Keywords

Introduction

Transcranial brain stimulation (TBS) is a term that denotes different noninvasive techniques which aim to modulate brain cortical activity through an external source, usually an electric or magnetic one. Currently, there are several techniques categorized as TBS. However, two are more used for scientific research, the transcranial magnetic stimulation (TMS) and the transcranial direct current stimulation (tDCS), which stimulate brain areas with a high-intensity magnetic field or a weak electric current on the scalp, respectively. They represent an enormous contribution to behavioral, cognitive, and social neuroscience since they reveal how delimited brain cortical areas contribute to some behavior or cognition. They have also been proposed as a feasible tool in the clinical setting since they can modulate abnormal cognition or behavior due to brain activity modulation. This chapter will present the standard methods of transcranial stimulation, their contributions to social and affective neuroscience through a few main topics, and the studies that adopted those techniques, also summing their findings.

Essentials of Transcranial Electrical and Magnetic Stimulation

Transcranial magnetic stimulation (TMS) first appeared in 1985, at the beginning, adopted to investigate nervous propagation along the corticospinal tract and peripheral nerves (Rossini & Rossi, 2007) and investigate the brain function excitability of different brain areas (Hallet, 2007). It consists of a coil and one or two generators (also called stimulators), which generate current pulses converted on the coil in a magnetic field. When positioned on the scalp, the coil delivers a magnetic pulse that creates a transient electric field in cortical areas underneath, activating neural networks through axonal depolarization or impairing neural activity through post-excitatory inhibition, i.e., “silent period” (Chen et al., 1999; Lefaucheur et al., 2014). It is possible to apply single or paired (e.g., double or triple pulses) TMS to investigate intracortical circuits and their relation to behavior and cognition (Ni et al., 2011). In addition to these procedures, there is also possible to use repetitive TMS (rTMS) to excite or inhibit a cortical area depending on the parameters adopted, mainly the frequency of pulses delivered. Studies with the motor cortex established that low-frequency stimulation (≤1 Hz) is usually inhibitory while high-frequency stimulation (≥5 Hz) is excitatory, but a variation in these effects can occur due to differences in intensity and duration of rTMS. It is also important to highlight those differences in effect depend on other parameters such as type of the coil, distance and orientation to the head and the waveform, intensity, and frequency of magnetic pulse (Lefaucheur et al., 2014).

Another common neuromodulator adopted in neuroscience similar to the TMS is the transcranial direct current stimulation (tDCS). An initial version of the tDCS (named “medical battery”) appeared during the nineteenth century to treat several ailments. Nevertheless, only at the beginning of the twenty-first century were its mechanisms deeply investigated and have been broadly adopted as a research tool (Wexler, 2017). tDCS consists of a low-intensity direct current (about 1 to 3 mA) applied on the brain by positioning two or more electrodes onto the scalp, forming an electrical circuit. While the minimum is two electrodes to close the circuit, it is possible to find assembles with more electrodes, just as in high-definition tDCS. The electrodes vary in format and size, usually ranging between 10 and 40 cm2 in a round or square format. As observed in studies investigating motor cortex excitability, the stimulation’s typical effect is enhanced excitability in cortical areas below anodic and inhibition below the cathodic electrode. However, differences in these effects can occur in brain areas other than the motor cortex, like those related to higher cognitive processing, where a linear effect between intensity and cortical excitability or inhibition seems not to be the rule. Also, the effect can vary accordingly to (i) the montage adopted, (ii) the size and orientation of the electrodes, (iii) the intensity and duration of stimulation, (iv) individual characteristics (e.g., gender, age, anatomical differences), and (v) if tDCS is applied during an active state (performing an activity of interest) or in a resting state (Giordano et al., 2017; Sellaro et al., 2017).

