Examining the Interactions Between Expectations and tDCS Effects on Motor and Cognitive Performance

The Effect of Brain Anodal and Cathodal Transcranial Direct Current Stimulation on Psychological Refractory Period at Different Stimulus-Onset Asynchrony in Non-Fatigue and Mental Fatigue Conditions

The psychological refractory period (PRP) effect occurs when two stimuli that require separate responses are presented sequentially, particularly with a short and variable time interval between them. Fatigue is a suboptimal psycho-physiological state that leads to changes in strategies. In recent years, numerous studies have investigated the effects of transcranial direct current stimulation (tDCS) on motor control. The present study aimed to investigate the effects of two tDCS methods, anodal and cathodal, on PRP in ten different conditions of stimulus-onset asynchronies (SOAs) under non-fatigue and mental fatigue conditions. The participants involved 39 male university students aged 19 to 25 years. In the pre-test, they were assessed using the PRP measurement tool under both non-fatigue and mental fatigue conditions. The mental fatigue was induced by a 30-min Stroop task. The test consisted of two stimuli with different SOAs (50, 75, 100, 150, 300, 400, 600, 900, 1200, and 1500 ms). The first was a visual stimulus with three choices (letters A, B, and C). After a random SOA, the second stimulus, a visual stimulus with three choices (colors red, yellow, and blue), was presented. Subsequently, participants were randomly assigned to the anodal, cathodal, and sham stimulation groups and underwent four consecutive sessions of tDCS stimulation. In the anodal and cathodal stimulation groups, 20 min of tDCS stimulation were applied to the PLPFC area in each session, while in the sham group, the stimulation was artificially applied. All participants were assessed using the same measurement tools as in the pre-test phase, in a post-test phase one day after the last stimulation session, and in a follow-up phase four days after that. Inferential statistics include mixed ANOVA, one-way ANOVA, independent, and dependent t-tests. The findings indicated that the response time to the second stimulus was longer at lower SOAs. However, there was no significant difference between the groups in this regard. Additionally, there was no significant difference in response time to the second stimulus between the fatigue and non-fatigue conditions, or between the groups. Therefore, tDCS had no significant effect. There was a significant difference between mental fatigue and non-fatigue conditions in the psychological refractory period. Moreover, at lower SOAs, the PRP was longer than at higher SOAs. In conditions of fatigue, the active stimulation groups (anodal and cathodal) performed better than the sham stimulation group at higher SOAs. Considering the difference in response to both stimuli at different SOAs, some central aspects of the response can be simultaneously parallel. Fatigue also affects parallel processing. This study supports the response integration phenomenon in PRP, which predicts that there will be an increase in response time to the first stimulus as the interval between the presentation of the two stimuli increases. This finding contradicts the bottleneck model. In this study, the effectiveness of cathodal and anodal tDCS on response time to the second stimulus and PRP was found to be very small.

Noninvasive brain stimulation in autism: review and outlook for personalized interventions in adult patients

Noninvasive brain stimulation (NIBS) has been increasingly investigated during the last decade as a treatment option for persons with autism spectrum disorder (ASD). Yet, previous studies did not reach a consensus on a superior treatment protocol or stimulation target. Persons with ASD often suffer from social isolation and high rates of unemployment, arising from difficulties in social interaction. ASD involves multiple neural systems involved in perception, language, and cognition, and the underlying brain networks of these functional domains have been well documented. Aiming to provide an overview of NIBS effects when targeting these neural systems in late adolescent and adult ASD, we conducted a systematic search of the literature starting at 631 non-duplicate publications, leading to six studies corresponding with inclusion and exclusion criteria. We discuss these studies regarding their treatment rationale and the accordingly chosen methodological setup. The results of these studies vary, while methodological advances may allow to explain some of the variability. Based on these insights, we discuss strategies for future clinical trials to personalize the selection of brain stimulation targets taking into account intersubject variability of brain anatomy as well as function.

Effect of a Neuromodulation Protocol Associated With Sports Training on the Precision Sports Performance of a Wheelchair Basketball Para-Athlete: A Case Study

OBJECTIVE

To investigate whether transcranial direct-current stimulation (tDCS) optimizes the performance of a wheelchair basketball player on precision tasks.

METHODS

A right-handed wheelchair basketball player (1.5 points functional class) with myelomeningocele (low lumbar level) participated in this case study. The tDCS neuromodulation protocol was applied throughout 10 interventions of 20 minutes with a current intensity of 2 mA, simultaneously with sport-specific training, 3 times a week for 4 weeks. Anodic stimulation was performed on the right cerebellar hemisphere (CB2) and cathodic stimulation in the left dorsolateral prefrontal cortex. A control participant was submitted to a sham-tDCS stimulation protocol for the same period. Functional performance was assessed before the intervention and after the 5th and 10th interventions using "pass accuracy," "free-throw shooting," and "spot shot" tests. Outcome measures were compared using percentage differences between preintervention, intermediate intervention, and postintervention values.

