Joon Ho Lee^a,d, Tae Lee Lee^a,d, Nyeonju Kang^a,b,c,d,*

^aDepartment of Human Movement Science, Incheon National University, Incheon, South Korea
^bDivision of Sport Science, Incheon National University, Incheon, South Korea
^cSport Science Institute & Health Promotion Center, Incheon National University, Incheon, South Korea
^dNeuromechanical Rehabilitation Research Laboratory, Incheon National University, Incheon, South Korea

^*Corresponding author

Background
This systematic review and meta-analysis investigated the effects of transcranial direct current stimulation (tDCS) on the cognitive functions of healthy older adults by focusing on the changes in reaction time during cognitive tasks.

Method
A total of 31 studies qualified for this meta-analysis, and we acquired 36 comparisons from the included studies for data synthesis. The individual effect sizes were calculated by comparing the altered reaction time during the performance of a specific cognitive task between the active tDCS and sham groups. In two moderator variable analyses, we examined the potentially different effects of the tDCS protocols on the cognition-related reaction time based on the tDCS protocol used (i.e., online vs. offline tDCS) and the five cognitive domains: (a) perceptual-motor function, (b) learning and memory, (c) executive function / complex attention, (d) language, and (e) social cognition. Meta-regression analyses were conducted to estimate the relationship between demographic and tDCS parameter characteristics and the changes in reaction time.

Results
The random-effects model meta-analysis revealed significant small effects of tDCS on cognition-related reaction time. Specifically, providing online tDCS significantly reduced the reaction time, and these patterns were observed during learning and memory and executive function / complex attention tasks. However, applying offline tDCS failed to find any significant reduction of reaction time across various cognitive tasks. The meta-regression analysis revealed that the effects of tDCS on the reaction time during the performance of cognitive tasks increased for the older people.

Conclusions
These findings suggest that providing online tDCS may effectively improve the ageing-induced reaction time related to specific cognitive functions of elderly people.

Keywords : Reaction time, cognitive functions, transcranial direct current stimulation, ageing