Despite the similarities here presented, there are also apparent differences between TMS and tDCS. For instance, TMS stimulation is more focal since the cortical target is circumscribed to an area about 2 or 3 cm2 when using a coil (Lozano & Hallett, 2013). Conversely, the usual tDCS montage’s cortical target is broader, but it is possible to target a narrow area employing high-definition tDCS. Regarding the TMS, its stimulation is more intense than tDCS, so it is possible to interrupt neural activity or stimulate an action potential with TMS, while tDCS can only modulate ongoing activity. Nevertheless, since TMS has a higher intensity, there is also a risk of seizures not present in tDCS stimulation, although reports in the scientific literature indicate it is rare (less than 1 seizure per 60,000 sessions) if safety guidelines are adopted (Lerner et al., 2019). Another critical question to research and clinics is that tDCS is easier to apply than TMS because it has fewer parameters. Moreover, tDCS is considerably inexpensive compared to TMS.

It is also essential to present some relevant limitations to both techniques. First, such techniques are more focused when modulating cortical areas of the brain but are not widely used to stimulate subcortical areas. The stimulation of subcortical areas is usually indirect, employing a tDCS current passing those areas (yet with limited focus) or in response to some cortical region’s stimulation by tDCS or TMS, such as stimulation of frontal areas to modulate the activity of subcortical areas as in the case of emotion regulation. TMS also has some coil models (e.g., H-coil, halo coil, or double-cone coil) that allow deep stimulation but also with less focus when compared to cortical targets.

Finally, concerning some practical aspects of using TBS, both techniques are usually applied prior (“offline”) or concomitant (“online”) to some cognitive or behavioral task. It is essential to consider safety aspects when using such techniques, such as avoiding applying such techniques in participants with epilepsy, metallic implants on the head, or pacemakers. Concerning tDCS, it is relevant to ascertain the skin’s integrity where the electrodes will be applied; besides, some participants report reactions of severe discomfort and skin irritation. Here, we present only a few more superficial aspects of both techniques. Bearing in mind that such techniques require different preparations and care, we recommend reading specific articles on practical aspects in applying and preparing experiments for tDCS (Woods et al., 2016) and TMS (Hannula & Ilmoniemi, 2017). In the following topics, we will address the use of both techniques in social and affective neuroscience, as well as their main findings.

Social Neuroscience

Social neuroscience is an interdisciplinary field that aims to understand the neurobiology of social cognition and behavior in humans and animals – first created from the merge of social psychology, neuroscience, and social sciences. It aims to investigate brain structures and their functioning on various social processes, such as communication, cooperation, empathy, moral judgment, prejudice, social learning, social decision-making, social perception, and so on (Cacioppo & Cacioppo, 2013; Lieberman, 2007). This section will present a few social neuroscience topics that adopted tDCS or TMS and demonstrate how these approaches clarified the brain processes related to prejudice, social decision-making, and moral judgment.

Prejudice

Prejudice is the attitude toward others based on their group membership, and it is intrinsically related to affective and cognitive processes, such as social categorization and stereotyping (Amodio, 2014). Negative beliefs about the outgroup influence choices, judgments, and behaviors (Sellaro et al., 2015) and can give rise to discrimination and prejudice to outgroup members (Amodio, 2014). In contrast, individuals judge more positively members of the same group when compared to another racial group, a phenomenon called ingroup favoritism (Taylor & Doria, 1981).

Different cerebral cortical regions are involved in prejudice, mainly associated with social perception and evaluation (Gamond et al., 2017). One of the primary brain areas associated with prejudice is the medial prefrontal cortex (MPFC), an area involved in several cognitive activities, such as social perception, categorization, stereotyping, and regulation/control of behavioral responses in social contexts (Amodio & Frith, 2006; Amodio, 2014; Sellaro et al., 2015). Sellaro et al. (2015) investigated the causal role of MPFC in stereotype neutralization using tDCS. In this study, participants performed the implicit racial attitude task (racial IAT) while submitted to a tDCS protocol (anodal, cathodal, or sham) of 1 mA intensity targeting the MPFC. Anodal stimulation decreased implicit bias when compared to cathodal stimulation or sham. Sellaro’s study was the first to demonstrate MPFC’s causal role in cognitive control in overcoming negative judgment concerning another social group.