RESULTS

There was a gradual increase in the athlete's total and average scores in all tests performed, with an overall improvement of 78% between the baseline and final assessments, while the control participant had an overall improvement of 6.5%.

CONCLUSION

The tDCS protocol was effective in improving performance in precision activities in a wheelchair basketball player.

Placebo effects of transcranial direct current stimulation on motor skill acquisition

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique used in neurorehabilitation to enhance motor training. However, its benefits to motor training can be difficult to reproduce across research studies. It is possible that the observed benefits of tDCS are not directly related to the intervention itself but rather to the brain-mind responses elicited by the treatment context, commonly known as a placebo effect. This study investigated the presence of a placebo effect of tDCS on motor training and explored potential underlying factors. Sixty-eight participants who were right-handed were randomly assigned to active tDCS, sham tDCS, or a no-stimulation control group. Double-blind active or sham tDCS was applied to the right primary motor cortex, while the unblinded control group received no stimulation. All participants completed 30 training trials of a functional upper-extremity motor task. Participants' beliefs of tDCS, along with their prior knowledge of tDCS, were also collected. There was no significant difference in the amount of improvement on the motor task between the active and sham tDCS groups; however, both active and sham tDCS groups improved more than the control group, indicating a placebo effect. More motor task improvement was also associated with higher beliefs of tDCS (regardless of whether active or sham tDCS was received). This demonstrates a measurable placebo effect of tDCS on motor training, driven at least in part by treatment expectations or beliefs. Future tDCS studies should control for beliefs and other placebo-related factors.

Reverse effect of home-use binaural beats brain stimulation

Binaural beats brain stimulation is a popular strategy for supporting home-use cognitive tasks. However, such home-use brain stimulation may be neutral to cognitive processes, and any intellectual improvement may be only a placebo effect. Thus, without belief in it, it may bring no benefits. Here we test 1000 individuals at their homes as they perform a two-part fluid intelligence test. Some took the second part listening to binaural beats, while others took it in silence or listening to other sounds. The binaural beats group was divided into three subgroups. The first one was informed that they would listen to sounds that improve the brain's work, the second that neutral sounds, and the third that some sounds the nature of which was not defined. We found that listening to binaural beats was not neutral, as it dramatically deteriorated the score irrespective of the condition. Silence or other sounds had no effect. Thus, home-use binaural beats brain stimulation brings reverse effects to those assumed: instead of supporting the effectiveness of cognitive activities, it may weaken them.

Mind matters: A narrative review on affective state-dependency in non-invasive brain stimulation

Variability in findings related to non-invasive brain stimulation (NIBS) have increasingly been described as a result of differences in neurophysiological state. Additionally, there is some evidence suggesting that individual differences in psychological states may correlate with the magnitude and directionality of effects of NIBS on the neural and behavioural level. In this narrative review, it is proposed that the assessment of baseline affective states can quantify non-reductive properties which are not readily accessible to neuroscientific methods. Particularly, affective-related states are theorized to correlate with physiological, behavioural and phenomenological effects of NIBS. While further systematic research is needed, baseline psychological states are suggested to provide a complementary cost-effective source of information for understanding variability in NIBS outcomes. Implementing measures of psychological state may potentially contribute to increasing the sensitivity and specificity of results in experimental and clinical NIBS studies.

Can brain stimulation enhance cognition in clinical populations? A critical review

Many psychiatric and neurological conditions are associated with cognitive impairment for which there are very limited treatment options. Brain stimulation methodologies show promise as novel therapeutics and have cognitive effects. Electroconvulsive therapy (ECT), known more for its related transient adverse cognitive effects, can produce significant cognitive improvement in the weeks following acute treatment. Transcranial magnetic stimulation (TMS) is increasingly used as a treatment for major depression and has acute cognitive effects. Emerging research from controlled studies suggests that repeated TMS treatments may additionally have cognitive benefit. ECT and TMS treatment cause neurotrophic changes, although whether these are associated with cognitive effects remains unclear. Transcranial electrical stimulation methods including transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS) are in development as novel treatments for multiple psychiatric conditions. These treatments may also produce cognitive enhancement particularly when stimulation occurs concurrently with a cognitive task. This review summarizes the current clinical evidence for these brain stimulation treatments as therapeutics for enhancing cognition. Acute, or short-lasting, effects as well as longer-term effects from repeated treatments are reviewed, together with potential putative neural mechanisms. Areas of future research are highlighted to assist with optimization of these approaches for enhancing cognition.

