Anodal Transcranial Direct Current Stimulation Induces High Gamma-Band Activity in the Left Dorsolateral Prefrontal Cortex During a Working Memory Task: A Double-Blind, Randomized, Crossover Study

A mathematical model and experimental procedure to analyze the cognitive effects of audio frequency magnetic fields

Audio frequency magnetic fields (20 Hz-20 kHz) are magnetic fields in extremely low frequency-very low frequency (ELF-VLF) bands that are present near audio equipment and acoustic transducers. These devices transform and operate the electrical signal from the recordings or other devices into acoustic and audio signals. The cognitive influence of sound and noise has been widely studied and recognized since the times of ancient Rome; however, the cognitive effects of the magnetic fields of these frequencies have not been studied. Due to the extensive use of audio devices that use this type of transducer near the temporal-parietal area, we believe that it is of interest to study their impact on short-term memory or working memory (WM) and to analyze their potential as they operate as a transcranial magnetic stimulation. In this study, a mathematical model and an experimental tool are introduced to analyze memory performance. The model dissociates the reaction time of a cognitive task. We analyze the model in data from a group of 65 young, healthy subjects. WM is assessed in our experimental setup by means of the Sternberg test (ST), whereby during the ST, one subgroup was exposed to an audio frequency magnetic stimulus, and the other subgroup received a sham stimulus. The magnetic stimulus was ~0.1 μT and was applied to both sides of the head at the frontal cortex near the temporal-parietal area, which is where WM is expected to be located. The ST records reaction times when determining whether an object displayed on the computer screen is one of the objects to be remembered. The results are analyzed within the mathematical model and changes are observed, including the deterioration of WM, which could affect 32% of its operability.

Transcranial Direct Current Stimulation Modulates Working Memory Maintenance Processes in Healthy Individuals

The effects of transcranial direct current stimulation (tDCS) at the pFC are often investigated using cognitive paradigms, particularly working memory tasks. However, the neural basis for the neuromodulatory cognitive effects of tDCS, including which subprocesses are affected by stimulation, is not completely understood. We investigated the effects of tDCS on working memory task-related spectral activity during and after tDCS to gain better insights into the neurophysiological changes associated with stimulation. We reanalyzed data from 100 healthy participants grouped by allocation to receive either sham (0 mA, 0.016 mA, and 0.034 mA) or active (1 mA or 2 mA) stimulation during a 3-back task. EEG data were used to analyze event-related spectral power in frequency bands associated with working memory performance. Frontal theta event-related synchronization (ERS) was significantly reduced post-tDCS in the active group. Participants receiving active tDCS had slower RTs following tDCS compared with sham, suggesting interference with practice effects associated with task repetition. Theta ERS was not significantly correlated with RTs or accuracy. tDCS reduced frontal theta ERS poststimulation, suggesting a selective disruption to working memory cognitive control and maintenance processes. These findings suggest that tDCS selectively affects specific subprocesses during working memory, which may explain heterogenous behavioral effects.

Underpinning the neurological source of executive function following cross hemispheric tDCS stimulation

Transcranial direct current stimulation (tDCS) is a promising technique for enhancement of executive functions in healthy as well as neurologically disturbed patients. However, the evidence regarding the neuropsychological and behavioral change with neurophysiological shifts as well as the mechanism of tDCS action as evidenced by activation of neuronal sources important for executive functions have remained unaddressed. The study thereby endeavors to (1) determine the neuropsychological, behavioral, and neurophysiological change induced with five sessions of bilateral tDCS stimulation and (2) identify putative neuronal sources related to the executive functions responsible for neuropsychological and behavioral change. For this single blinded study, a total of 40 healthy participants, randomly allocated to active (n = 19) or sham (n = 21) groups completed five sessions of 2 mA tDCS stimulation administered over Dorso-Lateral Prefrontal Cortex (DLPFC) (F3 as anode, F4 as cathode). Repeated measure analysis was performed on neuropsychological (Everyday Memory Questionnaire and Mindful Attention Awareness Scale), and behavioral assessment (n-Back and Stroop tests) to investigate within and between group differences. Pre and post neurophysiological (Electroencephalogram) results showed that bilateral tDCS stimulation activates cortical regions responsible for executive functions including updation (working memory) and inhibition (interference control or attention). Multiple sessions of bilateral tDCS stimulation results in a significant increase in theta, alpha, and beta-band activity in the DLPFC, cingulate and parietal cortex. This study provides evidence that tDCS can be used for performance enhancement of executive functions in able-bodied people.

