When brain stimulation backfires: the effects of prefrontal cortex stimulation on impulsivity

Transcranial direct current stimulation (tDCS) can sometimes cause the opposite of its intended effect. These reverse effects may be related in part to individual differences in personality and neurochemistry. Previous studies have demonstrated that dopamine levels can impact the effects of tDCS. In the present study, 124 healthy participants took the UPPS impulsive behavior scale. Participants then underwent a single, randomized anodal or sham tDCS session on the prefrontal cortex. While the effects of tDCS were still active, they performed the Stop Signal Task, a measure of state impulsivity. tDCS was associated with increased errors on this task in people who had higher scores on the UPPS in two facets of impulsivity that correlate with dopamine levels. tDCS had no effects on people who are low in trait impulsivity. These results suggest that the reverse effects of tDCS could be associated with inter-individual differences in personality and neurochemistry.

Is value-based choice repetition susceptible to medial frontal transcranial direct current stimulation (tDCS)? A preregistered study

In value-based decision making, people have to weigh different options based on their subjective value. This process, however, also is influenced by choice biases, such as choice repetition: in a series of choices, people are more likely to repeat their decision than to switch to a different choice. Previously, it was shown that transcranial direct current stimulation (tDCS) can affect such choice biases. We applied tDCS over the medial prefrontal cortex to investigate whether tDCS can alter choice repetition in value-based decision making. In a preregistered study, we applied anodal, cathodal, and sham tDCS stimulation to 52 participants. While we found robust choice repetition effects, we did not find support for an effect of tDCS stimulation. We discuss these findings within the larger scope of the tDCS literature and highlight the potential roles of interindividual variability and current density strength.

The neural mechanisms of social reward in early psychosis

In chronic psychosis, reduced trust is associated with a neural insensitivity to social reward and reduced theory of mind (ToM). Here we investigate whether these mechanisms could underlie emerging social impairments in early psychosis. Twenty-two participants with early psychosis and 25 controls (male, 13–19 years) participated in two interactive trust games against a cooperative and unfair partner. Region of interest neuroimaging analyses included right caudate, medial prefrontal cortex (mPFC) and right temporoparietal junction (rTPJ), involved in reward and ToM processing. Both groups showed similar levels of trust (i.e. investments). However, individuals with psychosis failed to activate the caudate differentially in response to cooperation and unfairness while making decisions to trust. During cooperative returns, patients showed reduced and controls increased caudate activation. Patients demonstrated greater rTPJ activation than controls, possibly pointing towards compensatory mechanisms. Effects were associated with Wechsler Abbreviated Scale of Intelligence vocabulary scores. No group differences emerged in mPFC activation. Early psychosis is associated with an aberrant neural sensitivity to social reward. This could foster reduced social motivation and social isolation. Absent behavioural differences in early, relative to chronic psychosis could indicate that trust is achieved through increased compensatory demand on ToM.

Modulation of income redistribution decisions by anodal tDCS over the medial prefrontal cortex

One cause of the persistence of income inequality may be rooted in people's resistance to change the existing income distribution. Prior studies have shown that the medial prefrontal cortex (mPFC) may be associated with the decision making that influences income distribution. However, it is unclear whether the mPFC is involved in income redistribution tasks when third-party decision makers are unaffected by the outcome of the decision. In this study, we elucidate the neural mechanism underlying the tolerance of income inequality and the decision making that is related to income redistribution. By applying the transcranial direct current stimulation (tDCS) over the mPFC, we investigate whether the change in the activation of the mPFC can influence a subject's inclination to expropriate a rich person's endowment and transfer it to a poor person. The main finding is that the anodal stimulation significantly reduced the subject's inclination to redistribute wealth from the rich to the poor, and lowered the rate of accepting options for redistribution. However, the willingness of income redistribution did not change following the cathodal stimulation compared with the sham condition. The effect of the anodal stimulation was constant across three types of initial inequality. The stimulation effect is likely caused by the subject's enhanced loss aversion or desire to reinforce social hierarchies.