Another area associated with prejudice is the cerebellum. Recent studies have demonstrated the cerebellum’s functional connectivity to the MPFC and other cortical regions like the temporoparietal junction during social judgments related to body reading, action sequencing, and mentalizing behavior (see Van Overwalle et al. 2015). One study by Gamond et al. (2017) evaluated the cerebellum and dorsomedial prefrontal cortex (dMPFC) roles in participants’ implicit attitudes, where Caucasian participants had to categorize valence of positive/negative primed by ingroup or outgroup faces while receiving TMS. The behavioral experiment (without neuromodulation) showed ingroup bias with faster categorization for positive adjectives primed by the ingroup faces. However, both the dMPFC and the right cerebellum modulation interfered with this effect, preventing the ingroup bias. The results suggest that both brain areas play a causal role in social cognition processes, such as implicit social attitudes for ingroup members.

Finally, another study demonstrated TMS over dMPFC interfered with one’s ability to discriminate emotions expressed by ingroup members. These findings suggest a causal role of dMPFC in recognizing ingroup emotions (Gamond & Cattaneo, 2016). In summary, those studies have demonstrated, using tDCS and TMS, the crucial role of MPFC and related cortical areas (e.g., cerebellum) in social cognitive processes such as social group categorization and recognition.

Social Decision-Making

Social decision-making is a social neuroscience topic that aims to comprehend the neural mechanisms of choosing between alternatives in a social context (Sanfey, 2007). tDCS and TMS have been adopted in social decision-making to investigate brain areas’ causal role (mainly prefrontal) during cooperation or competition situations simulated through simple games derived from behavioral economics. The selection of brain targets to modulate through tDCS or TMS is usually based on correlational studies previously conducted with neuroimage techniques, pointing to the probable involvement of a cortical area in some aspect of social decision-making.

Two main areas investigated through tDCS and TMS in social decision-making are the dorsolateral prefrontal cortex (DLPFC) and medial prefrontal cortex (MPFC). Several tDCS and TMS studies targeted DLPFC in social decision-making through ultimatum game (Knoch et al., 2006, 2008; Ruff et al., 2013), Trust Game (Knoch et al., 2009; Wang et al., 2016) and public goods game (Li et al., 2018; Liu et al., 2017). Overall, the findings point to the right DLPFC role in implementing controlled cognition to identify contextual social norms or expectations and orient adaptive behavior to comply with those norms (Sanfey et al., 2014). One study investigated left DLPFC role in supporting people, showing that this area’s enhanced excitability led to increased prosocial behavior. The authors hypothesized that this area could be related to the management of emotional information by controlled cognition (Balconi & Canavesio, 2014). Although these studies have been clearly showing the involvement of DLPFC in social decision-making, it is still not clear the specific role of right and left areas in social decision-making.

Another area investigated in social decision-making is the MPFC, usually detected in neuroimage studies. One study with MPFC investigated its role on unfairness acceptance when unfair proposals were committed to oneself compared to a third party, showing that inhibition of this area led to a higher acceptance rate of unfair proposals and implying a causal role of MPFC in process fairness in situations involving self (Civai et al., 2014). In another study, Klucharev et al. (2011) evaluated the role of MPFC in social conforming on an attractiveness decision task, where participants should rate the attractiveness of models presented in photos. In this task, the downregulation of MFPC diminished social conformation, indicating that this area is related to social learning related to others’ expectations in decision-making, as indicated by recent studies (Apps & Sallet, 2017; Sanfey et al., 2015). In summary, it appears that MFPC recruits controlled cognition to implement decisions related to oneself and calculate others’ expectations in the context.