Theta tACS impairs episodic memory more than tDCS

Episodic memory deficits are a common consequence of aging and are associated with a number of neurodegenerative disorders (e.g., Alzheimer's disease). Given the importance of episodic memory, a great deal of research has investigated how we can improve memory performance. Transcranial electrical stimulation (TES) represents a promising tool for memory enhancement but the optimal stimulation parameters that reliably boost memory are yet to be determined. In our double-blind, randomised, sham-controlled study, 42 healthy adults (36 females; 23.3 ± 7.7 years of age) received anodal transcranial direct current stimulation (tDCS), theta transcranial alternating current stimulation (tACS) and sham stimulation during a list-learning task, over three separate sessions. Stimulation was applied over the left temporal lobe, as encoding and recall of information is typically associated with mesial temporal lobe structures (e.g., the hippocampus and entorhinal cortex). We measured word recall within each stimulation session, as well as the average number of intrusion and repetition errors. In terms of word recall, participants recalled fewer words during tDCS and tACS, compared to sham stimulation, and significantly fewer words recalled during tACS compared with tDCS. Significantly more memory errors were also made during tACS compared with sham stimulation. Overall, our findings suggest that TES has a deleterious effect on memory processes when applied to the left temporal lobe.

Electric Field Strength From Prefrontal Transcranial Direct Current Stimulation Determines Degree of Working Memory Response: A Potential Application of Reverse-Calculation Modeling?

BACKGROUND

Transcranial direct current stimulation (tDCS) for working memory is an enticing treatment, but there is mixed evidence to date.

OBJECTIVES

We tested the effects of electric field strength from uniform 2 mA dosing on working memory change from prestimulation to poststimulation. Second, we statistically evaluated a reverse-calculation method of individualizing tDCS dose and its effect on normalizing electric field at the cortex.

MATERIALS AND METHODS

We performed electric field modeling on a data set of 28 healthy older adults (15 women, mean age = 73.7, SD = 7.3) who received ten sessions of active 2 mA tDCS (N = 14) or sham tDCS (N = 14) applied over bilateral dorsolateral prefrontal cortices (DLPFC) in a triple-blind design. We evaluated the relationship between electric field strength and working memory change on an N-back task in conditions of above-median, high electric field from active 2 mA (N = 7), below-median, low electric field from active 2 mA (N = 7), and sham (N = 14) at regions of interest (ROI) at the left and right DLPFC. We then determined the individualized reverse-calculation dose to produce the group average electric field and measured the electric field variance between uniform 2 mA doses vs individualized reverse-calculation doses at the same ROIs.

RESULTS

Working memory improvements from pre- to post-tDCS were significant for the above-median electric field from active 2 mA condition at the left DLPFC (mixed ANOVA, p = 0.013). Furthermore, reverse-calculation modeling significantly reduced electric field variance at both ROIs (Levene's test; p < 0.001).

CONCLUSIONS

Higher electric fields at the left DLPFC from uniform 2 mA doses appear to drive working memory improvements from tDCS. Individualized doses from reverse-calculation modeling significantly reduce electric field variance at the cortex. Taken together, using reverse-calculation modeling to produce the same, high electric fields at the cortex across participants may produce more effective future tDCS treatments for working memory.

Dependence of Working Memory on Coordinated Activity Across Brain Areas

Neural signatures of working memory (WM) have been reported in numerous brain areas, suggesting a distributed neural substrate for memory maintenance. In the current manuscript we provide an updated review of the literature focusing on intracranial neurophysiological recordings during WM in primates. Such signatures of WM include changes in firing rate or local oscillatory power within an area, along with measures of coordinated activity between areas based on synchronization between oscillations. In comparing the ability of various neural signatures in any brain area to predict behavioral performance, we observe that synchrony between areas is more frequently and robustly correlated with WM performance than any of the within-area neural signatures. We further review the evidence for alteration of inter-areal synchrony in brain disorders, consistent with an important role for such synchrony during behavior. Additionally, results of causal studies indicate that manipulating synchrony across areas is especially effective at influencing WM task performance. Each of these lines of research supports the critical role of inter-areal synchrony in WM. Finally, we propose a framework for interactions between prefrontal and sensory areas during WM, incorporating a range of experimental findings and offering an explanation for the observed link between intra-areal measures and WM performance.

