Transcranial direct current stimulation as an adjunct to cognitive training for older adults with mild cognitive impairment : a randomized controlled trial

Abstract

Background: Cognitive training (CT) for individuals with mild cognitive impairment (MCI) may not be optimal for enhancing cognitive functioning. Coupling CT with transcranial direct current stimulation (tDCS) may maximize the strength of transmission across synaptic circuits in pathways that are stimulated by CT. The synergistic effects arising from this combination could be superior to those with administration of CT alone. Objectives: To investigate whether the receiving tDCS combined with CT is superior to CT alone on domain-specific and task-specific cognitive outcomes in older adults with MCI. Methods: This double-blind, sham-controlled randomized trial included 67 older adults with MCI assigned to 3 groups: 1) tDCS combined with CT (tDCS + CT), 2) sham tDCS combined with CT (sham tDCS + CT) and 3) CT alone. Nine sessions of computerized CT were administered to the 3 groups for 3 weeks. In addition, tDCS and sham tDCS was delivered to the left dorsolateral prefrontal cortex to the tDCS + CT and sham tDCS + CT groups, respectively, simultaneously with CT. Standardized cognitive assessments were performed at baseline, post-intervention, and at 6-week follow-up. Participants’ performance in the CT tasks was rated every session. Results: The 3 groups showed improvements in global cognition and everyday memory (P < 0.017) after the intervention and at follow-up, with larger effect sizes in the tDCS + CT than other groups (d > 0.94) but with no significant differences between groups. Regarding CT outcomes, the groups showed significant differences in favour of the tDCS + CT group in decreasing the completion and reaction times of working memory and attention activities (P < 0.017). Conclusions: tDCS combined with CT was not superior to sham tDCS with CT and CT alone in its effects on domain-specific cognitive outcomes, but it did provide comparatively larger effect sizes and improve the processing speed of task-specific outcomes.