Brain Stimulation for Foot-sole Sensation in Older Adults With Foot-sole Somatosensory Deficits

Inclusion Criteria: 1. Aged ≥65 years. 2. Self-reported feeling of unsteadiness or difficulty when standing and walking. 3. Mild-to-moderate foot-sole somatosensory impairment: the ability to perceive 75g monofilament but inability to perceive 10g monofilament. Exclusion Criteria: 1. self-reported inability to stand or walk continuously for one minute without personal assistance (canes or walkers allowed); 2. history or presence of foot ulceration, amputation, or deformities; 3. self-reported uncontrolled pain or pain that is associated with mobility disability; 4. uncontrolled diabetes mellitus; 5. hospitalization within the past three months due to acute illness, or as the result of a musculoskeletal injury significantly affecting balance; 6. persistent severe pain of lower extremity when standing or walking; 7. diagnosis of dementia, Parkinson's disease, or stroke that affects balance; 8. unstable medical condition; 9. legal blindness or deafness; 10. uncontrolled hypertension (i.e., systolic BP \>180, diastolic BP \>100 mm Hg, or prescription of ≥3 anti-hypertensive medications); 11. functionally limiting nephropathy, severe diseases or transplant of the kidney or liver, renal or congestive heart failure; 12. active cancer treatment; 13. balance disorders due to past use of chemotherapy or history of Guillain-Barré syndrome; 14. use of neuro-active or recreational drugs (e.g., sedatives, anti-psychotics), or alcohol abuse, which may affect the brain excitability; 15. contraindications to MRI or tDCS (e.g., personal or family history of seizures or epilepsy, metallic or electric bio-implants, claustrophobia, brain surgery); 16. persistent vertigo; 17. history of Charcot-Marie-Tooth nerve disease.