Offline tDCS modulates prefrontal-cortical-subcortical-cerebellar fear pathways in delayed fear extinction

Transcranial direct current stimulation (tDCS) has been studied to enhance extinction-based treatments for anxiety disorders. However, the field shows conflicting results about its anxiolytic effect and only a few studies have observed the extinction of consolidated memories. We looked to study the effect of offline 1 mA tDCS over the right dorsolateral pre-frontal cortex across the fear pathways, in consolidated fear response during delayed extinction. Participants (N = 34 women) underwent in a two-day fear conditioning procedure. On day 1, participants were assigned to the control group (N = 18) or the tDCS group (N = 16) and went through a fear acquisition procedure. On day 2, the tDCS group received 20 min tDCS before extinction and while inside the MRI scanner. The control group completed the extinction procedure only. The tDCS session (for the tDCS group) and the fMRI scan (for both groups) were completed just on the second day. Univariate fMRI analysis showed stimulation-dependent activity during late extinction with the tDCS group showing decreased neural activity during the processing of threat cues (CS +) and increased activity during the processing of safety cues (CS -), in prefrontal, postcentral and paracentral regions, during late extinction. ROI to whole-brain psychophysiological interaction (PPI) analysis showed the tDCS effect on the connectivity between the left dorsolateral prefrontal cortex three cortical-amygdalo-hippocampal-cerebellar pathway clusters during the processing of the CS + in late extinction (TFCE corrected; p < 0.05). Increased neuronal activity during the processing of safety cues and stronger coupling during the processing of threat cues might be the mechanisms by which tDCS contributes to stimuli discrimination.

Combining proactive transcranial stimulation and cardiac biofeedback to substantially manage harmful stress effects

BACKGROUND

Previous studies have identified the dorsolateral prefrontal cortex (dlPFC) as a core region in cognitive emotional regulation. Transcranial direct current stimulations of the dlPFC (tDCS) and heart-rate variability biofeedback (BFB) are known to regulate emotional processes. However, the effect of these interventions applied either alone or concomitantly during an anticipatory stress remains unexplored.

OBJECTIVE

The study investigated the effect of anodal tDCS and BFB, alone or combined, on psychophysiological stress responses and cognitive functioning.

METHODS

Following a stress anticipation induction, 80 participants were randomized into four groups and subjected to a 15-min intervention: neutral video viewing (ctrl), left dlPFC anodal tDCS (tdcs), heart-rate variability biofeedback (bfb), or a combined treatment (bfb + tdcs). Participants were then immediately confronted with the stressor, which was followed by an assessment of executive functions. Psychophysiological stress responses were assessed throughout the experiment (heart rate, heart-rate variability, salivary cortisol).

RESULTS

The tdcs did not modulate stress responses. Compared with both ctrl and tdcs interventions, bfb reduced physiological stress and improved executive functions after the stressor. The main finding revealed that bfb + tdcs was the most effective intervention, yielding greater reduction in psychological and physiological stress responses than bfb.

CONCLUSIONS

Combining preventive tDCS with BFB is a relevant interventional approach to reduce psychophysiological stress responses, hence offering a new and non-invasive treatment of stress-related disorders. Biofeedback may be particularly useful for preparing for an important stressful event when performance is decisive.

The Role of the Dorsolateral Prefrontal Cortex for Speech and Language Processing

This review article summarizes various functions of the dorsolateral prefrontal cortex (DLPFC) that are related to language processing. To this end, its connectivity with the left-dominant perisylvian language network was considered, as well as its interaction with other functional networks that, directly or indirectly, contribute to language processing. Language-related functions of the DLPFC comprise various aspects of pragmatic processing such as discourse management, integration of prosody, interpretation of nonliteral meanings, inference making, ambiguity resolution, and error repair. Neurophysiologically, the DLPFC seems to be a key region for implementing functional connectivity between the language network and other functional networks, including cortico-cortical as well as subcortical circuits. Considering clinical aspects, damage to the DLPFC causes psychiatric communication deficits rather than typical aphasic language syndromes. Although the number of well-controlled studies on DLPFC language functions is still limited, the DLPFC might be an important target region for the treatment of pragmatic language disorders.

The neurobiology of prefrontal transcranial direct current stimulation (tDCS) in promoting brain plasticity: A systematic review and meta-analyses of human and rodent studies

The neurobiological mechanisms underlying prefrontal transcranial direct current stimulation (tDCS) remain elusive. Randomized, sham-controlled trials in humans and rodents applying in vivo prefrontal tDCS were included to explore whether prefrontal tDCS modulates resting-state and event-related functional connectivity, neural oscillation and synaptic plasticity. Fifty studies were included in the systematic review and 32 in the meta-analyses. Neuroimaging meta-analysis indicated anodal prefrontal tDCS significantly enhanced bilateral median cingulate activity [familywise error (FWE)-corrected p < .005]; meta-regression revealed a positive relationship between changes in median cingulate activity after tDCS and current density (FWE-corrected p < .005) as well as electric current strength (FWE-corrected p < .05). Meta-analyses of electroencephalography and magnetoencephalography data revealed nonsignificant changes (ps > .1) in both resting-state and event-related oscillatory power across all frequency bands. Applying anodal tDCS over the rodent hippocampus/prefrontal cortex enhanced long-term potentiation and brain-derived neurotrophic factor expression in the stimulated brain regions (ps <.005). Evidence supporting prefrontal tDCS administration is preliminary; more methodologically consistent studies evaluating its effects on cognitive function that include brain activity measurements are needed.