Moral Judgment

Moral judgment is the topic studying the judgment of right and wrong mainly respective to situations involving harm. Thus, most of the studies investigate moral judgment considering dilemmas such as the trolley problem (Thomson, 1984), in which the participant should decide between preserving individual rights from a single person and saving many others. This kind of task typically evaluates moral judgment in a utilitarian-deontological axis, considering cognitive reasoning relative to harm aversion. Furthermore, considering cortical brain regions, the majority of the studies investigated the modulation of two cortical structures, DLPFC and ventral MPFC (or just VMPFC), considering their role in other social phenomena (Boggio et al., 2016a; Darby & Pascual-Leone, 2017; Di Nuzzo et al., 2018).

Considering DLPFC modulation, one study by Tassy et al. (2011) investigated brain neuromodulation during moral judgment. The authors performed low-frequency rTMS (known to generate cortical inhibition) over the right DLPFC during a moral dilemma judgment task, where the participant should judge whether he or she considered an immoral attitude acceptable. The authors observed a significant increase in utilitarian judgments (i.e., “the most good for most people”) during active TMS compared to shame. In this way, this finding points out the significant role of this structure in moral judgment, specifically, in controlling emotional processes usually related to decreased utilitarian decisions. Similarly, Jeurissen et al. (2014) also demonstrated that low-frequency rTMS over right DLPFC was associated with moral judgment modulation in personal dilemmas (leading to less utilitarian responses), but not in impersonal or nonmoral dilemmas. The authors explained that the personal moral dilemmas are more emotionally salient; thus, the study suggested the right DPLFC role in cognitive control, probably dampening emotion processing and consequently enhancing utilitarian responses.

Regarding the use of tDCS, Kuehne et al. (2015) performed a task very similar to Jeurissen et al. (2014) concerning dilemmas with personal involvement nevertheless sought to modulate contralateral homologous region, that is, the left DLPFC. For this purpose, they performed three experimental conditions: two active conditions with target electrode (anodal or cathodal) over the left DPLFC and reference over the right parietal cortex and one sham stimulation. The authors found that only anodal condition presented significant moral judgment modulation compared to sham condition, showing a decrease in utilitarian judgments (greater frequency of deontological judgments), thus highlighting this structure’s role in the left hemisphere in the process of moral reasoning. However, in a recent study conducted by Zheng et al. (2018), an opposite effect was found, where the authors performed balanced bilateral tDCS over the DLPFC, with left anodal, left cathodal, and sham conditions. A significant decrease in the utilitarian judgment was observed for dilemmas with personal involvement during the anodal on the right hemisphere (cathodal at left hemisphere), which is compatible with the results found by Jeurissen et al. (2014). However, these findings revealed the need for experimental standardization since the positioning of the reference electrode can significantly impact tDCS effects.

Considering VMPFC role, many social neuroscience studies had shown this area involvement in empathy processes (Shamay-Tsoory et al., 2003), theory of mind (Shamay-Tsoory et al., 2005), and moral judgment (Greene, 2007; Moll & de Oliveira-Souza, 2007).

The first work with neuromodulation of ventral medial prefrontal cortex (VMPFC) and moral judgment conducted by Fumagalli et al. (2010) investigated VMPFC modulation employing tDCS, with anodal and cathodal over this area or over the occipital cortex (control condition) and reference electrode over right deltoid. They performed a judgment task of moral dilemmas with personal involvement, without personal involvement, and nonmoral dilemmas. The authors observed that brain modulation was only effective in female participants (which already presented low levels of utilitarian judgments in comparison to the male participants at the baseline), who presented a greater frequency of utilitarian responses after anodal tDCS on VPFC and lower frequency after cathodal tDCS, compared to baseline trials (Fumagalli et al., 2010). It is worth noting that the authors did not find any significant effect for tDCS in the occipital cortex or sham condition. In this way, these findings indicate that the neuromodulation of the VPFC may impact moral judgment and also highlights probably differences in brain circuitry for emotion processing between men and women (Fumagalli et al., 2010), as previously presented in the literature (Boggio et al., 2008). More recently, in a complementary way, Yuan et al. (2017) used a picture judgment task to assess moral judgment and arousal rating. Participants who received anodal tDCS on VMPFC (with reference electrode in the right deltoid) significantly increased moral judgment and arousal rating compared to sham condition. The authors did not evaluate differences in sex that could complement (Fumagalli et al., 2010) findings. Finally, recent work by Riva et al. (2018) investigated VMPFC modulation during a moral dilemma task, with the active electrode (anodal or cathodal) over VMPFC and the reference electrode over the occipital area. The findings revealed similar effects to Fumagalli et al. (2010) and Yuan et al. (2017), where participants receiving anodal tDCS over VMPFC had a higher frequency of utility judgments.

