS mainly affects the most excitable motoneurons could explain the observed antikinetic effect of the tACS at β-rhythm applied in the motor regions.

Electrical stimulation of cranial nerves in cognition and disease

The cranial nerves are the pathways through which environmental information (sensation) is directly communicated to the brain, leading to perception, and giving rise to higher cognition. Because cranial nerves determine and modulate brain function, invasive and non-invasive cranial nerve electrical stimulation methods have applications in the clinical, behavioral, and cognitive domains. Among other neuromodulation approaches such as peripheral, transcranial and deep brain stimulation, cranial nerve stimulation is unique in allowing axon pathway-specific engagement of brain circuits, including thalamo-cortical networks. In this review we amalgamate relevant knowledge of 1) cranial nerve anatomy and biophysics; 2) evidence of the modulatory effects of cranial nerves on cognition; 3) clinical and behavioral outcomes of cranial nerve stimulation; and 4) biomarkers of nerve target engagement including physiology, electroencephalography, neuroimaging, and behavioral metrics. Existing non-invasive stimulation methods cannot feasibly activate the axons of only individual cranial nerves. Even with invasive stimulation methods, selective targeting of one nerve fiber type requires nuance since each nerve is composed of functionally distinct axon-types that differentially branch and can anastomose onto other nerves. None-the-less, precisely controlling stimulation parameters can aid in affecting distinct sets of axons, thus supporting specific actions on cognition and behavior. To this end, a rubric for reproducible dose-response stimulation parameters is defined here. Given that afferent cranial nerve axons project directly to the brain, targeting structures (e.g. thalamus, cortex) that are critical nodes in higher order brain networks, potent effects on cognition are plausible. We propose an intervention design framework based on driving cranial nerve pathways in targeted brain circuits, which are in turn linked to specific higher cognitive processes. State-of-the-art current flow models that are used to explain and design cranial-nerve-activating stimulation technology require multi-scale detail that includes: gross anatomy; skull foramina and superficial tissue layers; and precise nerve morphology. Detailed simulations also predict that some non-invasive electrical or magnetic stimulation approaches that do not intend to modulate cranial nerves per se, such as transcranial direct current stimulation (tDCS) and transcranial magnetic stimulation (TMS), may also modulate activity of specific cranial nerves. Much prior cranial nerve stimulation work was conceptually limited to the production of sensory perception, with individual titration of intensity based on the level of perception and tolerability. However, disregarding sensory emulation allows consideration of temporal stimulation patterns (axon recruitment) that modulate the tone of cortical networks independent of sensory cortices, without necessarily titrating perception. For example, leveraging the role of the thalamus as a gatekeeper for information to the cerebral cortex, preventing or enhancing the passage of specific information depending on the behavioral state. We show that properly parameterized computational models at multiple scales are needed to rationally optimize neuromodulation that target sets of cranial nerves, determining which and how specific brain circuitries are modulated, which can in turn influence cognition in a designed manner.

Human Sensation of Transcranial Electric Stimulation

Noninvasive transcranial electric stimulation is increasingly being used as an advantageous therapy alternative that may activate deep tissues while avoiding drug side-effects. However, not only is there limited evidence for activation of deep tissues by transcranial electric stimulation, its evoked human sensation is understudied and often dismissed as a placebo or secondary effect. By systematically characterizing the human sensation evoked by transcranial alternating-current stimulation, we observed not only stimulus frequency and electrode position dependencies specific for auditory and visual sensation but also a broader presence of somatic sensation ranging from touch and vibration to pain and pressure. We found generally monotonic input-output functions at suprathreshold levels, and often multiple types of sensation occurring simultaneously in response to the same electric stimulation. We further used a recording circuit embedded in a cochlear implant to directly and objectively measure the amount of transcranial electric stimulation reaching the auditory nerve, a deep intercranial target located in the densest bone of the skull. We found an optimal configuration using an ear canal electrode and low-frequency (<300 Hz) sinusoids that delivered maximally ~1% of the transcranial current to the auditory nerve, which was sufficient to produce sound sensation even in deafened ears. Our results suggest that frequency resonance due to neuronal intrinsic electric properties need to be explored for targeted deep brain stimulation and novel brain-computer interfaces.

State-Dependent Effects of Transcranial Oscillatory Currents on the Motor System during Action Observation

We applied transcranial alternating current stimulation (tACS) to the primary motor cortex (M1) at different frequencies during an index–thumb pinch-grip observation task. To estimate changes in the corticospinal output, we used the size of motor evoked potentials (MEPs) obtained by transcranial magnetic stimulation (TMS) of M1 using an online MRI-guided simultaneous TMS-tACS approach. The results of the beta-tACS confirm a non-selective increase in corticospinal excitability in subjects at rest; an increase was observed for both of the tested hand muscles, the first dorsal interosseous (FDI) and the abductor digiti minimi (ADM). However, during action observation of the pinch-grip movement, the increase of corticospinal excitability was only observed for the prime mover FDI muscle and took place during alpha-tACS, while gamma-tACS affected both the FDI and control muscle (ADM) responses. These phenomena likely reflect the hypothesis that the mu and gamma rhythms specifically index the downstream modulation of primary sensorimotor areas by engaging mirror neuron activity. The current neuromodulation approach confirms that tACS can be used to induce neurophysiologically detectable state-dependent enhancement effects, even in complex motor-cognitive tasks.