Noninvasive brain stimulation to lateral prefrontal cortex alters the novelty of creative idea generation

Theories of the processes involved in creative cognition posit that cognitive control has a negative effect on creative idea generation but a positive effect on creative idea evaluation. Brain stimulation research has started to examine empirically the effects of cognitive control, with several reports of decreased cognitive control facilitating creative ideation. Such studies have shown how decreased cognitive control mechanisms facilitate creative idea generation, potentially by allowing participants access to less inhibited weaker-related associations, thereby increasing novelty. In the current study, we advance this line of work by investigating how cognitive control affects creative thinking, potentially inhibiting or facilitating novel idea generation based on task demands. Participants read sentences with the final word missing and were instructed to complete the sentence with an uncommon (but appropriate) ending. Participants performed this task while undergoing either anodal (excitatory), cathodal (inhibitory), or sham (control) transcranial direct current stimulation over their left prefrontal cortex. These responses were then rated for their novelty and appropriateness by an independent sample of raters. We found that anodal stimulation increased the appropriateness and decreased the novelty of participants' responses. Contrary to previous studies, we did not find that cathodal stimulation increased the novelty of participants' responses, which may be due to the nature of our task. Overall, we demonstrate how cognitive control mechanisms may inhibit novel idea generation.

[Review of cognitive enhancement techniques based on the combination of cognitive training and transcranial direct current stimulation]

Cognitive enhancement refers to the technology of enhancing or expanding the cognitive and emotional abilities of people without psychosis based on relevant knowledge of neurobiology. The common methods of cognitive enhancement include transcranial direct current stimulation (tDCS) and cognitive training (CT). tDCS takes effect quickly, with a short effective time, while CT takes longer to work, requiring several weeks of training, with a longer effective time. In recent years, some researchers have begun to use the method of tDCS combined with CT to regulate the cognitive function. This paper will sort out and summarize this topic from five aspects: perception, attention, working memory, decision-making and other cognitive abilities. Finally, the application prospect and challenges of technology are prospected.

Unskilled shooters improve both accuracy and grouping shot having as reference skilled shooters cortical area: An EEG and tDCS study

Transcranial direct current stimulation (tDCS) has been used as a non-invasive method for enhanced motor and cognitive abilities. However, no previous study has investigated if the tDCS application in unskilled shooters on cortical sites, selected based on the cortical activity of skilled shooters, improves the accuracy and shot grouping. Sixty participants were selected, which included 10 skilled shooters and 50 unskilled shooters. After we identified the right dorsolateral prefrontal cortex (DLPFC) as the area with the highest activity in skilled shooters, we applied anodal tDCS over the right DLPFC in the unskilled shooters under two conditions: sham-tDCS (placebo) and real-tDCS (anodal tDCS). We also analyzed electroencephalography. Our results indicated that anodal tDCS application enhanced the shot accuracy (p = 0.001). Furthermore, the beta power in the EEG recording was higher in the left DLPFC, left and right parietal cortex (p = 0,001) after applying anodal tDCS, while the low-gamma power was higher in the right DLPFC in sham-tDCS (p = 0.001) and right parietal cortex after anodal-tDCS (p = 0.001). Our findings indicate that anodal tDCS can improve accuracy and shot grouping when applied over the unskilled shooters' right DLPFC. Furthermore, beta and low-gamma bands are influenced by anodal tDCS over the right DLPFC, which may be predictive of skill improvement.

Educational fMRI: From the Lab to the Classroom

Functional MRI (fMRI) findings hold many potential applications for education, and yet, the translation of fMRI findings to education has not flowed. Here, we address the types of fMRI that could better support applications of neuroscience to the classroom. This 'educational fMRI' comprises eight main challenges: (1) collecting artifact-free fMRI data in school-aged participants and in vulnerable young populations, (2) investigating heterogenous cohorts with wide variability in learning abilities and disabilities, (3) studying the brain under natural and ecological conditions, given that many practical topics of interest for education can be addressed only in ecological contexts, (4) depicting complex age-dependent associations of brain and behaviour with multi-modal imaging, (5) assessing changes in brain function related to developmental trajectories and instructional intervention with longitudinal designs, (6) providing system-level mechanistic explanations of brain function, so that useful individualized predictions about learning can be generated, (7) reporting negative findings, so that resources are not wasted on developing ineffective interventions, and (8) sharing data and creating large-scale longitudinal data repositories to ensure transparency and reproducibility of fMRI findings for education. These issues are of paramount importance to the development of optimal fMRI practices for educational applications.