Overall, the findings regarding DLPFC and VMPFC’s neuromodulation highlight these structures’ essential causal role in moral judgment processes. However, all these findings represent tasks of moral dilemmas, such as the trolley/train problem (Thomson, 1984), which only measures the participant’s judgment concerning a deontological-utilitarian axis, without taking into account the different moral foundations (Graham et al., 2013).

Affective Neuroscience

Besides social phenomena, some studies have sought to understand several cortical brain structures’ specific role on affective phenomena, such as facial expression recognition and emotion regulation. It is a consensus that social and affective phenomena are closely intertwined (Boggio et al., 2016a, b), thus hindering the exclusive study of one of them. The following topics present the main findings regarding neuromodulation to understand two of the main topics from affective neuroscience:

Emotional Face Recognition

One crucial use of neuromodulation was to investigate brain networks involved in the recognition of emotional facial expressions. One of the main areas investigated is the medial prefrontal cortex (MPFC). Some of the studies assessed low-frequency TMS on dorsal MPFC (Balconi et al., 2011; Balconi & Bortolotti, 2012; Harmer et al., 2001), where inhibition of this area by neuromodulation specifically impaired recognition of facial expressions of anger and fear. TMS may have interfered with the dorsal anterior cingulate cortex’s activity, usually responsive to negative valence emotions. Another possibility is related to the role of MFPC on other brain areas via top-down regulation, as indicated by one study coupling TMS and EEG where magnetic pulses delivered over right MPFC led to altered electroencephalographic early evoked potentials detected at temporal and occipital regions (Mattavelli et al., 2013).

In addition to the MPFC, other studies also assessed orbital and dorsolateral prefrontal areas to investigate their role in processing facial expressions. For example, Nitsche et al. (2012) applied anodal and cathodal tDCS over the left DLPFC (reference electrode positioned on the contralateral supraorbital region). They found enhanced performance in healthy subjects answering a facial expression identification task markedly for positive valence emotions and anodal tDCS. In another study, Willis et al. (2015) applied anodal tDCS over the right orbitofrontal cortex with reference over P3 (left parietal cortex). Compared to sham, active tDCS enhanced performance on facial expression recognition. It is essential to notice that implying those specific prefrontal regions in emotion recognition is not so straightforward since tDCS is not so focal and the reference electrode could also interfere in the results. For example, Heberlein et al. (2008) investigated patients with prefrontal lesions in diverse regions, where they found that only patients with ventromedial lesions had impaired facial expression recognition and emotional expression. Besides, tDCS over prefrontal regions could indirectly act over ventromedial regions, leading to confounding results about what region is related to emotion recognition. One way to solve this problem is to use TMS, which is more focal. In a study by Ferrari et al. (2017), TMS was applied over the right or left DLPFC, and they found that both stimulations interfered in recognition of facial expressions, irrespective of emotion, similar to what Nitsche et al. (2012) found with tDCS. Thus, it is possible to implicate DLPFC in emotion recognition.