Safety Aspects, Tolerability and Modeling of Retinofugal Alternating Current Stimulation

BACKGROUND

While alternating current stimulation (ACS) is gaining relevance as a tool in research and approaching clinical applications, its mechanisms of action remain unclear. A review by Schutter and colleagues argues for a retinal origin of transcranial ACS' neuromodulatory effects. Interestingly, there is an alternative application form of ACS specifically targeting α-oscillations in the visual cortex via periorbital electrodes (retinofugal alternating current stimulation, rACS). To further compare these two methods and investigate retinal effects of ACS, we first aim to establish the safety and tolerability of rACS.

OBJECTIVE

The goal of our research was to evaluate the safety of rACS via finite-element modeling, theoretical safety limits and subjective report.

METHODS

20 healthy subjects were stimulated with rACS as well as photic stimulation and reported adverse events following stimulation. We analyzed stimulation parameters at electrode level as well as distributed metric estimates from an ultra-high spatial resolution magnetic resonance imaging (MRI)-derived finite element human head model and compared them to existing safety limits.

RESULTS

Topographical modeling revealed the highest current densities in the anterior visual pathway, particularly retina and optic nerve. Stimulation parameters and finite element modeling estimates of rACS were found to be well below existing safety limits. No serious adverse events occurred.

CONCLUSION

Our findings are in line with existing safety guidelines for retinal and neural damage and establish the tolerability and feasibility of rACS. In comparison to tACS, retinofugal stimulation of the visual cortex provides an anatomically circumscribed model to systematically study the mechanisms of action of ACS.

Transorbital alternating current stimulation modifies BOLD activity in healthy subjects and in a stroke patient with hemianopia: A 7 Tesla fMRI feasibility study

BACKGROUND

Modifying brain activity using non-invasive, low intensity transcranial electrical brain stimulation (TES) has rapidly increased during the past 20 years. Alternating current stimulation (ACS), for example, has been shown to alter brain rhythm activities and modify neuronal functioning in the visual system. Daily application of transorbital ACS to patients with optic nerve damage induces functional connectivity reorganization, and partially restores vision. While ACS is thought to mainly modify neuronal mechanisms, e.g. changes in brain oscillations that can be detected by EEG, it is still an open question, whether and how it may alter BOLD activity.

OBJECTIVE

We evaluated whether transorbital ACS modulates BOLD activity in early visual cortex using high-resolution 7 Tesla functional magnetic resonance imaging (fMRI).

METHODS

In this feasibility study transorbital ACS in the alpha range and sham ACS was applied in a random block design in five healthy subjects for 20 min at 1 mA. Brain activation in the visual areas V1, V2 and V3 were measured using 7 Tesla fMRI-based retinotopic mapping at the time points before (baseline) and after stimulation. In addition, we collected data from one hemianopic stroke patient with visual cortex damage after ten daily sessions with 25-50 min stimulation duration.

RESULTS

In healthy subjects transorbital ACS increased the activated cortical surface area, decreased the fMRI response amplitude and increased coherence in the visual cortex, which was most prominent in the full field task. In the patient, stimulation improved contrast sensitivity in the central visual field. BOLD amplitudes and coherence values were increased in most early visual areas in both hemispheres, with the most pronounced activation detected during eccentricity testing in retinotopic mapping.

CONCLUSIONS

This feasibility study showed that transorbital ACS modifies BOLD activity to visual stimulation, which outlasts the duration of the AC stimulation. This is in line with earlier neurophysiological findings of increased power in EEG recordings and functional connectivity reorganization in patients with impaired vision. Accordingly, the larger BOLD response area after stimulation can be explained by more coherent activation and lower variability in the activation. Alternatively, increased neuronal activity can also be taken into account. Controlled trials are needed to systematically evaluate the potential of repetitive transorbital ACS to improve visual function after visual pathway stroke and to determine the cause-effect relationship between neural and BOLD activity changes.

Working memory revived in older adults by synchronizing rhythmic brain circuits

Understanding normal brain aging and developing methods to maintain or improve cognition in older adults are major goals of fundamental and translational neuroscience. Here we show a core feature of cognitive decline-working-memory deficits-emerges from disconnected local and long-range circuits instantiated by theta-gamma phase-amplitude coupling in temporal cortex and theta phase synchronization across frontotemporal cortex. We developed a noninvasive stimulation procedure for modulating long-range theta interactions in adults aged 60-76 years. After 25 min of stimulation, frequency-tuned to individual brain network dynamics, we observed a preferential increase in neural synchronization patterns and the return of sender-receiver relationships of information flow within and between frontotemporal regions. The end result was rapid improvement in working-memory performance that outlasted a 50 min post-stimulation period. The results provide insight into the physiological foundations of age-related cognitive impairment and contribute to groundwork for future non-pharmacological interventions targeting aspects of cognitive decline.

