Interaction of task-related learning and transcranial direct current stimulation of the prefrontal cortex in modulating executive functions

The effect of transcranial direct current stimulation associated with video game training on the postural balance of older women in the community: A blind, randomized, clinical trial

BACKGROUND

Falls are frequent in older adults and can cause trauma, injury, and death. Fall prevention with virtual reality presents good results in improving postural control. Transcranial Direct Current Stimulation (tDCS) has been used with the same aim; however, the combination of the two techniques has still been little studied.

PURPOSE

To assess whether tDCS can enhance the effect of video game training (VGT) on improving the postural balance of healthy older women.

METHOD

A blinded, randomized, controlled clinical trial was conducted with 57 older women who were randomized to three balance training groups: Control Group (VGT), Anodal Group (VGT combined with anodic tDCS-atDCS), and Sham Group (VGT combined with sham tDCS-stDCS). Balance training was performed twice a week for four weeks, totalizing eight 20-min sessions using VGT associated with tDCS. Postural balance was assessed pre-and post-training and 30 days after the end of the eight sessions using the Mini-Balance Evaluation Systems Test.

RESULTS

Compared to pre-intervention the Mini BEST test increased similarly in the three groups in post-intervention (control: pre 23.7 ± 2.8 to post 27.0 ± 2.2; anodal: pre 24.4 ± 1 to post 27.7 ± 0.8 and sham: pre 24.2 ± 1.9 to post 26.5 ± 1.6; p < 0.001) and follow-up (control: pre 23.7 ± 2.8 to follow-up 26.8 ± 2.3; anodal: pre 24.4 ± 1 to follow-up 27.3 ± 1.4 and sham: pre 24.2 ± 1.9 to follow-up 26.8 ± 1.5; p < 0.001).

CONCLUSION

There was an improvement in the postural balance of the three training groups that were independent of tDCS.

DISCUSSION

Some studies have shown the positive tDCS effects associated with other tasks to improve balance. However, these results convey the effects of only anodic-tDCS compared to sham-tDCS. Possibly, the effect of VGT surpassed the tDCS effects, promoting a ceiling effect from the combination of these two therapies. However, studies with other therapies combined with tDCS for older adults deserve to be investigated, as well as in frail older people.

Dissociable effects of music and white noise on conflict-induced behavioral adjustments

Auditory stimuli, encompassing a continually expanding collection of musical genres and sonic hues, present a safe and easily administrable therapeutic option for alleviating cognitive deficits associated with neuropsychological disorders, but their effects on executive control are yet to be completely understood. To better understand how the processing of certain acoustic properties can influence conflict processing, we had a large of cohort of undergraduate students complete the Stroop colour and word test in three different background conditions: classical music, white noise, and silence. Because of pandemic guidelines and the necessity to run the experiment remotely, participants also completed the Wisconsin card sorting test (WCST), so that the reliability and consistency of acquired data could be assessed. We found that white noise, but not classical music increased the response time difference between congruent (low conflict) and incongruent (high conflict) trials (conflict cost), hence impairing performance. Results from the WCST indicated that home-based data collection was reliable, replicating a performance bias reported in our previous laboratory-based experiments. Both the auditory stimuli were played at a similar intensity, thus their dissociable effects may have resulted from differing emotional responses within participants, where white noise, but not music elicited a negative response. Integrated with previous literature, our findings indicate that outside of changes in tempo and valence, classical music does not affect cognitive functions associated with conflict processing, whilst white noise impairs these functions in a manner similar to other stressors, and hence requires further research before its implementation into neuropsychiatric care.

Dimension of visual information interacts with working memory in monkeys and humans

Humans demonstrate behavioural advantages (biases) towards particular dimensions (colour or shape of visual objects), but such biases are significantly altered in neuropsychological disorders. Recent studies have shown that lesions in the prefrontal cortex do not abolish dimensional biases, and therefore suggest that such biases might not depend on top-down prefrontal-mediated attention and instead emerge as bottom-up processing advantages. We hypothesised that if dimensional biases merely emerge from an enhancement of object features, the presence of visual objects would be necessary for the manifestation of dimensional biases. In a specifically-designed working memory task, in which macaque monkeys and humans performed matching based on the object memory rather than the actual object, we found significant dimensional biases in both species, which appeared as a shorter response time and higher accuracy in the preferred dimension (colour and shape dimension in humans and monkeys, respectively). Moreover, the mnemonic demands of the task influenced the magnitude of dimensional bias. Our findings in two primate species indicate that the dichotomy of top-down and bottom-up processing does not fully explain the emergence of dimensional biases. Instead, dimensional biases may emerge when processed information regarding visual object features interact with mnemonic and executive functions to guide goal-directed behaviour.

