Background: Subjective cognitive decline (SCD) is considered to be the early risk stage of dementia. Untreated SCD comorbid with sleep disturbances may accelerate the progression of neurodegeneration (β-amyloid and tau) and lead to cognitive deficits. At present, non-pharmacological interventions for managing SCD and sleep disturbances are very limited. High-definition slow-oscillatory transcranial direct current stimulation (so-tDCS) is a newly developed frequency-specific modality of brain stimulation for promoting brain health and cognition. Notably, neurophysiological feature (impedance) during tDCS was found to be related to treatment outcomes and adverse effects (skin injury). Objectives: We propose to 1) develop a high-performing neurophysiological signal detector for tDCS and test its feasibility and flexibility in a randomized clinical trial; 2) investigate the short-term and long-term effects of high-definition so-tDCS on SCD, sleep quality and plasma Aβ and p-tau levels; 3) examine the values of neurophysiological signals in predicting the treatment outcomes at individual level. Design: A randomized, double-blind, sham-controlled trial. Methods: Chinese right-handed older adults with SCD and sleep disturbances will be randomly assigned to a 4-week intervention of either high-definition 0.75 Hz so-tDCS or sham tDCS, with 40 participants per arm. Pre-treatment magnetic resonance imaging (MRI) scans will be collected to exclude the cases with major neurological disease and quantify individual's brain features. Galvanic skin response, subjective cognitive complaints, sleep quality, plasma p-tau and β-amyloid levels and domain-specific cognition will be assessed at baseline, 4th week, 8th week and 12th week. Program adherence and adverse effects will be monitored throughout the whole intervention.

This study aims to investigate the efficacy and sustainability of high-definition so-tDCS for managing SCD and sleep disturbances in older adults. The neurophysiological signal detector will be featured with a compact and high-performing GSR assessor with the state-of-the-art personalized measurement during so-tDCS. The successful implement of this signal detector will provide researchers assess to an "add-on" setting that allows them easily, quickly and quantitatively evaluate real-time neurophysiological signals and further incorporate transcranial brain stimulation in clinical practice.

Inclusion Criteria:

• Chinese, right-handed, aged from 60 to 85 years.
• Normal global cognition defined as a total score of the Montreal Cognitive Assessment Hong Kong version (HK MoCA) > 26.
• Subjective cognitive decline (SCD) is evaluated by a 10-item questionnaire that covers subjective cognitive complaints (SCC) in the core cognitive domains.
• Sleep disturbances or poor sleep quality is assessed and defined with a total score of the Pittsburgh Sleep Quality Index (PSQI) greater than 5.

Exclusion Criteria:

• Previous diagnosis of age-related neurodegenerative diseases, such as Alzheimer's disease, Parkinson disease.
• History of neurological diseases including brain tumor, and stroke.
• History of psychiatric disorders including schizophrenia and depression.
• Presence of positive neurological diseases, such as stroke, brain tumor, dementia.
• Physically unable to attend the sessions of intervention.
• Currently enrolled in another intervention study.
• Currently taking a psychiatric or other medication that may affect sleep quality and cognition.