Other regions in the frontal lobe also investigated through neuromodulation methods are the supplementary motor area’s anterior region (pre-SMA) and primary motor area (M1). Regarding the pre-SMA, Rochas et al. (2013) inhibited its activity through low-frequency TMS and investigated recognition of faces expressing happiness, anger, or fear. In this case, left pre-SMA disruption impaired recognition of happy faces but did not affect fear or angry faces. In addition to Nitsche et al. (2012), it is possible to hypothesize the left hemisphere’s implication in processing positive valence, in line with previous neuroimaging and behavioral studies (Root et al., 2006). However, Ferrari et al. (2017) did not detect this, and there is still controversy in the literature supporting this lateralized valence theory (Root et al., 2006). The study by Rochas et al. (2013) hypothesized that disrupting pre-SMA led to impaired emotion recognition due to the mirror neuron system, i.e., disrupting the motor simulation of an expression in motor areas could also impair emotion recognition, similar to presented in simulation theories of emotion recognition (Gallese & Sinigaglia, 2011; Goldman & Sripada, 2005). Another study indicated the role of MNS in emotional face recognition, which found a positive correlation between cortex excitability of M1 (assessed by TMS) in response to movement observation and performance in facial expression recognition (Enticott et al., 2008).

Another critical region investigated in facial expression recognition is the temporal lobe, given the vital role of the superior temporal sulcus in processing dynamic facial features, such as eye gaze and facial expressions (Furl et al., 2014). Three studies by the same group investigated the contribution of the right occipital face area (rOFCA) compared to the right somatosensory cortex (rSC) (Pitcher et al., 2008) or the right posterior superior temporal sulcus (rpSTS) (Pitcher, 2014; Pitcher et al., 2014) in dynamic face processing. They found that all those areas contribute to recognizing facial expressions, with rOFA responsible for early processing of facial features (less than 100 ms), while rSC and rpSTS were responsible for posterior processing, despite still in the automatic domain (between 100 and 170 ms). Furthermore, although rOFA stimulation disrupted facial expression perception, this area appeared to be more related to the processing of static facial features, whereas rpSTS stimulation disrupted precisely dynamic face recognition. Summing, these results indicate a network of the occipital and temporal area responsible for processing dynamic features of facial expressions (Pitcher et al., 2008, 2014; Pitcher, 2014).

Another relevant study that neuromodulated the temporal lobe is from Boggio et al. (2008), using tDCS and finding opposite effects between women and men. In this study, they applied anodal tDCS over the left temporal and the reference over the contralateral region, which led to women’s enhanced performance in detecting sad faces, while men performed worse due to stimulation. This study indicates differences among men and women in how the brain processes recognize basic emotions, which is specifically problematic given other studies have not evaluated gender as a factor in their analysis.

Finally, two other studies investigated emotional face recognition. Ferrucci et al. (2012) stimulated the cerebellum through anodal and cathodal tDCS, where both polarities led to better performance in recognition of faces expressing emotions of negative valence. In another experiment, Cecere et al. (2013) inhibited the left occipital region through cathodal tDCS, while participants responded to a go/no-go task with images of fearful and happy faces. This experiment investigated the occipital cortex’s role in integrating explicit and implicit stimuli (i.e., subliminal visual stimuli) showed to the left and right visual fields, respectively; it also investigated how unconscious emotional stimuli could facilitate behavior in a go/no-go task (correctly react to targets pressing a button). This study demonstrated a facilitation effect in the go/no-go task when explicit and implicit were congruent (showing the same expression of happiness or fear). However, after occipital cortex disruption by tDCS, this congruent facilitation disappeared, and implicit detection of fearful faces facilitated behavior, but only when the target was happy faces (similar to hemianopsia patients). The study demonstrated cortical (occipital cortex) role and subcortical routes in processing implicit visual information, showing occipital role in processing high-order level information regarding congruence, while subcortical routes’ role was relevant for processing implicit fear stimuli.

In sum, neuromodulation studies indicate the existence of different systems between basic emotions, as suggested by neuroimage studies (Tettamanti et al., 2012; Diano et al., 2017), and it can vary between men and women. Modulation techniques also helped to elucidate the role of several brain areas (e.g., cerebellum, temporal, occipital, and frontal lobes) and of the MNS system in emotion recognition.

