High-definition Transcranial Electrical Stimulation for Upper Extremity Rehabilitation

Upper limb hemiparesis is the most common sequelae in patients, severely impacting their independence and quality of life. Transcranial electrical stimulation (tCES) is a non-invasive and safe treatment, which uses a low direct current or alternating current to change the excitability of the cerebral cortex. It can induces long-term potentiation-like or long-term depression-like effects, thereby modulating the cortical excitability. In recent years, researchers have developed high-definition (HD) devices, which integrate high definition ring electrode configurations and incorporate direct current with theta burst stimulation waveforms. Diverging from traditional transcranial direct current stimulation (tDCS), which applies weak currents (0.5-2 mA) through two large sponge electrodes (25\~35 cm\^2) externally to the scalp for widespread non-specific cortical stimulation, HD-tES employs an array of small-area electrodes (1 cm\^2) to control current distribution over localized cortical regions, thereby enhancing spatial accuracy. However, there is a lack of studies validating the optimal waveform for HD-tES, as well as clinical evidence in subacute stroke populations. The optimal unilateral versus bilateral stimulation modes and their neurological mechanisms for stroke rehabilitation also remain uncertain.