Rebound or Entrainment? The Influence of Alternating Current Stimulation on Individual Alpha

Alternating current stimulation (ACS) is an established means to manipulate intrinsic cortical oscillations. While working towards clinical impact, ACS mechanisms of action remain unclear. For ACS’s well-documented influence on occipital alpha, hypotheses include neuronal entrainment as well as rebound phenomena. As a retinal origin is also discussed, we employed a novel form of ACS with the advantage that it specifically targets occipital alpha-oscillations via retinofugal pathways retinofugal ACS (rACS). We aimed to confirm alpha-enhancement outlasting the duration of stimulation with 10 Hz rACS. To distinguish entrainment from rebound effects, we investigated the correlation between alpha peak frequency change and alpha-enhancement strength. We quantified the alpha band power before and after 10 Hz rACS in 15 healthy subjects. Alpha power enhancement and alpha peak frequency change were assessed over the occipital electrodes and compared to sham stimulation. RACS significantly enhanced occipital alpha power in comparison to sham stimulation (p < 0.05). Alpha peak frequency changed by a mean 0.02 Hz (± 0.04). A greater change in alpha peak frequency did not correlate with greater effects on alpha power. Our findings show an alpha-enhancement consistent with studies conducted for transcranial ACS (tACS) and contribute evidence for a retinal involvement in tACS effects on occipital alpha. Furthermore, the lack of correlation between alpha peak frequency change and alpha-enhancement strength provides an argument against entrainment effects and in favor of a rebound phenomenon.

tACS motor system effects can be caused by transcutaneous stimulation of peripheral nerves

Transcranial alternating current stimulation (tACS) is a noninvasive neuromodulation method which has been shown to modulate hearing, motor, cognitive and memory function. However, the mechanisms underpinning these findings are controversial, as studies show that the current reaching the cortex may not be strong enough to entrain neural activity. Here, we propose a new hypothesis to reconcile these opposing results: tACS effects are caused by transcutaneous stimulation of peripheral nerves in the skin and not transcranial stimulation of cortical neurons. Rhythmic activity from peripheral nerves then entrains cortical neurons. A series of experiments in rats and humans isolated the transcranial and transcutaneous mechanisms and showed that the reported effects of tACS on the motor system can be caused by transcutaneous stimulation of peripheral nerves. Whether or not the transcutaneous mechanism will generalize to tACS effects on other systems is debatable but should be investigated.

Transcranial alternating current stimulation (tACS) uses weak electrical currents, applied to the head, to modulate brain activity. Here, the authors show that contrary to previous assumptions, the effects of tACS on the brain may be mediated by its effect on peripheral nerves in the skin, not direct.

Methods to Compare Predicted and Observed Phosphene Experience in tACS Subjects

Background

Phosphene generation is an objective physical measure of potential transcranial alternating current stimulation (tACS) biological side effects. Interpretations from phosphene analysis can serve as a first step in understanding underlying mechanisms of tACS in healthy human subjects and assist validation of computational models.

Objective/Hypothesis

This preliminary study introduces and tests methods to analyze predicted phosphene occurrence using computational head models constructed from tACS recipients against verbal testimonies of phosphene sensations. Predicted current densities in the eyes and the occipital lobe were also verified against previously published threshold values for phosphenes.

Methods

Six healthy subjects underwent 10 Hz tACS while being imaged in an MRI scanner. Two different electrode montages, T7-T8 and Fpz-Oz, were used. Subject ratings of phosphene experience were collected during tACS and compared against current density distributions predicted in eye and occipital lobe regions of interest (ROIs) determined for each subject. Calculated median current densities in each ROI were compared to minimum thresholds for phosphene generation.

Main Results

All subjects reported phosphenes, and predicted median current densities in ROIs exceeded minimum thresholds for phosphenes found in the literature. Higher current densities in the eyes were consistently associated with decreased phosphene generation for the Fpz-Oz montage. There was an overall positive association between phosphene perceptions and current densities in the occipital lobe.

Conclusions

These methods may have promise for predicting phosphene generation using data collected during in-scanner tACS sessions and may enable better understanding of phosphene origin. Additional empirical data in a larger cohort is required to fully test the robustness of the proposed methods. Future studies should include additional montages that could dissociate retinal and occipital stimulation.

Low-Frequency Conductivity Tensor Imaging of the Human Head In Vivo Using DT-MREIT: First Study

We present the first in vivo images of anisotropic conductivity distribution in the human head, measured at a frequency of approximately 10 Hz. We used magnetic resonance electrical impedance tomography techniques to encode phase changes caused by current flow within the head via two independent electrode pairs. These results were then combined with diffusion tensor imaging data to reconstruct full anisotropic conductivity distributions in 5-mm-thick slices of the brains of two participants. Conductivity values recovered in this paper were broadly consistent with literature values. We anticipate that this technique will be of use in many areas of neuroscience, most importantly in functional imaging via inverse electroencephalogram. Future studies will involve pulse sequence acceleration to maximize brain coverage and resolution.

Modulation of the frontal-parietal network by low intensity anti-phase 20 Hz transcranial electrical stimulation boosts performance in the attentional blink task

Performance in the attentional blink task has been demonstrated to be directly influenced by alpha and beta neural oscillatory activity. In two experiments we stimulated the right parietal cortex and left frontal cortex with transcranial alternating current stimulation. For the first experiment we targeted only the right parietal cortex and found a non-significant increase in performance from 20 Hz stimulation. In the second experiment we applied two stimulators to the right parietal and left frontal cortex and found a significant increase in performance from 20 Hz tACS with a phase difference of 180°. Since low intensity stimulation has been shown to inhibit cortical excitability, and anti-phasic stimulation has been hypothesized to decrease presynaptic activation in one region and drive postsynaptic spikes in the other, we suggest that low intensity anti-phasic 20 Hz stimulation inhibited the parietal cortex, thereby disinhibiting the frontal cortex. This visual attention mechanism supposedly reduces processing of distractor stimuli and enhances processing of target stimuli. This study reveals that the frontal-parietal visual attention network may be modulated with low intensity 20 Hz anti-phase tACS.

