This study aimed to investigate if peripheral nerve stimulation (PNS) paired with action observation (AO) would augment motor training in promoting corticomotor excitability for the paretic hand in people with stroke. Twenty subjects in chronic stage of stroke were exposed to 3 different interventions involving one-hour PNS or sham PNS paired with 30 minutes of AO or sham AO, all followed by 30 minutes of finger abduction training in 3 separate sessions.

The investigator's previous study (Liu & Au-Yeung, 2017; PMID: 28383292) has demonstrated that 1-hour peripheral nerve stimulation (PNS) applied to the radial and ulnar nerves of the paretic UL of people with chronic stroke can increase transcranial magnetic stimulation (TMS)-evoked excitability of corticomotor pathways in both hemispheres projected to the contralateral first dorsal interosseous (FDI) hand muscle, and also improve dexterity in the paretic hand. This bi-hemispheric effect of PNS suggests that certain additional therapies such as action observation (AO) and motor training should be incorporated to strengthen the plastic corticomotor adaptation specifically for paretic UL control. The hypothesis of this study is that PNS paired with AO would augment task training of the paretic hand by increasing the corticomotor excitability in the lesioned hemisphere for the trained hand muscle in people with stroke, with associated dexterity function improvement. Twenty subjects were recruited by convenience in the community. They then attended an assessment session to evaluate the baseline sensorimotor status of their paretic upper limb. After the clinical tests，the subjects were introduced the assessment procedure for measuring corticomotor excitability with transcranial magnetic stimulation (TMS). After 1-week intermission, the subjects returned for 3 intervention sessions with different protocols including (1)PNS+AO+Training; (2)PNS+AOsham+Training; and (3)PNSsham+AOsham+Training. These 3 intervention sessions were arranged in random sequence for subjects. For "PNS+AO+Training" session, electrical stimulation was applied to the radial and ulnar nerve (PNS) of the paretic arm for 60 minutes. During the last 30 minutes of PNS, action observation (AO) was introduced in which subjects were required to watch a series of video clips. After one hour's PNS+AO, the subjects then proceeded to a 30-minute period of motor training of paretic hand. During "PNS+AOsham+Training" session, PNS was given as described before. During the last 30 minutes of the PNS, subjects were shown different photos illustrating a letter, a number or a hand as AOsham. The protocol of motor training was the same as described before. In the session of "PNSsham+AOsham+Training", The 1-hour PNSsham was conducted with the electrical stimulation unit turned on but without current output. The protocols for AOsham and motor training were the same as those described before.

Inclusion Criteria:

• age between 40 and 80 years;
• has history of first hemispheric stroke of 6or more months;
• presence of electromyographic signals in first dorsal interosseous (FDI) during active abduction of the index finger contralateral to the lesioned hemisphere;
• medically stable;
• able to comprehend instructions.

Exclusion Criteria:

• past history of musculoskeletal or neurological impairment in the paretic upper limb;
• recurrent stroke;
• cognitive impairment (Mini-mental state examination (MMSE) < 24);
• visual impairment not correctable by optical lens;
• presence of metal in the head region or a cardiac pacemaker as contraindications for TMS (Rossi et al., 2009).