Emotion Regulation

Emotion regulation is the capacity to modify oneself or someone else emotional responses in order to intensify (upregulation) or diminish (downregulation) current emotion (Gross, 2014). Many studies on this topic focused on the emotional reappraisal strategy, i.e., a technique to change the cognitive label of specific emotional content. This preference is because this strategy is more effective in modulating the long-term emotional response (Gross, 2014), besides presenting a direct relation with cognitive control and the brain structures involved in this control (Ochsner et al., 2012), mainly the dorsolateral prefrontal cortex (DLPFC) and ventrolateral prefrontal cortex (VLPFC), due to the critical role of these structures on cognitive control, attentional orientation, response inhibition (Ochsner et al., 2012), and mediating amygdala’s activity (Wager et al., 2008).

One relevant study on this topic is by Feeser et al. (2014). They investigated the role of right DLPFC anodal tDCS in using emotion reappraisal strategy (cathodal electrode positioned at contralateral supraorbital region). They found a significant increase in cognitive control measured by arousal ratings and skin conductance response (SCR). The typical variation according to reappraisal, i.e., higher for upregulation and lower for downregulation compared to observation only, was potentialized with anodal stimulation of DLPFC. These findings clarify the significant role of the right DLPFC in cognitive control and emotion regulation through a reappraisal of negative valence content. In the same line, Pripfl and Lamm (2015) and Rêgo et al. (2015) also found a significant impact of right DLPFC anodal stimulation on cognitive control. However, contrary to Pripfl and Lamm (2015), Rêgo et al. (2015) also found that left anodal DLPFC condition significantly modulates emotion regulation, possibly due to increased attentional control, following Plewnia et al. (2015).

Thus, it seems that there is a misunderstanding between studies and relative to the neuromodulation of hemispheric sides. With this in mind, Marques et al. (2018) performed a study in order to investigate bilateral balanced DLPFC in two conditions compared to sham: (i) anodal left and cathodal right and (ii) anodal right and cathodal left. They did not find any significant impact of DLPFC tDCS on the emotional reappraisal of negative pictures. Notwithstanding, in a second study, they performed the same experimental procedures; however, over VLPFC, they found that left anodal VLPFC tDCS significantly impacted emotion reappraisal of negative pictures, increasing valence (more positive) regardless of emotion regulation strategy. Furthermore, they found a significant impact of left anodal VLPFC tDCS on the cardiac inter-beat interval, increasing cardiac recruitment on the first seconds of emotional processing, indicating that this neuromodulation condition significantly increased participants’ cognitive engagement, and also leading to an increased valence estimation.

Thus, following the discussion of Paulo S Boggio et al. (2016b), these findings indicate several particularities of each mentioned brain structure on emotion regulation, as the role of DLPFC on cognitive control (Ochsner et al., 2012) and VLPFC on attentional control (Wager et al., 2008). Future studies should standardize the experimental protocol between studies due to significant discrepancies in the literature related to electrode size, current intensity, cathode positioning, and emotion regulation tasks. Moreover, as highlighted by Kim et al. (2019), future studies should also use TMS as an exciting technique to address both DLPFC and VLPFC’s role in emotion regulation.

Conclusions

To conclude, the transcranial stimulation methods, tDCS, and TMS have been an important tool to investigate cortical circuits’ role in several social, like prejudice, social decision-making, and moral judgment, and affective processes, like emotion recognition and regulation. Those techniques were essential to demonstrate several brain areas’ role in a plethora of previously described processes in neuroimaging studies. TMS studies could also demonstrate the role of different areas in a brain network across time, which is very relevant to indicate how the brain integrates complex information among several cortical areas.

The observed cognitive and behavioral effects in response to brain modulation are of great relevance since they can indicate the future use of these neuromodulation techniques to modulate brain activity noninvasively in clinical patients with social or affective disorders to ameliorate their clinical condition.