Pre-stimulus Brain Activity Is Associated With State-Anxiety Changes During Single-Session Transcranial Direct Current Stimulation

Transcranial direct current stimulation is a promising neuromodulation method for treating depression. However, compared with pharmacological treatment, previous studies have reported that a relatively limited proportion of patients respond to tDCS treatment. In addition, the neurophysiological mechanisms underlying tDCS treatment remain unclear, making it difficult to identify response predictors for tDCS treatment based on neurophysiological function. Because treatment effects are achieved by repetitive application of tDCS, studying the immediate effects of tDCS in depressive patients could extend understanding of its treatment mechanisms. However, immediate changes in a single session of tDCS are not well documented. Thus, in the current study, we focused on the immediate impact of tDCS and its association with pre-stimulus brain activity. To address this question, we applied anodal tDCS to the left dorsolateral prefrontal cortex (DLPFC) or dorsomedial prefrontal cortex (DMPFC) in 14 patients with major depressive disorder (MDD) and 19 healthy controls (HCs), at an intensity of 1.0 mA for 20 min in a single session. To evaluate anxiety, the state trait anxiety inventory was completed before and after tDCS. We recorded resting electroencephalography before tDCS, and calculated electrical neuronal activity in the theta and alpha frequency bands using standardized low-resolution electromagnetic tomography. We found that, during application of left DLPFC tDCS to patients with MDD, the anxiety reduction effect of tDCS was related to higher baseline theta-band activity in the rostral anterior cingulate cortex (rACC) and no medication with benzodiazepine used as hypnotic. For DMPFC stimulation in MDD, the anxiety reduction effect was associated with lower baseline alpha-band activity in the left inferior parietal lobule. In contrast, in HCs, the anxiety reduction effect was associated with higher baseline alpha activity in the precuneus during DMPFC stimulation. The current results suggest that the association between pre-tDCS brain activity and the anxiety reduction effect of tDCS depends on psychopathology (depressed or non-depressed) as well as the site of stimulation (DMPFC or left DLPFC) and insomnia. Furthermore, the results suggest that tDCS response might be associated with baseline resting state electrophysiological neural activity.

The Neural Basis of and a Common Neural Circuitry in Different Types of Pro-social Behavior

Pro-social behaviors are voluntary behaviors that benefit other people or society as a whole, such as charitable donations, cooperation, trust, altruistic punishment, and fairness. These behaviors have been widely described through non self-interest decision-making in behavioral experimental studies and are thought to be increased by social preference motives. Importantly, recent studies using a combination of neuroimaging and brain stimulation, designed to reveal the neural mechanisms of pro-social behaviors, have found that a wide range of brain areas, specifically the prefrontal cortex, anterior insula, anterior cingulate cortex, and amygdala, are correlated or causally related with pro-social behaviors. In this review, we summarize the research on the neural basis of various kinds of pro-social behaviors and describe a common shared neural circuitry of these pro-social behaviors. We introduce several general ways in which experimental economics and neuroscience can be combined to develop important contributions to understanding social decision-making and pro-social behaviors. Future research should attempt to explore the neural circuitry between the frontal lobes and deeper brain areas.

Transcranial direct current stimulation of the medial prefrontal cortex modulates the propensity to help in costly helping behavior

Social decision-making engages traditional decision-making processes (e.g. valuation), as well as social cognition processes (e.g. inferring the affective and mental states of another person). Neuroimaging and neuro-stimulation studies have suggested the involvement of the medial prefrontal cortex (mPFC) in a variety of social decision-making tasks. Yet no study has investigated the effect of the cortical excitability of mPFC in the decision-making of costly helping behavior. Here, we used tDCS to demonstrate the causal relationship between the cortical excitability of mPFC and costly helping decision-making. Subjects assigned to the anodal, cathodal and sham groups were required to decide whether they would like to cost their own money to relieve another subject (a confederate actually) from painful electrical shocks with a certain probability of success. Results showed that the subjects receiving anodal stimulation acted more prosaically than the subjects receiving cathodal stimulation. And this effect was only significant when the probability of success was high. We proposed that tDCS induced modulation of the cortical excitability, targeting the mPFC, can affect the prosocial propensity in costly helping behavior, and the possible underlying mechanisms were discussed.

