Gambling disorder (GD) and internet gaming disorder (IGD) are formally recognized behavioral addictions with a rapidly growing prevalence and limited treatment options. Transcranial electrical stimulation (tES) techniques have emerged as potentially promising interventions for improving treatment outcomes by ameliorating cognitive functions implicated in addictive behaviors.
Gambling and gaming are reward-based activities that many people engage in for fun and leisure. However, it has long been known that excessive engagement in certain rewarding behaviors may lead to addiction-like symptoms, often grouped under the term non-substance or behavioral addictions (BAs). Gambling and gaming represent behaviors considered to be addictive, as attested by the ever-rising prevalence of problematic gaming and gambling across the world 
. This is why gambling disorder (GD) and internet gaming disorder (IGD) have recently been added to the International Classification of Diseases, 11th edition (ICD-11), as formal diagnoses of BAs alongside substance use disorders (SUDs) 
. The reclassification in ICD-11 has made GD and IGD the first and only two officially recognized BAs. They are defined as patterns of gambling/gaming behaviors characterized by loss of control over the activity, prioritizing gambling/gaming over other activities, and continuation of gambling/gaming despite the negative consequences 
Gambling and gaming share common mechanisms that keep people engaged. The aesthetical and functional components of gambling and video games activate dopaminergic reward circuitry, which also plays a significant role in SUDs 
. The similarities between gambling and gaming are reflected in the large overlap between the definition and symptomatology of GD and IGD. At the neural level, individuals with GD and IGD show alterations in fronto-striatal and prefrontal brain regions 
. Accordingly, studies show that GD and IGD affect the same cognitive functions, including decision making, cognitive control, and reward sensitivity 
. In addition, typical cognitive distortions responsible for the maintenance of GD (e.g., near-miss effect and loss-chasing behaviors) can be present during disordered gaming 
. Thus, recent findings about the co-occurrence of problematic gambling and gaming among youth do not come as a surprise 
The overlap between gambling and gaming is nowadays even more pronounced, with the expansion of online games offering options to pay for additional features. Furthermore, the Internet has brought significant changes to gambling patterns, with many online gambling activities resembling video games. This rapid growth of the gambling and gaming industries has led to an increased number of people demonstrating symptoms of GD or IGD 
. While the prevalence of both disorders continues to grow, there is a need for more efforts to develop effective treatment options.
Recently, transcranial electrical stimulation (tES) has emerged as a promising tool for the complementary treatment of various psychiatric disorders, including addiction disorders 
. TES techniques are safe, inexpensive, painless, and easy-to-use forms of non-invasive brain stimulation (NIBS), which makes them particularly appealing 
. During tES, a weak electrical field is formed between two or more electrodes placed on the person’s head 
. The induced electrical field causes local changes in neuronal membrane potentials, bringing neurons closer or further from their firing thresholds 
. The modulation of neuronal firing patterns can influence cognitive functions controlled by the brain networks targeted with tES 
. There are different types of tES, depending on the electrode montage and the current waveform that is delivered: transcranial direct current stimulation (tDCS), high definition tDCS (HD-tDCS), transcranial alternating current stimulation (tACS), etc.
2. Effects of Transcranial Electrical Stimulation on Gambling and Gaming
The tES Effects on Gambling and Gaming-Related Cognitive Tasks in Healthy Participants
Overall, most studies showed that tES can affect gambling and gaming-related cognitive tasks in healthy participants. However, there was a large variability in the results when it came to the direction of the effects. Studies that used bipolar or multifocal tDCS, for instance, showed that modulation frontal activity could lead to more advantageous decision making and reduction of risk taking behaviors during tasks that involved gambling/gaming . On the other hand, in certain studies that used the same or similar electrode configuration, tDCS disrupted decision making and increased risk taking in these tasks . These opposing effects were observed even within the same study, that is, some of the studies reported both increases as well as decreases in risk taking, depending on the outcome measures or participants’ characteristics . Finally, several studies showed no significant effect of tDCS on gambling and gaming task performance . There was no indication that the variability of the effects could be attributed to differences in either stimulation intensity, tDCS set-up (including the stimulation side), or protocol (offline vs. online).
The tES Effects in Studies on GD and IGD
Soyata et al. 
, using bilateral right-to-left montage found that participants with GD improved decision making in IGT and cognitive flexibility in the Wisconsin card sorting task, following three sessions of bilateral tDCS. Further, Martinotti et al. 
, using the same type of montage, showed that five sessions of tDCS could reduce craving, in a mixed sample of patients with SUDs and GD.
As for the research on IGD, Wu et al. 
found that a single tDCS session targeting the right DLPFC could result in an improvement of inhibitory control over gaming-related cues and could also facilitate the regulation of craving and emotions. Using a bilateral left–right montage, Jeong et al. 
found that 12 active tDCS sessions may decrease time spent on gaming, as well as improve self-control and reward seeking. Conversely, Lee et al. 