Executive function in obesity and anorexia nervosa: Opposite ends of a spectrum of disordered feeding behaviour?

Higher-order executive functions such as decision-making, cognitive flexibility and behavioural control are critical to adaptive success in all aspects of life, including the maintenance of a healthy body weight by regulating food intake. Performance on tasks designed to assess these aspects of cognition is impaired in individuals with obesity and anorexia nervosa (AN); conditions at either end of a spectrum of body weight disturbance. While the conceptualisation of obesity and AN as mirror images of each other makes some sense from a metabolic point of view, whether or not these conditions also reflect opposing states of executive function is less clear. Here, we review evidence from neurocognitive and neuroimaging studies to compare the direction and extent of executive dysfunction in subjects with obesity and AN and how these are underpinned by changes in structure and function of subregions of the prefrontal cortex (PFC). Both conditions of extreme body weight disturbance are associated with impaired decision-making and cognitive inflexibility, however, impulsive behaviour presents in opposing directions; obesity being associated with reduced behavioural control and AN being associated with elevated control over behaviour with respect to food and feeding. Accordingly, the subregions of the PFC that guide inhibitory control and valuation of action outcomes (dorsolateral prefrontal cortex and orbitofrontal cortex) show opposite patterns of activation in subjects with obesity compared to those with AN, whereas the subregions implicated in cognitive and behavioural flexibility (ventromedial prefrontal cortex and anterior cingulate cortex) show alterations in the same direction in both conditions but with differential extent of dysfunction. We synthesise these findings in the context of the utility of animal models of obesity and AN to interrogate the detail of the neurobiological contributions to cognition in patient populations and the utility of such detail to inform future treatment strategies that specifically target executive dysfunction.

Investigating the sex-dependent effects of prefrontal cortex stimulation on response execution and inhibition

Context-dependent execution or inhibition of a response is an important aspect of executive control, which is impaired in neuropsychological and addiction disorders. Transcranial direct current stimulation (tDCS) of the dorsolateral prefrontal cortex (DLPFC) has been considered a remedial approach to address deficits in response control; however, considerable variability has been observed in tDCS effects. These variabilities might be related to contextual differences such as background visual-auditory stimuli or subjects' sex. In this study, we examined the interaction of two contextual factors, participants' sex and background acoustic stimuli, in modulating the effects of tDCS on response inhibition and execution. In a sham-controlled and cross-over (repeated-measure) design, 73 participants (37 females) performed a Stop-Signal Task in different background acoustic conditions before and after tDCS (anodal or sham) was applied over the DLPFC. Participants had to execute a speeded response in Go trials but inhibit their response in Stop trials. Participants' sex was fully counterbalanced across all experimental conditions (acoustic and tDCS). We found significant practice-related learning that appeared as changes in indices of response inhibition (stop-signal reaction time and percentage of successful inhibition) and action execution (response time and percentage correct). The tDCS and acoustic stimuli interactively influenced practice-related changes in response inhibition and these effects were uniformly seen in both males and females. However, the effects of tDCS on response execution (percentage of correct responses) were sex-dependent in that practice-related changes diminished in females but heightened in males. Our findings indicate that participants' sex influenced the effects of tDCS on the execution, but not inhibition, of responses.

Effects of single-session transcranial direct current stimulation on reactive response inhibition

Transcranial direct current stimulation (tDCS) is widely used to explore the role of various cortical regions for reactive response inhibition. In recent years, tDCS studies reported polarity-, time- and stimulation-site dependent effects on response inhibition. Given the large parameter space in which study designs, tDCS procedures and task procedures can differ, it is crucial to systematically explore the existing tDCS literature to increase the current understanding of potential modulatory effects and limitations of different approaches. We performed a systematic review on the modulatory effects of tDCS on response inhibition as measured by the Stop-Signal Task. The final dataset shows a large variation in methodology and heterogeneous effects of tDCS on performance. The most consistent result across studies is a performance enhancement due to anodal tDCS over the right prefrontal cortex. Partially sub-optimal choices in study design, methodology and lacking consistency in reporting procedures may impede valid conclusions and obscured the effects of tDCS on response inhibition in some previous studies. Finally, we outline future directions and areas to improve research.