Transcranial Alternating Current Stimulation Modulates Risky Decision Making in a Frequency-Controlled Experiment

In this study, we investigated the effect of transcranial alternating current stimulation (tACS) on voluntary risky decision making and executive control in humans. Stimulation was delivered online at 5 Hz (θ), 10 Hz (α), 20 Hz (β), and 40 Hz (γ) on the left and right frontal area while participants performed a modified risky decision-making task. This task allowed participants to voluntarily select between risky and certain decisions associated with potential gains or losses, while simultaneously measuring the cognitive control component (voluntary switching) of decision making. The purpose of this experimental design was to test whether voluntary risky decision making and executive control can be modulated with tACS in a frequency-specific manner. Our results revealed a robust effect of a 20-Hz stimulation over the left prefrontal area that significantly increased voluntary risky decision making, which may suggest a possible link between risky decision making and reward processing, underlined by β-oscillatory activity.

Imaging of current flow in the human head during transcranial electrical therapy

BACKGROUND

It has been assumed that effects caused by tDCS or tACS neuromodulation are due to electric current flow within brain structures. However, to date, direct current density distributions in the brains of human subjects have not been measured. Instead computational models of tDCS or tACS have been used to predict electric current and field distributions for dosimetry and mechanism analysis purposes.

OBJECTIVE/HYPOTHESIS

We present the first in vivo images of electric current density distributions within the brain in four subjects undergoing transcranial electrical stimulation.

METHODS

Magnetic resonance electrical impedance tomography (MREIT) techniques encode current flow in phase images. In four human subjects, we used MREIT to measure magnetic flux density distributions caused by tACS currents, and then calculated current density distributions from these data. Computational models of magnetic flux and current distribution, constructed using contemporaneously collected T₁-weighted structural MRI images, were co-registered to compare predicted and experimental results.

RESULTS

We found consistency between experimental and simulated magnetic flux and current density distributions using transtemporal (T7-T8) and anterior-posterior (Fpz-Oz) electrode montages, and also differences that may indicate a need to improve models to better interpret experimental results. While human subject data agreed with computational model predictions in overall scale, differences may result from factors such as effective electrode surface area and conductivities assumed in models.

CONCLUSIONS

We believe this method may be useful in improving reproducibility, assessing safety, and ultimately aiding understanding of mechanisms of action in electrical and magnetic neuromodulation modalities.

Frequency-specific insight into short-term memory capacity

The digit span is one of the most widely used memory tests in clinical and experimental neuropsychology for reliably measuring short-term memory capacity. In the forward version, sequences of digits of increasing length have to be reproduced in the order in which they are presented, whereas in the backward version items must be reproduced in the reversed order. Here, we assessed whether transcranial alternating current stimulation (tACS) increases the memory span for digits of young and midlife adults. Imperceptibly weak electrical currents in the alpha (10 Hz), beta (20 Hz), theta (5 Hz), and gamma (40 Hz) range, as well as a sham stimulation, were delivered over the left posterior parietal cortex, a cortical region thought to sustain maintenance processes in short-term memory through oscillatory brain activity in the beta range. We showed a frequency-specific effect of beta-tACS that robustly increased the forward memory span of young, but not middle-aged, healthy individuals. The effect correlated with age: the younger the subjects, the greater the benefit arising from parietal beta stimulation. Our results provide evidence of a short-term memory capacity improvement in young adults by online frequency-specific tACS application.

Evaluation of a Modified High-Definition Electrode Montage for Transcranial Alternating Current Stimulation (tACS) of Pre-Central Areas

OBJECTIVE

To evaluate a modified electrode montage with respect to its effect on tACS-dependent modulation of corticospinal excitability and discomfort caused by neurosensory side effects accompanying stimulation.

METHODS

In a double-blind cross-over design, the classical electrode montage for primary motor cortex (M1) stimulation (two patch electrodes over M1 and contralateral supraorbital area) was compared with an M1 centre-ring montage. Corticospinal excitability was evaluated before, during, immediately after and 15 minutes after tACS (10 min., 20 Hz vs. 30 s low-frequency transcranial random noise stimulation).

RESULTS

Corticospinal excitability increased significantly during and immediately after tACS with the centre-ring montage. This was not the case with the classical montage or tRNS stimulation. Level of discomfort was rated on average lower with the centre-ring montage.

CONCLUSIONS

In comparison to the classic montage, the M1 centre-ring montage enables a more focal stimulation of the target area and, at the same time, significantly reduces neurosensory side effects, essential for placebo-controlled study designs.

