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

• transcranial direct current stimulation (Device): tDCS can safely and selectively modulate cortical excitability (specifically neuronal firing likelihood) by transferring weak electrical currents between scalp electrodes. The direct current delivered by any one electrode will not exceed 2.0 mA and the total amount of current from all electrodes will not exceed 4 mA in this study.
• active sham stimulation (Device): sham stimulation will implement the same protocol of the tDCS intervention; however, only very low-level currents (no more than 0.5 mA) are transferred between the same electrodes used for the tDCS throughout the 20-minute session. This strategy effectively mimics the cutaneous sensations and skin redness induced by creating only micro cortical electric fields.

Primary outcomes

• postural sway area when standing with eyes closed on the foam [Before and immediately after intervention]: This metric assesses the degree to which the soft support and cut-off of the vision diminishes the control of standing posture.
• Timed Up-and-Go (TUG) time [Before and immediately after intervention]: This metric assesses mobility.
• On-target Blood oxygen level dependent (BOLD) signal intensity in response to foot sole stimulation [Before and immediately after intervention]: This metric assesses the degree to which brain cortical regions activated by the walking-related foot-sole stimulation.
• Standing vibratory thresholds of each foot sole [Before and immediately after intervention]: This metric assesses the degree to which the foot soles can sense the vibro-tactile stimuli when standing.

Secondary outcomes

• Postural sway speed when standing with eyes closed on the foam [Before and immediately after intervention]: This metric assesses the degree to which the soft support and cut-off of the vision diminishes the control of standing posture.
• Gait speed of 10m walking test [Before and immediately after intervention]: This metric assesses mobility.

Other outcomes

• BOLD signal intensity within other relevant cortical regions in response to foot sole stimulation [Before and immediately after intervention]: This metric assesses the degree to which brain cortical regions activated by the walking-related foot-sole stimulation.

Sex: All, ages: 65 Years to any

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.

NIH grants: K01AG075180