The effects of emotional stimuli and oxytocin on inhibition ability and response execution in macaque monkeys

Social and emotional content of environmental stimuli influence executive control of behavior. There has been a great variability in the behavioral effects of emotional stimuli in humans. These variabilities might arise from other contextual factors, such as specific task demands or natural hormones, which potentially interact with emotional stimuli in modulating executive functions. This study aimed at examining the effects of social-emotional visual stimuli and a natural hormone (oxytocin) on inhibition ability and response execution of macaque monkeys. In a crossover design, monkeys received inhaled oxytocin or its vehicle before performing a stop-signal task in which they must respond rapidly to a visual go-cue in Go trials but inhibit the initiated response following the onset of a stop-cue in Stop trials. The social-emotional content (negative, positive or neutral) of the go-cue was changed trial-by-trial. We found that monkeys' inhibition ability was significantly influenced by the social-emotional content of stimuli and appeared as an enhanced inhibition ability when monkeys were exposed to negative stimuli. However, response execution was not influenced by the emotional content of stimuli in the current or preceding trials. The same dose of oxytocin, which modulated working memory in monkeys, had no significant effect on the inhibition ability, but significantly decreased monkeys' response time regardless of the stimulus valence. Our findings indicate that emotional stimuli, valence dependently, influence monkeys' inhibition ability but not their response execution and suggest that oxytocin might attenuate reorientation of cognitive resources to the task irrelevant emotional information.

Effects of tDCS during inhibitory control training on performance and PTSD, aggression and anxiety symptoms: a randomized-controlled trial in a military sample

BACKGROUND

Post-traumatic stress disorder (PTSD), anxiety, and impulsive aggression are linked to transdiagnostic neurocognitive deficits. This includes impaired inhibitory control over inappropriate responses. Prior studies showed that inhibitory control can be improved by modulating the right inferior frontal gyrus (IFG) with transcranial direct current stimulation (tDCS) in combination with inhibitory control training. However, its clinical potential remains unclear. We therefore aimed to replicate a tDCS-enhanced inhibitory control training in a clinical sample and test whether this reduces stress-related mental health symptoms.

METHODS

In a preregistered double-blind randomized-controlled trial, 100 active-duty military personnel and post-active veterans with PTSD, anxiety, or impulsive aggression symptoms underwent a 5-session intervention where a stop-signal response inhibition training was combined with anodal tDCS over the right IFG for 20 min at 1.25 mA. Inhibitory control was evaluated with the emotional go/no-go task and implicit association test. Stress-related symptoms were assessed by self-report at baseline, post-intervention, and after 3-months and 1-year follow-ups.

RESULTS

Active relative to sham tDCS neither influenced performance during inhibitory control training nor on assessment tasks, and did also not significantly influence self-reported symptoms of PTSD, anxiety, impulsive aggression, or depression at post-assessment or follow-up.

CONCLUSIONS

Our results do not support the idea that anodal tDCS over the right IFG at 1.25 mA enhances response inhibition training in a clinical sample, or that this tDCS-training combination can reduce stress-related symptoms. Applying different tDCS parameters or combining tDCS with more challenging tasks might provide better conditions to modulate cognitive functioning and stress-related symptoms.

Dimensional bias and adaptive adjustments in inhibitory control of monkeys

Humans and macaque monkeys, performing a Wisconsin Card Sorting Test (WCST), show a significant behavioral bias to a particular sensory dimension (e.g. color or shape); however, lesions in prefrontal cortical regions do not abolish the dimensional biases in monkeys and, therefore, it has been proposed that these biases emerge in earlier stages of visual information processing. It remains unclear whether such dimensional biases are unique to the WCST, in which attention-shifting between dimensions are required, or affect other aspects of executive functions such as 'response inhibition' and 'error-induced behavioral adjustments'. To address this question, we trained six monkeys (Macaca mulatta) to perform a stop-signal task in which they had to inhibit their response when an instruction for inhibition was given by changing the color or shape of a visual stimulus. Stop Signal Reaction Time (SSRT) is an index of inhibitory processes. In all monkeys, SSRT was significantly shorter, and the probability of a successful inhibition was significantly higher, when a change in the shape dimension acted as the stop-cue. Humans show a response slowing following a failure in response inhibition and also adapt a proactive slowing after facing demands for response inhibition. We found such adaptive behavioral adjustments, with the same pattern, in monkeys' behavior; however, the dimensional bias did not modulate them. Our findings, showing dimensional bias in monkey, with the same pattern, in two different executive control tasks support the hypothesis that the bias to shape dimension emerges in early stages of visual information processing.