A meta-analytic study of exogenous oscillatory electric potentials in neuroenhancement

The assumption that transcranial alternating current stimulation (tACS) enhances perceptual and cognitive ability in healthy volunteers by exposing the brain to exogenous oscillatory electric fields is increasingly finding its way into society and commercial parties. The aim of the present study is to quantify the effects of exogenous oscillatory electric field potentials on neuroenhancement in healthy volunteers. The meta-analysis included fifty-one sham controlled experiments that investigated the effects of tACS on perception and cognitive performance. Results from random effects modelling of the cumulative effect size showed small, but robust perceptual and cognitive enhancement in healthy participants to weak exogenous oscillatory electric field potentials. Analyses of tACS parameters indicate that simultaneous stimulation of the anterior and posterior locations of the scalp at >1mA intensity currently has the highest probability of increasing performance. However, technical and methodological issues currently limit the applicability of tACS in neuroenhancement. Additional research is needed to further evaluate the potential of tACS in perception and cognitive ability, and to establish the contexts and parameters under which tACS is effective.

Beta Band Transcranial Alternating (tACS) and Direct Current Stimulation (tDCS) Applied After Initial Learning Facilitate Retrieval of a Motor Sequence

The primary motor cortex (M1) contributes to the acquisition and early consolidation of a motor sequence. Although the relevance of M1 excitability for motor learning has been supported, the significance of M1 oscillations remains an open issue. This study aims at investigating to what extent retrieval of a newly learned motor sequence can be differentially affected by motor-cortical transcranial alternating (tACS) and direct current stimulation (tDCS). Alpha (10 Hz), beta (20 Hz) or sham tACS was applied in 36 right-handers. Anodal or cathodal tDCS was applied in 30 right-handers. Participants learned an eight-digit serial reaction time task (SRTT; sequential vs. random) with the right hand. Stimulation was applied to the left M1 after SRTT acquisition at rest for 10 min. Reaction times were analyzed at baseline, end of acquisition, retrieval immediately after stimulation and reacquisition after eight further sequence repetitions. Reaction times during retrieval were significantly faster following 20 Hz tACS as compared to 10 Hz and sham tACS indicating a facilitation of early consolidation. tDCS yielded faster reaction times, too, independent of polarity. No significant differences between 20 Hz tACS and tDCS effects on retrieval were found suggesting that 20 Hz effects might be associated with altered motor-cortical excitability. Based on the behavioral modulation yielded by tACS and tDCS one might speculate that altered motor-cortical beta oscillations support early motor consolidation possibly associated with neuroplastic reorganization.

Application of Transcranial Electric Stimulation (tDCS, tACS, tRNS)

Abstract. Low-intensity transcranial electrical stimulation (tES) techniques are a group of noninvasive brain stimulation approaches, where currents are applied with intensities ranging between 0.4 and 2 mA through the human scalp. The most frequently used tES methods are transcranial direct current (tDCS), alternating current (tACS), and random noise stimulation (tRNS). These methods have been shown to induce changes in cortical excitability and activity during and after the stimulation in a reversible manner. It was observed that while anodal and cathodal tDCS acts on the membrane potentials by depolarizing or hyperpolarizing them, tACS probably modifies cortical oscillations. tRNS, that is a special form of tACS, might act through affecting the signal-to-noise ratio in the brain. Currently, an exponentially increasing number of studies have been published regarding the effects of tES on physiological processes and cognition. The aim of this review is to summarize the basic aspects of tES methods.

Individual differences and specificity of prefrontal gamma frequency-tACS on fluid intelligence capabilities

Emerging evidence suggests that transcranial alternating current stimulation (tACS) is an effective, frequency-specific modulator of endogenous brain oscillations, with the potential to alter cognitive performance. Here, we show that reduction in response latencies to solve complex logic problem indexing fluid intelligence is obtained through 40 Hz-tACS (gamma band) applied to the prefrontal cortex. This improvement in human performance depends on individual ability, with slower performers at baseline receiving greater benefits. The effect could have not being explained by regression to the mean, and showed task and frequency specificity: it was not observed for trials not involving logical reasoning, as well as with the application of low frequency 5 Hz-tACS (theta band) or non-periodic high frequency random noise stimulation (101-640 Hz). Moreover, performance in a spatial working memory task was not affected by brain stimulation, excluding possible effects on fluid intelligence enhancement through an increase in memory performance. We suggest that such high-level cognitive functions are dissociable by frequency-specific neuromodulatory effects, possibly related to entrainment of specific brain rhythms. We conclude that individual differences in cognitive abilities, due to acquired or developmental origins, could be reduced during frequency-specific tACS, a finding that should be taken into account for future individual cognitive rehabilitation studies.

Effects of alternating current stimulation on the healthy and diseased brain

Cognitive and neurological dysfunctions can severely impact a patient's daily activities. In addition to medical treatment, non-invasive transcranial alternating current stimulation (tACS) has been proposed as a therapeutic technique to improve the functional state of the brain. Although during the last years tACS was applied in numerous studies to improve motor, somatosensory, visual and higher order cognitive functions, our knowledge is still limited regarding the mechanisms as to which type of ACS can affect cortical functions and altered neuronal oscillations seem to be the key mechanism. Because alternating current send pulses to the brain at predetermined frequencies, the online- and after-effects of ACS strongly depend on the stimulation parameters so that “optimal” ACS paradigms could be achieved. This is of interest not only for neuroscience research but also for clinical practice. In this study, we summarize recent findings on ACS-effects under both normal conditions and in brain diseases.

