Description:

Throughout the study, participants are required to maintain stable pharmacological and psychotherapeutic regimens, defined as:

No changes in medications or therapy for at least 6 weeks after tDCS initiation.

TAU is provided by participants' regular treating physician or mental health professional, independent of the study team.

TAU includes psychotropic medication, psychotherapy, and case management, as clinically indicated.

All participants receive access to a structured online psychoeducation program before randomization.

This includes:

• Short videos (8-14 minutes each) covering BPD, depression, neuromodulation, therapeutic success factors, and tDCS.
• Multiple-choice quizzes (MCQs) to ensure comprehension.
• Delivered in French with English subtitles. Designed to improve understanding of BPD and associated treatments, including the rationale for tDCS.

Description:

Participants randomized to this arm will receive 14 sessions of sham tDCS, with identical scheduling and device setup as the active condition. The session mimics real tDCS (same SNAPstrap montage, ramp-up and ramp-down of 30 seconds), but no current is delivered after the initial 30 seconds.

Enhancement component:

Participants follow the same emotional regulation script and Lumosity cognitive training during

Quick Inventory of Depressive Symptomatology (QIDS): The QIDS is a 16-item self-report questionnaire based on DSM-IV criteria for major depression. It assesses symptom severity over the past 7 days (higher is more severe) and takes approximately 5 to 10 minutes to complete. The assessment has good internal consistency, with a Cronbach's alpha of 0.69 to 0.89 (75). It has been translated into French.

The BPDSI is a 70-item semi-structured interview designed to assess the frequency and severity of BPD symptoms over the past three months. It captures critical symptoms and behaviours associated with BPD, such as self-harm, emotional dysregulation, and interpersonal conflicts. The interview takes approximately 45 to 60 minutes to complete. The BPDSI yields highly reliable (ICC = .93) and internally consistent (Cronbach's α = .85) scores. The BPDSI has been translated into French and used in clinical research, although its psychometric properties have not yet been validated for the French version.

The PDS-ICD-11 is a clinician-rated instrument designed to assess the severity of personality disorder features by the International Classification of Diseases, 11th Revision (ICD-11). This scale evaluates impairments in self-functioning (identity and self-direction) and interpersonal functioning (empathy and intimacy), the core components of personality pathology defined by the ICD-11 framework. The PDS-ICD-11 provides a dimensional assessment, categorizing severity into five levels: no personality dysfunction, personality difficulty, mild personality disorder, moderate personality disorder, and severe personality disorder. This nuanced approach allows for a standardized and comprehensive evaluation of personality dysfunction, facilitating both diagnosis and the monitoring of treatment outcomes. A version has been translated into French.

The BIS is a 30-item self-report questionnaire that evaluates various dimensions of impulsivity, including attentional, motor, and non-planning impulsivity. It takes approximately 10 minutes to complete. The BIS has good internal consistency, with a Cronbach's alpha ranging from 0.69 to 0.83, indicating satisfactory scale homogeneity. A French version is available.

The WHODAS 2.0 (12-item) is a brief self-report questionnaire designed to assess disability and functioning across six domains: cognition, mobility, self-care, getting along with others, life activities, and participation in society. Higher scores indicate greater functional impairment. Developed by the World Health Organization, it has good internal consistency, with a Cronbach's alpha ranging from 0.87 to 0.94, indicating excellent scale homogeneity. The scale is available in French (85)and English and takes approximately 5-10 minutes to complete.

The BAI is a 21-item self-report questionnaire designed to assess the severity of anxiety symptoms experienced over the past week. The items focus on physical symptoms of anxiety, such as nervousness and heart palpitations. The BAI takes approximately 10 minutes to complete. It has excellent internal consistency, with a Cronbach's alpha of 0.91.

The ATHF-SF is a clinician-administered tool designed to assess a patient's history of antidepressant treatments systematically. It provides a concise and structured way to document the type, dosage, duration, and adequacy of antidepressant trials and the patient's response to these treatments. The ATHF-SF helps determine the extent of previous antidepressant exposure and can identify potential treatment-resistant depression. It will enable us to classify patients into five stages, from no treatment to ECT.

ASSIST is a widely used tool developed by the World Health Organization (WHO) for the screening and measurement of addiction across various substances, including alcohol, tobacco, cannabis, and opioids. ASSIST is designed for use in clinical and research settings to assess both current and lifetime use, and associated risks of substance use disorders. The tool consists of eight questions that measure the frequency of use, cravings, and negative impacts of substances on physical, mental, social, and legal aspects of the individual's life, allowing for a quick and comprehensive screening. Psychometric evaluations of ASSIST demonstrate high reliability and validity across various settings and populations. Studies report good internal consistency, with Cronbach's alpha values ranging from 0.80 to 0.88 and high test-retest reliability (ICC = 0.90), indicating consistent results over time.

The PSQI is a 19-item self-report questionnaire designed to assess subjective sleep quality across seven components, including sleep latency, duration, sleep disturbances, and daytime dysfunction. Higher scores indicate poorer sleep quality. The PSQI takes approximately 10 minutes to complete. It has good internal consistency, with a fair or reasonable Cronbach's alpha coefficient (ranging from 0.64 to 0.83. The instrument has been validated in French.

In this study, participants will wear FitBit watches to objectively measure several key outcomes related to sleep, physical activity, and physiological metrics. Sleep data will include the total sleep duration and the time spent in each stage (light, deep, and REM sleep). Physical activity will be assessed by tracking the number of steps taken per day and the number of active minutes. Additionally, the investigators will monitor physiological activity, focusing on heart rate variability (HRV) and heart rate (HR) frequency, providing insights into participants' cardiovascular and autonomic functioning.

The PID-5 is a validated psychometric tool designed to assess maladaptive personality traits described in the DSM-5 Alternative Model for Personality Disorders. In clinical research, the PID-5 is an outcome measure by capturing dimensional traits across five broad domains: Negative Affectivity, Detachment, Antagonism, Disinhibition, and Psychoticism. Each domain consists of more specific facets that provide a detailed profile of personality pathology. Using the PID-5, researchers can quantify changes in these traits over time, making it a valuable tool for evaluating the effectiveness of therapeutic interventions. Additionally, its use in cross-sectional and longitudinal studies allows for assessing personality trait stability or improvement following treatment.

The N-back task is designed to evaluate working memory and will be administered online. Participants will view a sequence of stimuli and indicate when the current stimulus matches one presented "n" steps earlier. The task duration is typically 15-20 minutes, depending on the difficulty level (e.g., 2-back or 3-back).

The Go/No-Go task measures impulsivity and inhibitory control. Participants respond to "go" stimuli while inhibiting responses to "no-go" stimuli. This task generally takes around 10 minutes to complete. These assessments are integrated into the protocol to evaluate key cognitive domains (planning, working memory, and inhibitory control). They will be conducted remotely, ensuring accessibility and participant convenience while collecting valid and reliable data.

The MADRS is a clinician-administered scale with ten items designed to assess the severity of depressive symptoms. Each item is scored from 0 to 6, yielding a total score of 0 to 60 (higher scores indicate greater severity). The assessment requires approximately 15 to 20 minutes to complete. It exhibits high internal consistency, as evidenced by a Cronbach's alpha (α) of .85-.89, and demonstrates sensitivity to change, making it suitable for evaluating treatment efficacy. Remission will be defined as a MADRS total score ≤ 10. The proportion of participants meeting this criterion will be calculated separately for each treatment group. The scale has been validated in French and can be administered over the phone.