Declines in several cognitive domains, including processing speed, occur in non-pathological aging. Studies have shown that cognitive training using the Useful Field of View (UFOV) paradigm can improve cognition. Furthermore, meta-analyses have found that transcranial direct current stimulation (tDCS), a non-invasive form of brain stimulation, can strengthen functional connectivity networks and improve cognition. However, few studies have looked at the synergistic effects of tDCS and cognitive training. The current study leverages data from the largest phase III clinical trial assessing the efficacy of a combined tDCS (active versus sham) and a cognitive training intervention. The current study assessed changes in task-based functional connectivity during completion of the UFOV task between tDCS groups following intervention.

155 healthy older adults completed a three-month double-blind randomized controlled tDCS and cognitive training intervention. Cognitive training was comprised of 8 attention/processing speed and working memory tasks from the POSIT Brain HQ suite. Additionally, participants were randomized into either active or sham tDCS groups. For both stimulation groups, two 5x7cm² electrodes were placed at F3 (cathode) and F4 (anode) using the 10-20 measurement system. The active group received 2mA of current for 20 minutes; conversely, the sham group received 2mA for 30 seconds, then no current for the remaining 20 minutes. The intervention comprised of 20 minutes of tDCS paired with 40 minutes of cognitive training five days a week for the first two weeks. Subsequently, participants would receive combined tDCS/cognitive training for one day a week and would complete 40 minutes of cognitive training at home for the remaining 4 days a week. Participants underwent functional MRI prior to and immediately following intervention and completed the UFOV task in-scanner. We used a generalized psychophysiological interaction approach to model task-based functional connectivity prior to and after intervention during each portion of the UFOV task. We assessed pre/post functional connectivity between 10 a priori ROIs and assessed behavioral changes associated with active versus sham tDCS.

There was no significant main effect of timepoint or tDCS group on task-based functional connectivity. However, there was a significant tDCS by timepoint interaction, controlling for covariates. Those in the active tDCS group had significantly increased functional connectivity between the left and right pars triangularis at the three-month timepoint compared to baseline, whereas those in the sham group had significantly decreased functional connectivity between these ROIs at the three-month timepoint. Additionally, we found a trending UFOV change score by functional connectivity change score interaction between active and sham tDCS groups. Within the sham group, a greater UFOV change score was associated with decreased functional connectivity, whereas within the active group, a greater UFOV change score was associated with increased functional connectivity.

These findings provide evidence that tDCS and cognitive training can be effective modulators of functional connectivity. Allocation to the active tDCS group was associated with increased task-based functional connectivity in the ROIs closest to the electrodes, compared to sham, and these findings are associated with trending improvements in cognitive performance. Future research should assess long-term neuroplastic changes following tDCS interventions.