Dorsolateral Prefrontal Cortex and Task-Switching Performance: Effects of Anodal Transcranial Direct Current Stimulation

Boosting proactive motor control via statistical learning with brain stimulation

Visual statistical regularities are nested patterns of information extracted to build a predictive internal model that guides attentional and motor decisions. Here, we sought to understand the contributions of the left and right frontoparietal areas in modulating the effect of this expectancy implementation on premotor preparation. Healthy subjects were asked to detect a high-contrast stimulus target presented simultaneously with a distractor, with preceding color cues indicating, trial by trial, the pairing between the response hand and the upcoming stimuli locations. Performance was measured at baseline, and immediately after a one-session training on the task. During the training target locations appeared 75% of the time to the right of the distractor, a regularity unnoticed by participants. The training session was paired with unilateral transcranial random noise stimulation (tRNS) or sham stimulation over the left or right frontoparietal cortex in a counterbalanced design. Results showed a significant response bias in reaction times after training, with faster responses for targets to the right of the distractor. This bias was enhanced by right, but not left, frontoparietal stimulation, highlighting a hemispheric asymmetry in proactive motor control. The implicit nature of learning, as evidenced by subjects' unawareness of probability distributions, underscores how proactive motor control quickly adapts to statistical regularities. Results suggest a dominant role for the right hemisphere in mediating attentional learning effects, with implications for understanding lateralized functions in adaptation of the motor control.

Mitochondria and Cognition: An [18F]BCPP-EF Positron Emission Tomography Study of Mitochondrial Complex I Levels and Brain Activation During Task Switching

BACKGROUND

Mitochondrial complex I is the largest enzyme complex in the respiratory chain and can be noninvasively measured using [18F]BCPP-EF positron emission tomography (PET). Neurological conditions associated with mitochondria complex I pathology are also associated with altered blood oxygen level-dependent (BOLD) response and impairments in cognition. In this study, we aimed to investigate the relationship between mitochondrial complex I levels, cognitive function, and associated neural activity during task switching in healthy humans.

METHODS

Cognitively healthy adults (N = 23) underwent [18F]BCPP-EF PET scans and functional magnetic resonance imaging (fMRI) while performing a task-switching exercise. Task performance metrics included switch cost and switching accuracy. Data were analyzed using linear mixed-effects models and partial least squares regression (PLS-R).

RESULTS

We found significant positive associations between [18F]BCPP-EF volume of distribution (VT) and the task-switching fMRI response (β = 3.351, SE = 1.01, z = 3.249, p = .001). Positive Pearson's correlations between [18F]BCPP-EF VT and the fMRI response were observed in the dorsolateral prefrontal cortex (r = 0.61, p = .0019), insula (r = 0.46, p = .0264), parietal precuneus (r = 0.51, p = .0139), and anterior cingulate cortex (r = 0.45, p = .0293). [18F]BCPP-EF VT across task-relevant regions was associated with task switching accuracy (PLS-R, R2 = 0.48, root mean square error [RMSE] = 0.154, p = .011) and with switch cost (PLS-R, R2 = 0.38, RMSE = 0.07, p = .048).

CONCLUSIONS

Higher mitochondrial complex I levels may underlie an individual's ability to exhibit a stronger BOLD response during task switching and are associated with better task-switching performance. This provides the first evidence linking the BOLD response with mitochondrial complex I and suggests a possible biological mechanism for the aberrant BOLD response in conditions associated with mitochondrial complex I dysfunction that should be tested in future studies.

Cognitive Remediation in Patients With Bipolar Disorder: A Randomized Trial by Sequential tDCS and Navigated rTMS Targeting the Primary Visual Cortex

BACKGROUND

Non-invasive brain stimulation (NIBS), such as transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS), has emerged as a promising alternative in the precise treatment of clinical symptoms, such as the cognitive impairment of bipolar disorder (BD). Optimizing the neurocognitive effects by combining tDCS and rTMS to strengthen the clinical outcome is a challenging research issue.

