Investigating the Effectiveness of a 2-week High-frequency Transcranial Pulsed-current Stimulation on Cognitive Outcomes in Healthy Adults: A Randomised, Double Blinded, Sham-controlled Study

The goal of this randomised controlled trial is to investigate the effects of a 2-week high-frequency transcranial pulsed current stimulation (tPCS) on cognitive outcomes in older and healthy adults. The main questions it aims to answer are: 1. Would a single session of anodal tPCS over the left prefrontal cortex improve working memory, task inhibition, and task switching in healthy young and older adults, compared to anodal transcranial direct current stimulation (tDCS) and sham-tPCS? 2. Would repeated session of anodal-tPCS (2-weeks) lead to improvements in working memory, task inhibition, and task switching in healthy young and older adults, compared to tDCS and sham-tPCS? 3. Will tPCS be better tolerated among healthy adults, compared to tDCS? Researchers will compare tPCS / tDCS / sham-tPCS to see if there are any differences in cognitive outcomes after 1 session and after repeated sessions, as well as to compare the tolerance of tPCS against tDCS and sham-tPCS. Participants will be requested to: * Undergo 1 of 3 of the following conditions: tDCS / tPCS / sham-tPCS * Complete three sessions of cognitive tasks testing working memory, inhibition and task switching at baseline (pre-stimulation), after day 1 of brain stimulation and after 10 sessions of brain stimulation (post- repeated stimulation) * Undergo fNIRS-EEG brain measurements concurrently with the cognitive tasks