Ankle control is essential to safe over-ground navigation for humans. Middle- aged and older adults and patients with stroke whose ankle control is poor often lose their balance or fall. Transcranial direct current stimulation (tDCS) is an emerging non-invasive brain stimulation technology that has great potential to be applied to neurorehabilitation; however, the optimization of its applications still needs further studies. The aims of this project are to compare the effects of anodal tDCS (AtDCS) applied to the primary motor cortex (M1) contralateral to the moving leg (cM1), posterior parietal cortex (PPC) contralateral to the moving leg (cPPC), and cerebellar cortex (CBM) ipsilateral to the moving leg (iCBM) on motor learning, motor adaptation, and brain connectivity in healthy middle-aged and older adults and hemiparetic patients with chronic subcortical stroke.

Inclusion Criteria: Part 1: healthy middle-aged and older adults

1. age between 40 and 80 years old
2. intact cognitive function (MMSE ≧ 27)
3. normal ankle dorsiflexor and plantarflexor strength (manual muscle strength testing= 5) and passive range of motion (ankle dorsiflexion ≧10 degrees; ankle plantarflexion ≧ 45 degrees)
4. corrected far vision ≥ 0.8 (Landolt C test) and uncorrected near vision ≥ 0.04 (Comprehensive Color Blindness Checklist)

Exclusion Criteria: Part 1: healthy middle-aged and older adults

1. having any contraindications for MRI or tDCS;
2. serious or uncontrolled systematic diseases;
3. symptoms or history of neurological diseases, including transient ischemic attack, stroke, epilepsy, history of abnormal electroencephalogram (EEG), meningitis, encephalitis, brain tumors, brain surgery, and sensory disorders, etc.;
4. severe musculoskeletal problems that would affect lower limb functions;
5. visual spatial perception disorders and hearing loss;
6. color blindness;
7. depression and psychiatric disorders;
8. use of any medication that could affect the central nervous system function;
9. drug, substance, or alcohol addiction;
10. those participating in research involving invasive or non-invasive brain stimulation;
11. those deemed unsuitable for MRI or tDCS after evaluation by the attending physician;
12. those affiliated with any research-conducting institution.

Inclusion Criteria: Part 2: patients with chronic subcortical stroke

1. aged between 40 and 80 years old;
2. intact cognitive function (MMSE ≥ 27);
3. first-ever onset of subcortical stroke occurring at least 3 months prior to enrollment, with brain lesions involving unilateral subcortical regions only;
4. hemiplegic or hemiparetic;
5. no spatial neglect;
6. able to actively perform at least 5 degrees of ankle dorsiflexion and 10 degrees of ankle plantarflexion with the affected ankle;
7. no excessive spasticity;
8. no severe contracture (passive range of motion ≥ 10 degrees for ankle dorsiflexion and ≥ 20 degrees for ankle plantarflexion) in the affected leg;
9. mild-to moderate disability;
10. can walk independently or under supervision without assistance;
11. corrected far vision ≥ 0.8 (Landolt C test) and uncorrected near vision ≥ 0.04 (Comprehensive Color Blindness Checklist.)

Exclusion Criteria: Part 2: patients with chronic subcortical stroke The exclusion criteria will be the same as those listed for healthy participants described in Part 1 except for items (3) and (4). Item (3) will be changed to "(3a) symptoms or history of other neurological diseases, including epilepsy, history of abnormal EEG, meningitis, encephalitis, brain tumors, brain surgery, and sensory disorders, etc." and item (4) will be changed to "(4a) severe musculoskeletal problems of the non-hemiparetic limbs that would affect mobility functions".