OBJECTIVE

In this randomized, controlled trial, we first combined tDCS and neuronavigated rTMS targeting the V1 region to explore the efficacy on neurocognitive function in BD patients with depressive episodes.

METHODS

Eligible individuals (n = 105) were assigned into three groups, Group A (active tDCS-active rTMS), Group B (sham tDCS-active rTMS), and Group C (active tDCS-sham rTMS). All participants received 3-week treatment in which every participant received 15 sessions of stimulation through the study, 5 sessions every week, with tDCS treatment followed by neuronavigated rTMS every session. We evaluated the cognitive, emotional, and safety outcomes at week-0 (w0, baseline), week-3 (w3, immediately post-treatment), and week-8 (w8, follow-up period). The THINC-integrated tool (THINC-it), 17-item Hamilton Depression Rating Scale, and Young Mania Rating Scale were applied for evaluating the cognitive function and emotional state, respectively. Data were analyzed by repeated measure ANOVA and paired t-test.

RESULTS

Eventually, 32 patients in Group A, 27 in Group B, and 23 in Group C completed the entire treatment. Compared to Groups B and C, Group A showed greater improvement in Symbol Check items (Time and Accuracy) at W3 and Symbol Check Accuracy at W8 (p < 0.01). The W0-W3 analysis indicated a significant improvement in depressive symptoms in both Group A and Group B (p < 0.01). Additionally, neuroimaging data revealed increased activity in the calcarine sulcus in Group A, suggesting potential neuroplastic changes in the visual cortex following the electromagnetic stimulation.

CONCLUSIONS

These findings provide preliminary evidence that the combination of navigated rTMS with tDCS targeting V1 region may serve as a potential treatment strategy for improving cognitive impairment and depressive symptoms in BD patients.

TRIAL REGISTRATION

Clinical Trial Registry number: NCT05596461.

Work performance in industry: The impact of mental fatigue and a passive back exoskeleton on work efficiency

Mental fatigue (MF) is likely to occur in the industrial working population. However, the link between MF and industrial work performance has not been investigated, nor how this interacts with a passive lower back exoskeleton used during industrial work. Therefore, to elucidate its potential effect(s), this study investigated the accuracy of work performance and movement duration through a dual task paradigm and compared results between mentally fatigued volunteers and controls, with and without the exoskeleton. No main effects of MF and the exoskeleton were found. However, when mentally fatigued and wearing the exoskeleton, movement duration significantly increased compared to the baseline condition (β_MF:Exo = 0.17, p = .02, ω² = .03), suggesting an important interaction between the exoskeleton and one's psychobiological state. Importantly, presented data indicate a negative effect on production efficiency through increased performance time. Further research into the cognitive aspects of industrial work performance and human-exoskeleton interaction is therefore warranted.

Effects of anodal tDCS stimulation in predictable and unpredictable task switching performance: The possible involvement of the parietal cortex

Transcranial direct current stimulation (tDCS) has been used to explore the causal relationship between specific brain regions and task switching. However, most studies have focused on the frontal cortex, and only few have examined other related cortices, e.g., the parietal cortex. However, no prior study has systematically explored the tDCS-induced effect of the parietal cortex in different task switching types. Therefore, the current study mainly used the unilateral anodal-tDCS (a-tDCS) stimulation setting to investigate the possible involvement of the parietal cortex in predictable and unpredictable task switching. It was noted that compared with sham group, significantly higher switch cost reaction time of right anode tDCS (RA) group was found in predictable task but not unpredictable task. No interaction effect was observed between congruence and tDCS groups in predictable task. These findings suggested that a-tDCS over right parietal cortex could markedly decrease the predictable task-switching performance in both congruent and incongruent trials, and indicated that parietal cortex is more likely to be involved in the proactive cognitive processes, such as endogenous preparation.