The stimulated social brain: effects of transcranial direct current stimulation on social cognition

Transcranial direct current stimulation (tDCS) is an increasingly popular noninvasive neuromodulatory tool in the fields of cognitive and clinical neuroscience and psychiatry. It is an inexpensive, painless, and safe brain-stimulation technique that has proven to be effective in modulating cognitive and sensory-perceptual functioning in healthy individuals and clinical populations. Importantly, recent findings have shown that tDCS may also be an effective and promising tool for probing the neural mechanisms of social cognition. In this review, we present the state-of-the-art of the field of tDCS research in social cognition. By doing so, we aim to gather knowledge of the potential of tDCS to modulate social functioning and social decision making in healthy humans, and to inspire future research investigations.

Enhancing the Activity of the DLPFC with tDCS Alters Risk Preference without Changing Interpersonal Trust

Interpersonal trust plays an essential role in economic interactions and social development. Extensive behavioral experiments have examined the nature of trust, particularly the question of whether trusting decisions are connected to risk preferences or risk attitudes. Various laboratory observations have been reported regarding the difference between trust and risk, and neural imaging studies have demonstrated that the right dorsolateral prefrontal cortex (rDLPFC) is more activated when individuals decide to trust other human beings compared with individuals decide to invest in a non-social risk condition. Moreover, the rDLPFC has been found to exhibit an intimate relationship with risk preference in previous neuroscience studies. However, the causal relationship between the rDLPFC and trust has rarely been revealed. Whether modulating the excitability of the rDLPFC, which shares roles in both trust and risk decisions, alters the trust or risk preference of participants remains unknown. In the present study, we aimed to provide evidence of a direct link between the neural and behavioral results through the application of transcranial direct current stimulation (tDCS) over the rDLPFC. We found that activating the rDLPFC altered the risk preferences of our participants, whereas no such significant effect over interpersonal trust was observed. Our findings indicate that enhancing the excitability of the rDLPFC using tDCS leads to more conservative decision-makings in a risk game, and this effect is specific to non-social risk rather than social-related trust.

Modulating the Activity of Ventromedial Prefrontal Cortex by Anodal tDCS Enhances the Trustee's Repayment through Altruism

Trust and trustworthiness are essential to an efficient economy and play crucial roles in social life. Previous evidence from behavioral experiments has revealed that the trustworthiness of individuals is closely related with their altruistic preference. It has been demonstrated that the ventromedial prefrontal cortex (vmPFC) is associated with decisions involving trustworthiness. Moreover, vmPFC lesion patients showed less trustworthiness and altruism than control subjects, indicating the indispensable role of this specific brain area in human social interactions. However, the causal relationship between this neural area and trustworthiness, as well as altruism, has not been fully revealed. The potential neural basis behind the behavior of trustees' repayment has also seldom been discussed. In the present study, we aimed to provide evidence of a direct link between the neural and behavioral results through the application of transcranial direct current stimulation (tDCS) over the vmPFC of our participants. We found that activating the vmPFC could promote both the trustworthiness and altruism of our participants. We also show that enhancing the excitability of the vmPFC using tDCS increased the trustworthiness of the participants, and this promoting effect might be attributable to the enhancement of individuals' altruistic preference. In addition, we revealed that the enhancing effect in trustworthiness and altruism might be specific to the activation of the vmPFC by applying tDCS over another brain region within the prefrontal cortex as a control site. Crucially, our findings provide direct evidence supporting the critical role of the vmPFC in cooperative behaviors in economic interactions, especially the trustees' repayment in the trust game and the dictators' altruistic transfer in the dictator game.