Cutaneous retinal activation and neural entrainment in transcranial alternating current stimulation: A systematic review

Transcranial alternating current stimulation (tACS) applies exogenous oscillatory electric field potentials to entrain neural rhythms and is used to investigate brain-function relationships and its potential to enhance perceptual and cognitive performance. However, due to current spread tACS can cause cutaneous activation of the retina and phosphenes. Several lines of evidence suggest that retinal phosphenes are capable of inducing neural entrainment, making the contributions of central and peripheral stimulation to the effects in the brain difficult to disentangle. In this literature review, the importance of this issue is further illustrated by the fact that photic stimulation can have a direct impact on perceptual and cognitive performance. This leaves open the possibility that peripheral photic stimulation can at least in part explain the central effects that are attributed to tACS. The extent to which phosphene perception contributes to the effects of exogenous oscillatory electric fields in the brain and influence perception and cognitive performance needs to be examined to understand the working mechanisms of tACS in neurophysiology and behaviour.

Montage matters: the influence of transcranial alternating current stimulation on human physiological tremor

BACKGROUND

Classically, studies adopting non-invasive transcranial electrical stimulation have placed greater importance on the position of the primary "stimulating" electrode than the secondary "reference" electrode. However, recent current density modeling suggests that ascribing a neutral role to the reference electrode may prove an inappropriate oversimplification.

HYPOTHESIS

We set out to test the hypothesis that the behavioral effects of transcranial electrical stimulation are critically dependent on the position of the return ("reference") electrode.

METHODS

We examined the effect of transcranial alternating current stimulation (sinusoidal waveform with no direct current offset at a peak-to-peak amplitude of 2000 μA and a frequency matched to each participant's peak tremor frequency) on physiological tremor in a group of healthy volunteers (N = 12). We implemented a sham-controlled experimental protocol where the position of the stimulating electrode remained fixed, overlying primary motor cortex, whilst the position of the return electrode varied between two cephalic (fronto-orbital and contralateral primary motor cortex) and two extracephalic (ipsilateral and contralateral shoulder) locations. We additionally controlled for the role of phosphenes in influencing motor output by assessing the response of tremor to photic stimulation, through self-reported phosphene ratings.

RESULTS

Altering only the position of the return electrode had a profound behavioral effect: only the montage with extracephalic return contralateral to the primary stimulating electrode significantly entrained physiological tremor (15.9% ± 6.1% increase in phase stability, 1 S.E.M.). Photic stimulation also entrained tremor (11.7% ± 5.1% increase in phase stability). Furthermore, the effects of electrical stimulation are distinct from those produced from direct phosphene induction, in that the latter were only seen with the fronto-orbital montage that did not affect the tremor.

CONCLUSION

The behavioral effects of transcranial alternating current stimulation appear to be critically dependent on the position of the reference electrode, highlighting the importance of electrode montage when designing experimental and therapeutic protocols.

The impact of electrical stimulation techniques on behavior

Low-intensity transcranial electrical stimulation (tES) methods are a group of noninvasive brain stimulation techniques, whereby currents are applied with intensities typically ranging between 1 and 2 mA, through the human scalp. These techniques have been shown to induce changes in cortical excitability and activity during and after the stimulation in a reversible manner. They include transcranial direct current simulation (tDCS), transcranial alternating current simulation (tACS), and transcranial random noise stimulation (tRNS). Currently, an increasing number of studies have been published regarding the effects of tES on cognitive performance and behavior. Processes of learning and increases in cognitive performance are accompanied by changes in cortical plasticity. tES can impact upon these processes and is able to affect task execution. Many studies have been based on the accepted idea that by increasing cortical excitability (e.g., by applying anodal tDCS) or coherence of oscillatory activity (e.g., by applying tACS) an increase in performance should be detected; however, a number of studies now suggest that the basic knowledge of the mechanisms of action is insufficient to predict the outcome of applied stimulation on the execution of a cognitive or behavioral task, and so far no standard paradigms for increasing cortical plasticity changes during learning or cognitive tasks have been established. The aim of this review is to summarize recent findings with regard to the effects of tES on behavior concentrating on the motor and visual areas. WIREs Cogn Sci 2014, 5:649-659. doi: 10.1002/wcs.1319 For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.

Emerging concepts in the physiological basis of dystonia

Work over the past 2 decades has led to substantial changes in our understanding of dystonia pathophysiology. Three general abnormalities appear to underlie the pathophysiological substrate. The first is a loss of inhibition. This makes sense considering that it may be responsible for the excess of movement and for the overflow phenomena seen in dystonia. A second abnormality is sensory dysfunction which is related to the mild sensory complaints in patients with focal dystonias and may be responsible for some of the motor dysfunction. Third, evidence from animal models of dystonia as well as from patients with primary dystonia has revealed significant alterations of synaptic plasticity characterized by a disruption of homeostatic plasticity, with a prevailing facilitation of synaptic potentiation, together with the loss of synaptic inhibitory processes. We speculate that during motor learning this abnormal plasticity may lead to an abnormal sensorimotor integration, leading to consolidation of abnormal motor engrams. If so, then removing this abnormal plasticity might have little immediate effect on dystonic movements because bad motor memories have already been ''learned'' and are difficult to erase. These considerations might explain the delayed clinical effects of deep brain stimulation (DBS) in patients with generalized dystonia. Current lines of research will be discussed from a network perspective. © 2013 Movement Disorder Society.