Standard Non-Personalized Electric Field Modeling of Twenty Typical tDCS Electrode Configurations via the Computational Finite Element Method: Contributions and Limitations of Two Different Approaches

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation procedure to modulate cortical excitability and related brain functions. tDCS can effectively alter multiple brain functions in healthy humans and is suggested as a therapeutic tool in several neurological and psychiatric diseases. However, variability of results is an important limitation of this method. This variability may be due to multiple factors, including age, head and brain anatomy (including skull, skin, CSF and meninges), cognitive reserve and baseline performance level, specific task demands, as well as comorbidities in clinical settings. Different electrode montages are a further source of variability between tDCS studies. A procedure to estimate the electric field generated by specific tDCS electrode configurations, which can be helpful to adapt stimulation protocols, is the computational finite element method. This approach is useful to provide a priori modeling of the current spread and electric field intensity that will be generated according to the implemented electrode montage. Here, we present standard, non-personalized model-based electric field simulations for motor, dorsolateral prefrontal, and posterior parietal cortex stimulation according to twenty typical tDCS electrode configurations using two different current flow modeling software packages. The resulting simulated maximum intensity of the electric field, focality, and current spread were similar, but not identical, between models. The advantages and limitations of both mathematical simulations of the electric field are presented and discussed systematically, including aspects that, at present, prevent more widespread application of respective simulation approaches in the field of non-invasive brain stimulation.

Enhancement of task-switching performance with transcranial direct current stimulation over the right lateral prefrontal cortex

Switching between two or more tasks is a key component in our modern world. Task switching, however, requires time-consuming executive control processes and thus produces performance costs when compared to task repetitions. While executive control during task switching has been associated with activation in the lateral prefrontal cortex (lPFC), only few studies so far have investigated the causal relation between lPFC activation and task-switching performance by modulating lPFC activation. In these studies, the results of lPFC modulation were not conclusive or limited to the left lPFC. In the present study, we aimed to investigate the effect of non-invasive transcranial direct current stimulation [tDCS; anodal tDCS (1 mA, 20 min) vs. cathodal tDCS (1 mA, 20 min) vs. sham tDCS (1 mA, 30 s)] over the right inferior frontal junction on task-switching performance in a well-established task-switching paradigm. In response times, we found a significant effect of tDCS Condition (atDCS, ctDCS vs. sham) on task-switching costs, indicating the modulation of task-switching performance by tDCS. In addition, we found a task-unspecific tDCS Condition effect in the first experimental session, in which participants were least familiar with the task, indicating a general enhancement of task performance in both task repetitions and task-switching trials. Taken together, our study provides evidence that the right lPFC is involved in task switching as well as in general task processing. Further studies are needed to investigate whether these findings can be translated into clinically relevant improvement in older subjects or populations with executive function impairment.

Anodal Transcranial Direct Current Stimulation-Induced Effects Over the Right Dorsolateral Prefrontal Cortex: Differences in the Task Types of Task Switching

Transcranial direct current stimulation (tDCS) has been previously used to investigate the causal relationships between the dorsolateral prefrontal cortex (DLPFC) and task switching but has delivered inconclusive results that may be due to different switching tasks involving different cognitive control processes. In the current study, we manipulated task types and task predictability to investigate the role of DLPFC in task-switching performances. Notably, we distinguished the specific effects of anodal-tDCS on two types of tasks (parity/magnitude and parity/vowel-consonant tasks). Forty-eight participants were randomly assigned to four task groups as follows; Group I who was assigned right anode (RA) parity/magnitude tasks, Group II who were assigned sham parity/magnitude tasks, Group III who were assigned RA parity/vowel-consonant tasks, and Group IV who were assigned sham parity/vowel-consonant tasks. Participants were asked to complete both predictable and unpredictable tasks. In the parity/magnitude task, we demonstrated a lower switch cost for the RA group compared to the sham group for unpredictable tasks. In contrast, in the parity/vowel-consonant task, the switch cost was higher for the RA group compared to the sham group for unpredictable and predictable tasks. These findings confirmed an anodal-tDCS-induced effect over the right DLPFC both in the parity/magnitude and parity/vowel-consonant tasks. Our data indicated that anodal tDCS may have a stronger influence on task-switching performance over the right DLPFC by changing the irrelevant task-set inhibition process. Also, the right DLPFC is unlikely to act by performing exogenous adjustment of predictable task switching.