Color-hierarchies in executive control of monkeys' behavior

Processing advantages for particular colors (color-hierarchies) influence emotional regulation and cognitive functions in humans and manifest as an advantage of the red color, compared with the green color, in triggering response inhibition but not in response execution. It remains unknown how such color-hierarchies emerge in human cognition and whether they are the unique properties of human brain with advanced trichromatic vision. Dominant models propose that color-hierarchies are formed as experience-dependent learning that associates various colors with different human-made conventions and concepts (e.g., traffic lights). We hypothesized that if color-hierarchies modulate cognitive functions in trichromatic nonhuman primates, it would indicate a preserved neurobiological basis for such color-hierarchies. We trained six macaque monkeys to perform cognitive tasks that required behavioral control based on colored cues. Color-hierarchies significantly influenced monkeys' behavior and appeared as an advantage of the red color, compared to the green, in triggering response inhibition but not response execution. For all monkeys, the order of color-hierarchies, in response inhibition and also execution, was similar to that in humans. In addition, the cognitive effects of color-hierarchies were not limited to the trial in which the colored cues were encountered but also persisted in the following trials in which there was no colored cue on the visual scene. These findings suggest that color-hierarchies are not resulting from association of colors with human-made conventions and that simple processing advantage in retina or early visual pathways does not explain the cognitive effects of color-hierarchies. The discovery of color-hierarchies in cognitive repertoire of monkeys indicates that although the evolution of humans and monkeys diverged in about 25 million years ago, the color-hierarchies are evolutionary preserved, with the same order, in trichromatic primates and exert overarching effects on the executive control of behavior.

Please, don't do it! Fifteen years of progress of non-invasive brain stimulation in action inhibition

The ability to inhibit prepotent responses is critical for survival. Action inhibition can be investigated using a stop-signal task (SST), designed to provide a reliable measure of the time taken by the brain to suppress motor responses. Here we review the major research advances using the combination of this paradigm with the use of non-invasive brain stimulation techniques in the last fifteen years. We highlight new methodological approaches to understanding and exploiting several processes underlying action control, which is critically impaired in several psychiatric disorders. In this review we present and discuss existing literature demonstrating i) the importance of the use of non-invasive brain stimulation in studying human action inhibition, unveiling the neural network involved ii) the critical role of prefrontal areas, including the pre-supplementary motor area (pre-SMA) and the inferior frontal gyrus (IFG), in inhibitory control iii) the neural and behavioral evidence of proactive and reactive action inhibition. As the main result of this review, the specific literature demonstrated the crucial role of pre-SMA and IFG as evidenced from the field of noninvasive brain stimulation studies. Finally, we discuss the critical questions that remain unanswered about how such non-invasive brain stimulation protocols can be translated to therapeutic treatments.

Negative Emotional Stimuli Enhance Conflict Resolution Without Altering Arousal

In our daily life, we frequently need to make decisions between competing behavioral options while we are exposed to various contextual factors containing emotional/social information. We examined how changes in emotional/arousal state influence resolving conflict between behavioral rules. Visual stimuli with emotional content (positive, negative and neutral) and music (High/Low tempo), which could potentially alter emotional/arousal states, were included in the task context while participants performed the Wisconsin Card Sorting Test (WCST). The WCST requires the application of abstract matching rules, to resolve conflict between competing behavioral options. We found that conflict influenced both accuracy and response time (RT) in implementing rules. Measuring event-related autonomic responses indicated that these behavioral effects were accompanied by concomitant alterations in arousal levels. Performance in the WCST was modulated by the emotional content of visual stimuli and appeared as a faster response and higher accuracy when trials commenced with negative emotional stimuli. These effects were dependent on the level of conflict but were not accompanied by changes in arousal levels. Here, we report that visual stimuli with emotional content influence conflict processing without trial-by-trial changes in arousal level. Our findings indicate intricate interactions between emotional context and various aspects of executive control such as conflict resolution and suggest that these interactions are not necessarily mediated through alterations in arousal level.