Cortico-muscular coupling and motor performance are modulated by 20 Hz transcranial alternating current stimulation (tACS) in Parkinson's disease

Parkinson’s disease (PD) is associated with pathologically altered oscillatory activity. While synchronized oscillations between 13 and 30 Hz are increased within a cortico-subcortical network, cortico-muscular coupling (CMC) is decreased. The present study aims at investigating the effect of non-invasive transcranial alternating current stimulation (tACS) of the primary motor cortex (M1) on motor symptoms and motor-cortical oscillations in PD. In 10 PD patients and 10 healthy control subjects, static isometric contraction, dynamic fast finger tapping, and diadochokinesia of the more severely affected hand were investigated prior to and shortly after tACS of the contralateral M1 at 10 Hz vs. 20 Hz vs. sham. During isometric contraction, neuromagnetic activity was recorded using magnetoencephalography. 20 Hz tACS attenuated beta band CMC during isometric contraction and amplitude variability during finger tapping in PD patients but not in healthy control subjects. 10 Hz tACS yielded no significant after-effects. The present data suggest that PD is associated with pathophysiological alterations which abet a higher responsiveness toward frequency-specific tACS – possibly due to pathologically altered motor-cortical oscillatory synchronization at frequencies between 13 and 30 Hz.

State-dependent effects of transcranial oscillatory currents on the motor system: what you think matters

Imperceptible transcranial alternating current stimulation (tACS) changes the endogenous cortical oscillatory activity in a frequency-specific manner. In the human motor system, tACS coincident with the idling beta rhythm of the quiescent motor cortex increased the corticospinal output. We reasoned that changing the initial state of the brain (i.e., from quiescence to a motor imagery task that desynchronizes the local beta rhythm) might also change the susceptibility of the corticospinal system to resonance effects induced by beta-tACS. We tested this hypothesis by delivering tACS at different frequencies (theta, alpha, beta, and gamma) on the primary motor cortex at rest and during motor imagery. Motor-evoked potentials (MEPs) were obtained by transcranial magnetic stimulation (TMS) on the primary motor cortex with an online-navigated TMS-tACS setting. During motor imagery, the increase of corticospinal excitability was maximal with theta-tACS, likely reflecting a reinforcement of working memory processes required to mentally process and "execute" the cognitive task. As expected, the maximal MEPs increase with subjects at rest was instead obtained with beta-tACS, substantiating previous evidence. This dissociation provides new evidence of state and frequency dependency of tACS effects on the motor system and helps discern the functional role of different oscillatory frequencies of this brain region. These findings may be relevant for rehabilitative neuromodulatory interventions.

The effect of 10 Hz transcranial alternating current stimulation (tACS) on corticomuscular coherence

Synchronous oscillatory activity at alpha (8-12 Hz), beta (13-30 Hz), and gamma (30-90 Hz) frequencies is assumed to play a key role for motor control. Corticomuscular coherence (CMC) represents an established measure of the pyramidal system's integrity. Transcranial alternating current stimulation (tACS) offers the possibility to modulate ongoing oscillatory activity. Behaviorally, 20 Hz tACS in healthy subjects has been shown to result in movement slowing. However, the neurophysiological changes underlying these effects are not entirely understood yet. The present study aimed at ascertaining the effects of tACS at 10 and 20 Hz in healthy subjects on CMC and local power of the primary sensorimotor cortex. Neuromagnetic activity was recorded during isometric contraction before and at two time points (2-10 min and 30-38 min) after tACS of the left primary motor cortex (M1), using a 306 channel whole head magnetoencephalography (MEG) system. Additionally, electromyography (EMG) of the right extensor digitorum communis (EDC) muscle was measured. TACS was applied at 10 and 20 Hz, respectively, for 10 min at 1 mA. Sham stimulation served as control condition. The data suggest that 10 Hz tACS significantly reduced low gamma band CMC during isometric contraction. This implies that tACS does not necessarily cause effects at stimulation frequency. Rather, the findings suggest cross-frequency interplay between alpha and low gamma band activity modulating functional interaction between motor cortex and muscle.

Computational analysis shows why transcranial alternating current stimulation induces retinal phosphenes

OBJECTIVE

Transcranial alternating current stimulation (tACS), which is a novel technique for the manipulation of cortical oscillations, can generate subjective visual sensations (phosphenes). In this work, we computationally investigate the current that reaches the eyes from tACS electrodes in order to show that phosphenes induced by tACS are retinal in origin.

APPROACH

The finite-element method is used for modelling the path of the current in an anatomically realistic model of the head for various electrode montages. The computational results are used for analysing previous experimental data to investigate the sensitivity of the eye to electrical stimulation.

MAIN RESULTS

Depending on the locations of both the stimulating and reference electrodes, a small portion of the stimulation current chooses a path that goes through the eyes. Due to the sensitivity of the retina to electrical stimulation, even distant electrodes can produce a sufficiently strong current at the eyes for inducing retinal phosphenes.

SIGNIFICANCE

The interference from retinal phosphenes needs to be considered in the design of tACS experiments. The occurrence of phosphenes can be reduced by optimizing the locations of the electrodes, or potentially increasing the number of reference electrodes to two or more. Computational modelling is an effective tool for guiding the electrode positioning.