Five-Session Dual-Transcranial Direct Current Stimulation With Task-Specific Training Does Not Improve Gait and Lower Limb Performance Over Training Alone in Subacute Stroke: A Pilot Randomized Controlled Trial

OBJECTIVE

To determine the effect of five-session dual-transcranial direct current stimulation (dual-tDCS) combined with task-specific training on gait and lower limb motor performance in individuals with subacute stroke.

MATERIALS AND METHODS

Twenty-five participants who had a stroke in the subacute phase with mild motor impairment were recruited, randomized, and allocated into two groups. The active group (n = 13) received dual-tDCS with anodal over the lesioned hemisphere M1 and cathodal over the nonlesioned hemisphere, at 2 mA for 20 min before training for five consecutive days, while the sham group (n = 12) received sham mode before training. Gait speed as a primary outcome, temporospatial gait variables, lower-limb functional tasks (sit-to-stand and walking mobility), and muscle strength as secondary outcomes were collected at preintervention and postintervention (day 5), one-week follow-up, and one-month follow-up.

RESULTS

The primary outcome and most of the secondary outcomes were improved in both groups, with no significant difference between the two groups, and most of the results indicated small to moderate effect sizes of active tDCS compared to sham tDCS.

CONCLUSION

The combined intervention showed no benefit over training alone in improving gait variables and lower-limb performance. However, some performances were saturated at some point, as moderate to high function participants were recruited in the present study. Future studies should consider recruiting participants with more varied motor impairment levels and may need to determine the optimal stimulation protocols and parameters to improve gait and lower-limb performance.

Modulating Cognitive–Motor Multitasking with Commercial-off-the-Shelf Non-Invasive Brain Stimulation

One growing area of multitasking research involves a focus on performing cognitive and motor tasks in tandem. In these situations, increasing either cognitive or motor demands has implications for performance in both tasks, an effect which is thought to be due to competing neural resources. Separate research suggests that non-invasive brain stimulation may offer a means to mitigate performance decrements experienced during multitasking. In the present study, we investigated the degree to which a commercially available non-invasive brain stimulation device (Halo Sport) alters balance performance in the presence of different types of cognitive demands. Specifically, we tested if performing a secondary cognitive task impacts postural sway in healthy young adults and if we could mitigate this impact using transcranial direct current stimulation (tDCS) applied over the primary motor cortex. Furthermore, we included conditions of unstable and stable surfaces and found that lower surface stability increased postural sway. In addition, we found that cognitive load impacted postural sway but in the opposite pattern we had anticipated, with higher sway found in the single-task control condition compared to executive function conditions. Finally, we found a small but significant effect of tDCS on balance with decreased sway for active (compared to sham) tDCS.

Blinding in tDCS Studies: Correct End-of-Study Guess Does Not Moderate the Effects on Associative and Working Memory

Transcranial direct current stimulation (tDCS) has become a valuable tool in cognitive neuroscience research as it enables causal inferences about neural underpinnings of cognition. However, studies using tDCS to modulate cognitive functions often yield inconsistent findings. Hence, there is an increasing interest in factors that may moderate the effects, one of which is the participants’ beliefs of the tDCS condition (i.e., real or sham) they received. Namely, whether participants’ correct guessing of sham condition may lead to false-positive tDCS effects. In this study, we aimed to explore if participants’ beliefs about received stimulation type (i.e., the success of blinding) impacted their task performance in tDCS experiments on associative (AM) and working memory (WM). We analyzed data from four within-subject, sham-controlled tDCS memory experiments (N = 83) to check if the correct end-of-study guess of sham condition moderated tDCS effects. We found no evidence that sham guessing moderated post-tDCS memory performance in experiments in which tDCS effects were observed as well as in experiments that showed null effects of tDCS. The results suggest that the correct sham guessing (i.e., placebo-like effect) is unlikely to influence the results in tDCS memory experiments. We discuss the results in light of the growing debate about the relevance and effectiveness of blinding in brain stimulation research.

The effects of monophasic anodal transcranial pulsed current stimulation on corticospinal excitability and motor performance in healthy young adults: A randomized double-blind sham-controlled study

INTRODUCTION

Transcranial pulsed current stimulation (tPCS) could be used to deliver electrical pulses at different frequencies to entrain the cortical neurons of the brain. Frequency dependence of these pulses in the induction of changes in corticospinal excitability (CSE) has not been reported.

OBJECTIVE

We aimed to assess the effect of anodal tPCS (a-tPCS) at theta (4 Hz), and gamma (75 Hz) frequencies on CSE as assessed by the peak-to-peak amplitude of transcranial magnetic stimulation (TMS) induced motor evoked potentials (MEPs) and motor performance.

METHOD

In a randomized double-blinded sham-controlled cross over design study, seventeen healthy participants attended three experimental sessions and received either a-tPCS at 4 Hz, 75 Hz, or sham a-tPCS with 1.5 mA for 15 min. The amplitude of TMS induced resting MEPs and time for completion of the grooved pegboard test were recorded at baseline, immediately after, and 30-min after a-tPCS.