, using the same montage, did not observe significant effects of 10 sessions of tDCS on any outcomes, including craving, cognitive control, the severity of addiction, behavioral activation, and inhibition. It might be of note that the intensity of stimulation in the latter was only 1 mA in comparison to 2 mA in the former.
The tES Effects on Gambling and Gaming-Related Cognitive Tasks in Participants with SUDs
It showed that the direction of tES effects on gambling/gaming-related tasks could depend on the sample type, the outcome measure, and/or the stimulation parameters. While Boggio et al.  showed that cannabis users increased risk taking in CGT during bilateral stimulation of DLPFC, regardless of whether the anode was left or right, Patel et al.  did not find tDCS effects when the anode was left, using the same montage and almost the same study design. In contrast, Pripfl et al.  showed that, during bilateral DLPFC stimulation, in the ‘’cold‘’ version of the CCT, both smokers and non-smokers reduced risk taking when the anode was left (when the anode was right, there was no effect), whereas, in the ‘’hot‘’ version of the same task, only smokers decreased, while non-smokers paradoxically increased risk taking, but only when the anode was right (when the anode was left, there was no effect). Moreover, Gorini et al.  showed that both cocaine users and healthy controls decreased risk taking following anodal tDCS over the right DLPFC (measured by BART and GDT), while anodal tDCS over the left DLPFC increased risk taking, but only in cocaine users (measured by GDT).
Using tES to Modulate Prefrontal Networks Relevant to GD and IGD
Most studies opted for conventional bipolar tDCS, which consists of placing two electrodes of opposite charge (anode and cathode) on the participant’s head to form the electrical field with unidirectional current flow. To modulate GD/IGD functions and behaviors, prefrontal brain areas have been the primary target in tDCS studies. This choice of stimulation site is driven by neuroimaging evidence pointing out DLPFC and OFC as some of the key structures involved in cognitive control 
, as well as in disorders due to substance use or addictive behaviors 
. The idea behind placing electrodes over the prefrontal cortex is that tDCS would improve cognitive control and lead to more favorable decision making. Building up on the hypothesis of an imbalanced activity between the left and right hemispheres in psychiatric disorders affecting decision making 
, numerous studies decided on the bilateral montage. While the left DLPFC is considered particularly important for regulating negative emotions and controlling impulsive behavior 
, the right DLPFC is thought to be more involved in reward processing and reward-related emotions and motivation 
. Thus, the imbalance in left/right DLPFC activity may result in cognitive–emotional patterns typically observed in gambling and gaming addiction, i.e., high sensitivity to reward, impulsivity, and impaired cognitive control.
Indeed, many studies found that bi-hemispheric frontal stimulation was associated with the adoption of risk-averse behaviors during gambling/gaming tasks in healthy participants, indicating that balancing the activity across the left and right frontal hemispheres may be crucial for taking control over risky gambling and gaming associated behavior 
. Moreover, this montage has been shown to be effective in improving cognition and reducing cravings in patients with GD and IGD 
, as well as in facilitating decision making, in studies focusing on gambling/gaming behaviors in participants with SUDs 
. Nevertheless, some studies showed null or even the opposite effects 
Diversity of Cognitive Functions Relevant for GD and IGD
People who demonstrate problematic gambling and gaming are characterized by impulsiveness, reduced inhibitory control, suboptimal decision making, heightened reward sensitivity, etc. Therefore, in tES studies, GD and IGD-relevant functions can be assessed with a wide range of outcome measures. The diversity and heterogeneity of the outcome measures (i.e., different types of tasks used) may be an important source of the results’ variability. Namely, a conflicting aspect of gambling/gaming tasks is their feature to recruit two distinctive cognitive processes: decision making under risk (when the probability to win is known, e.g., CGT) and decision making under ambiguity (when the probability to win is unknown, e.g., IGT, BART). While both forms of decision making are relevant to GD and IGD, they might engage different neural mechanisms. In fact, several studies found that the same stimulation protocol differently affected performance, depending on the decision-making type required 
. This is in line with neuroimaging evidence identifying distinct roles of cortical and subcortical regions associated with different types of decision making 
. For example, the meta-analysis by Krian and colleagues showed that risky decision making relies on OFC and rostral portions of the medial wall, while ambiguous decision making is associated with DLPFC and more caudal portions of ACC 
. Thus, it is important to be mindful of the type of decision-making task when deciding on electrode positioning i.e., maximizing current intensity in the relevant region of the brain.
TES can modulate prefrontal networks to induce changes in cognitive functions underlying gambling and gaming behaviors in healthy participants, as well as in people with GD/IGD and other addictions. However, further research is needed to determine the most effective stimulation protocols, as well as to assess the effectiveness of so far not-much-used tES techniques, such as tACS and HD-tDCS/tACS.