Introduction

Borderline personality disorder (BPD) is a mental disorder that exhibits instability in several domains, such as relationships, emotions, and self-perception.1 This disorder is also characterized by substantial impulsivity.1 Its prevalence is estimated at 1%–2% within the general population and around 15%–28% among patients with psychiatric conditions.2^–4 Clinical research has advanced considerably in the symptomatic treatment of BPD, primarily through psychotherapy.5 However, although these treatments show moderate effect sizes in reducing symptoms, their overall influence on psychosocial functioning is generally regarded as limited.5^,6

One possible explanation for this limited improvement is that these treatments primarily focus on emotions, relational patterns, and dysfunctional cognitive schemas without particularly targeting neuropsychological impairments.7 Although these factors are connected to psychosocial functioning, they may not fully account for it.7 Compared with the general population, people with BPD exhibit substantial neuropsychological impairments across multiple domains, particularly in cognitive flexibility, inhibition, and executive control.8 A meta-analysis by D’Iorio and colleagues8 found moderate-to-strong effect sizes in areas like executive functioning, working memory, decision-making, and sustained attention. Processing speed and visuospatial abilities showed smaller deficits, while memory — primarily verbal and spatial — exhibited significant levels of impairment.9^–11 A meta-analysis by Ruocco and colleagues11 and a systematic review by McClure and colleagues12 both found that patients with BPD also experience notable difficulties in planning.13 These neuropsychological impairments are associated with daily functional challenges.8^,14

Numerous studies have highlighted specific brain regions or networks that support these neuropsychological functions. 15 In BPD, neurobiological anomalies are linked to symptoms and cognitive dysfunctions, particularly corticolimbic alterations.16,17 Therefore, a central aspect of the neurobiological explanation for BPD involves functional and structural alterations of the dorsolateral prefrontal cortex (DLPFC), which plays a critical role in impulse control, cognitive functions, and emotional regulation.18,19 Findings about the lateralization of the DLPFC are yet to be specific to 1 side.18,19 The left DLPFC has been directly implicated in several studies specific to BPD, although the results are somewhat contradictory.18,20,21 Hypoactivation of the left DLPFC in BPD has been observed during reward-based tasks, reflecting a potential dysfunction in processing motivational salience or future-oriented behaviour, which contributes to impulsivity and difficulty in decision-making.18 This reduced activation impairs the DLPFC’s ability to modulate limbic and subcortical activity, which is essential for controlling impulsive behaviour.17,20–22 Interestingly, the left DLPFC may show heightened activation during tasks involving negative emotional stimuli and behavioural inhibition (e.g., aggression regulation), suggesting an overengaged cognitive control system in emotionally charged contexts, further complicating emotional regulation.18,20,21 Another study reported that people with BPD exhibit reduced bilateral DLPFC activation during negative emotion processing, suggesting a reduced capacity for cognitive control and the use of emotion regulation strategies, which are key features of the disorder’s cognitive dysfunction.18,23 In contrast, healthy individuals show more functionally distinct lateralization in the DLPFC, with the left DLPFC consistently engaged in working memory, stimulus interference control, planning, and proactive control, often without right DLPFC involvement, while the right DLPFC is associated with behavioural inhibition and impulse control during reactive tasks such as go/no-go or stop-signal paradigms.18,23,24

These findings have paved the way for new therapeutic approaches like neuromodulation. In the past decade, these methods have emerged to treat various mental health conditions.25 Notably, many neuromodulation studies in BPD have targeted the left DLPFC, particularly in protocols aiming to enhance regulatory capacities and cognitive functions.25 This focus highlights the potential functional relevance of the left DLPFC in treatment-oriented approaches. Among all these techniques, transcranial direct current stimulation (tDCS) is the most cost-efficient and logistically simple to administer, with the added advantage of potential for supervised home use.26 It applies a low electrical current to the scalp to modulate neuronal excitability and enhance cognitive functions.26,27 It has shown effectiveness for several mental illnesses, including major depressive disorder, anxiety disorders, obsessive–compulsive disorder, and bipolar disorder.26