RESULTS

Both a-tPCS at 75 Hz and 4 Hz showed significantly increased CSE compared to sham. The a-tPCS at 75 Hz induced significantly higher CSE changes compared to 4 Hz. There was a significant increase in intracortical facilitation and a significant reduction in short-interval intra-cortical inhibition with both 4 and 75 Hz stimulation. However, the inhibition and facilitation did not correlate with CSE. Motor performance was unaffected by the interventions.

CONCLUSION

The high CSE changes in M1 in a-tPCS at 75 Hz provides an initial understanding of the frequency-specific effect of a-tPCS. More research is needed to establish this concept and to assess its behavioural relevance.

The Role of Expectation and Beliefs on the Effects of Non-Invasive Brain Stimulation

Non-invasive brain stimulation (NIBS) techniques are used in clinical and cognitive neuroscience to induce a mild magnetic or electric field in the brain to modulate behavior and cortical activation. Despite the great body of literature demonstrating promising results, unexpected or even paradoxical outcomes are sometimes observed. This might be due either to technical and methodological issues (e.g., stimulation parameters, stimulated brain area), or to participants’ expectations and beliefs before and during the stimulation sessions. In this narrative review, we present some studies showing that placebo and nocebo effects, associated with positive and negative expectations, respectively, could be present in NIBS trials, both in experimental and in clinical settings. The lack of systematic evaluation of subjective expectations and beliefs before and after stimulation could represent a caveat that overshadows the potential contribution of placebo and nocebo effects in the outcome of NIBS trials.

Is the "end-of-study guess" a valid measure of sham blinding during transcranial direct current stimulation?

Studies using transcranial direct current stimulation (tDCS) typically incorporate a fade‐in, short‐stimulation, fade‐out sham (placebo) protocol, which is assumed to be indistinct from a 10–30 min active protocol on the scalp. However, many studies report that participants can dissociate active stimulation from sham, even during low‐intensity 1 mA currents. We recently identified differences in the perception of an active (10 min of 1 mA) and a sham (20 s of 1 mA) protocol that lasted for 5 min after the cessation of sham. In the present study we assessed whether delivery of a higher‐intensity 2 mA current would exacerbate these differences. Two protocols were delivered to 32 adults in a double‐blinded, within‐subjects design (active: 10 min of 2 mA, and sham: 20 s of 2 mA), with the anode over the left primary motor cortex and the cathode on the right forehead. Participants were asked “Is the stimulation on?” and “How sure are you?” at 30 s intervals during and after stimulation. The differences between active and sham were more consistent and sustained during 2 mA than during 1 mA. We then quantified how well participants were able to track the presence and absence of stimulation (i.e. their sensitivity) during the experiment using cross‐correlations. Current strength was a good classifier of sensitivity during active tDCS, but exhibited only moderate specificity during sham. The accuracy of the end‐of‐study guess was no better than chance at predicting sensitivity. Our results indicate that the traditional end‐of‐study guess poorly reflects the sensitivity of participants to stimulation, and may not be a valid method of assessing sham blinding.

Effects of transcranial direct current stimulation over frontal, parietal and cerebellar cortex for cognitive function during fasting in healthy adults

Background

Transcranial direct current stimulation (tDCS) is a neuromodulation tool used to modify the cognitive function in subjects. There is a paucity of data on tDCS' effect on cognitive function during Ramadan fasting. This paper aims to assess the effect of tDCS of three brain areas, including the right dorsolateral prefrontal cortex (DLPFC), posterior parietal cortex (PPC), and cerebellum on cognitive function, and obtain safety data in healthy adults during Ramadan fasting.

Methods and material

A total of 42 healthy, right-handed participants were randomly assigned to one of the 6 stimulation groups: active (anodal)-tDCS of right DLPFC, PPC, and cerebellum; or sham for DLPFC, PPC, and cerebellum after 8 h of fasting for Ramadan. Safety data and cognitive function, such as attention-switching tasks (AST), were obtained by employing the Cambridge Neuropsychological Test Automated Battery (CANTAB) before and after each tDCS session. The cognitive function outcome variables were the response time and the percentage of correct answers in AST. For sham stimulation, the placement of the electrodes was the same as for the active stimulation.

Results

An improvement in performance time in attention tasks was observed; however, it did not reach a significant level after anodal stimulation of the DLPFC, PPC, and cerebellum. Overall, there were no statistically significant differences between the active and sham tDCS groups in cognitive function. There were no significant side effects of tDCS during fasting for any group.

Conclusions

Our data suggest that there are variable effects of tDCS on attention tasks during Ramadan fasting. TDCS appears to be safe, well-tolerated and adhered to the international standard of safety in the local population during Ramadan fasting. Further large sample size studies should be conducted to validate the current study findings and reach better conclusions.