High loneliness is prevalent among older adults and is a significant risk factor for major depression and low recovery rates in patients. Current interventions for loneliness are limited. Previous research has demonstrated a link between loneliness, negative social emotions, and reduced resting-state functional connectivity in the lateral prefrontal cortex (LPFC) and the default mode network (DMN). Transcranial direct current stimulation (tDCS) has shown promise in enhancing emotional regulation and connectivity, potentially alleviating loneliness in older adults with depression.

Loneliness is a pervasive issue among older adults, significantly increasing the risk of major depression and contributing to low recovery rates in affected individuals. Despite its widespread impact, current interventions aimed at alleviating loneliness remain limited and often ineffective. Research has revealed a strong connection between feelings of loneliness, negative social emotions, and decreased resting-state functional connectivity in key brain regions, particularly the lateral prefrontal cortex (LPFC) and the default mode network (DMN). These findings suggest that the neural underpinnings of loneliness may be targeted to improve emotional well-being.

Transcranial direct current stimulation (tDCS) emerges as a promising intervention in this context, as it has been shown to enhance emotional regulation and improve functional connectivity within these neural networks. By applying low electrical currents to specific brain areas, tDCS may help mitigate feelings of loneliness, thereby fostering a greater sense of social connection among older adults with depression.

Objectives Evaluate the Effects of Repeated tDCS on Loneliness Levels: This trial aims to systematically assess how repeated sessions of tDCS influence loneliness in older adults with major depressive disorder. By comparing these groups, we hope to understand the differential impacts of tDCS on loneliness, taking into account the potential confounding effects of depression.

Elucidate the Neuro-Emotional Mechanisms: A key objective is to uncover the neuro-emotional mechanisms that underlie the effects of tDCS on loneliness. This involves examining changes in neural activity and connectivity patterns in the LPFC and DMN, as well as tracking shifts in negative social emotions that may accompany improvements in loneliness. Understanding these mechanisms could enhance the efficacy of tDCS as an intervention.

Develop Predictive Models for Individualized Treatment Outcomes: Another important aim of this trial is to create predictive models that utilize neuro-emotional data to tailor treatment approaches for individuals. By identifying specific neural and emotional markers that correlate with positive outcomes from tDCS, we can move toward more personalized interventions, potentially improving recovery rates and overall mental health in older adults with depression facing loneliness.

This trial represents a significant step toward addressing the critical issue of loneliness in older adults with depression, with the potential to offer a novel and effective intervention strategy that integrates neurobiology and emotional health.

Inclusion Criteria:

• High loneliness (UCLA Loneliness Scale > 43).
• Diagnosis of major depressive disorder confirmed by the Structured Clinical Interview for DSM-5 Disorders.
• absence of suicidal ideation (Hamilton Depression Scale item 3 score ≤ 1).

Exclusion Criteria:

• Severe neurological diseases.
• Bipolar disorder, schizophrenia, addiction, or impulse control disorders.
• Contraindications for tDCS or MRI (e.g., metal implants).
• Presence of suicidal ideation.

Participant eligibility is based on self-representation of gender identity.