Transcranial alternating current stimulation: a review of the underlying mechanisms and modulation of cognitive processes

Brain oscillations of different frequencies have been associated with a variety of cognitive functions. Convincing evidence supporting those associations has been provided by studies using intracranial stimulation, pharmacological interventions and lesion studies. The emergence of novel non-invasive brain stimulation techniques like repetitive transcranial magnetic stimulation (rTMS) and transcranial alternating current stimulation (tACS) now allows to modulate brain oscillations directly. Particularly, tACS offers the unique opportunity to causally link brain oscillations of a specific frequency range to cognitive processes, because it uses sinusoidal currents that are bound to one frequency only. Using tACS allows to modulate brain oscillations and in turn to influence cognitive processes, thereby demonstrating the causal link between the two. Here, we review findings about the physiological mechanism of tACS and studies that have used tACS to modulate basic motor and sensory processes as well as higher cognitive processes like memory, ambiguous perception, and decision making.

Transcranial electrical stimulation over visual cortex evokes phosphenes with a retinal origin

Transcranial electrical stimulation (tES) is a promising therapeutic tool for a range of neurological diseases. Understanding how the small currents used in tES spread across the scalp and penetrate the brain will be important for the rational design of tES therapies. Alternating currents applied transcranially above visual cortex induce the perception of flashes of light (phosphenes). This makes the visual system a useful model to study tES. One hypothesis is that tES generates phosphenes by direct stimulation of the cortex underneath the transcranial electrode. Here, we provide evidence for the alternative hypothesis that phosphenes are generated in the retina by current spread from the occipital electrode. Building on the existing literature, we first confirm that phosphenes are induced at lower currents when electrodes are placed farther away from visual cortex and closer to the eye. Second, we explain the temporal frequency tuning of phosphenes based on the well-known response properties of primate retinal ganglion cells. Third, we show that there is no difference in the time it takes to evoke phosphenes in the retina or by stimulation above visual cortex. Together, these findings suggest that phosphenes induced by tES over visual cortex originate in the retina. From this, we infer that tES currents spread well beyond the area of stimulation and are unlikely to lead to focal neural activation. Novel stimulation protocols that optimize current distributions are needed to overcome these limitations of tES.

Progressive enhancement of alpha activity and visual function in patients with optic neuropathy: a two-week repeated session alternating current stimulation study

INTRODUCTION

Repetitive transorbital alternating current stimulation (rtACS) can improve visual deficits in patients with optic nerve damage. Recent retrospective results suggest that rtACS enhances oscillatory brain activity. The exact mechanisms of rtACS are unclear and little is known about possibly frequency-specific neural-plastic mechanisms. An association between bandwidth-confined neural-entrainment and vision recovery maximization could offer a novel therapeutic option for patients with optic neuropathy.

OBJECTIVES

The goal of this prospective open-label study was to investigate if the enhancement of rhythmic brain activity over 10 days of consecutive rtACS stimulation is associated with visual field recovery. The secondary goal was to investigate neurophysiological mechanisms related to frequency dependent adaptive plasticity.

METHODS

18 Patients with visual field impairments resulting from pre-chiasmatic partial optic nerve damage received rtACS on 10 consecutive days. Daily, subject-specific treatment parameters (<500 μA, 9-37 Hz, 25-40 min/day) were defined and EEG-spectra collected prior to and after rtACS. Visual field data was collected at day 1 and 10. The change of spectral-power in classic bandwidths were investigated and correlated with visual field deficit recovery.

RESULTS

After 10 days of rtACS alpha-power over bilateral occipital electrodes was significantly larger than at baseline (F(Time x alpha-power)p < 0.01). This effect was progressive over subsequent days of stimulation (cubic-fit, R(2) 0.70, RMSE 0.008). Perimetric results improved significantly, but they were not associated with changes in alpha-synchronization.

DISCUSSION

rtACS can induce cumulative bandwidth-confined changes in brain rhythms over multiple sessions. These findings are in line with the notion of brain-state dependent [1] and bandwidth-confined entrainment [2] as well as rtACS facilitated visual recovery [3].

Transcranial alternating current stimulation increases risk-taking behavior in the balloon analog risk task

The process of evaluating risks and benefits involves a complex neural network that includes the dorsolateral prefrontal cortex (DLPFC). It has been proposed that in conflict and reward situations, theta-band (4–8 Hz) oscillatory activity in the frontal cortex may reflect an electrophysiological mechanism for coordinating neural networks monitoring behavior, as well as facilitating task-specific adaptive changes. The goal of the present study was to investigate the hypothesis that theta-band oscillatory balance between right and left frontal and prefrontal regions, with a predominance role to the right hemisphere (RH), is crucial for regulatory control during decision-making under risk. In order to explore this hypothesis, we used transcranial alternating current stimulation, a novel technique that provides the opportunity to explore the functional role of neuronal oscillatory activities and to establish a causal link between specific oscillations and functional lateralization in risky decision-making situations. For this aim, healthy participants were randomly allocated to one of three stimulation groups (LH stimulation/RH stimulation/Sham stimulation), with active AC stimulation delivered in a frequency-dependent manner (at 6.5 Hz; 1 mA peak-to-peak). During the AC stimulation, participants performed the Balloon Analog Risk Task. This experiment revealed that participants receiving LH stimulation displayed riskier decision-making style compared to sham and RH stimulation groups. However, there was no difference in decision-making behaviors between sham and RH stimulation groups. The current study extends the notion that DLPFC activity is critical for adaptive decision-making in the context of risk-taking and emphasis the role of theta-band oscillatory activity during risky decision-making situations.