This randomized clinical trial aims to test the effects of 10 sessions of transcranial direct current stimulation (tDCS) on social and emotional functions in lonely older adults.

The main objectives include:

• Understand whether tDCS could successfully boost positive social processing and social motivation in older adults, paralleling its effect on non-social reward processing;
• Understand whether the reduced negative and increased positive social processing in older adults would translate to an increase in real-life social activity and a decrease in loneliness feeling;
• Examine the intervention efficacy of tDCS among the aged population;
• Understand whether the tDCS effect could be long-lasting (i.e., 1 and 3 months after intervention).

Participants will be lonely older adults aged 60 or above and screened on inclusion and exclusion criteria. Eligible participants will be randomly allocated to the left dorsolateral prefrontal cortex (DLPFC) tDCS group, the right ventrolateral prefrontal cortex (VLPFC) tDCS group, or the sham control group. Participants will complete an Emotion Rating Task, questionnaires assessing their psychosocial functions, and neuropsychological tests assessing their cognitive functions at baseline, after the 5th stimulation session, immediately after the 10th stimulation session, and 1 month and 3 months after the 10th stimulation session.

Loneliness, or perceived social isolation, results from discrepancies between an individual's desired and achieved social needs. A substantial proportion of older adults experience loneliness. With the social distancing policies implemented during the COVID-19 pandemic, older adults, who are more likely to be functionally dependent on their caregivers, have been primarily affected, contributing to adverse impacts on their social and emotional functions. As current loneliness interventions have shown limited efficacy, it is of great significance to test novel interventions for lonely older adults.

Transcranial direct current stimulation (tDCS) is a neuromodulatory technique that has demonstrated its safety and efficacy in modulating cortical excitability and associated behaviors in older adults. It has been suggested that repeated tDCS sessions show sustained therapeutic efficacy in chronic disorders. Additionally, the efficacy of tDCS as an intervention for lonely older adults has not been investigated. As a result, it is crucial to test repeated tDCS sessions as a potential intervention to reduce loneliness and improve socioemotional functions in lonely older adults.

Inclusion Criteria:

1. Aged ≥ 60 years old;
2. Fluent in reading and speaking Traditional Chinese (Cantonese or Mandarin);
3. Right-handed as assessed with the Edinburgh Handedness Inventory;
4. Normal or corrected-to-normal vision and hearing;
5. Normal general cognitive function as determined by the Hong Kong version of the Montreal Cognitive Assessment (HK-MoCA) ≥ 22;
6. ≥ 9 years of formal education as lexical comprehension is required in the Emotion Rating Task.

Exclusion Criteria:

1. Current or past major physical or neurological illness that significantly affects the individual's current cognitive, motor, or brain function;
2. Medication or other treatment received within 2 weeks before the study that may affect the individual's current cognitive, motor, or brain function;
3. Receiving maintained therapy that may affect cognitive, motor, or brain function and cannot be discontinued for the entire course of study;
4. Past or present major psychological conditions, including affective disorders, anxiety disorder, psychotic disorder, addiction, and impulse control disorder;
5. Any contraindication of tDCS, such as a cerebral implant or history (either personal or family) of seizure.
6. Participation in similar brain stimulation studies within the past six months.

Participants will be unaware of their group assignment.