High-definition transcranial direct current stimulation (HD-tDCS) can modulate neuronal activity and may hold promise as a treatment in those with mild cognitive impairment (MCI). Mnemonic strategy training (MST) has also been shown to be effective for enhancing memory in those with MCI. Our previous studies demonstrated that mnemonic strategy training (MST) enhanced activation within the left inferior frontal gyrus (IFG) during memory encoding. Here, we present training-session data from a recently completed 2x2 double-blind randomized controlled trial examining the effects of concurrent active or sham HD-tDCS over the IFG during MST. In this exploratory analysis, we hypothesized that the active HD-tDCS group would have greater performance (i.e., recall of MST cues/stimuli) within sessions compared to sham as a result of stimulation being delivered to the IFG.

Fifty-four of 107 older adults with amnestic MCI (M_age = 73.19±6.74) were randomized to 5 consecutive daily sessions of either active (2mA for 20 minutes) or sham HD-tDCS and received concurrent MST (the other fifty-three participants were randomized to participate in autobiographical memory recall during stimulation as opposed to MST). MST required the participants to use a previously validated 3-step process when encountering each of 12 novel stimuli (6 object-location associations and 6 face-name associations) during each session. During the MST training sessions, participants developed and recalled their own feature (step 1) and reason (step 2) cues for each object or face stimulus they were shown; these cues aided their recall of the target location/name (step 3) of the stimulus image (i.e., our FRI process). All participants used this 3-step MST process during each of 9 trials per stimulus in each session. Participants’ MST performance was measured by their ability to correctly recall the specific feature, reason, and target location/name for each stimulus on each trial. We used independent samples t-tests to evaluate the effects of HD-tDCS on summated performances.

The number of FRI cues correctly recalled was the highest in the third session and lowest in the fifth session. There were no statistically significant differences between the groups for the average number of cues correctly recalled during each session. Average MST performance within each session ranged from an average of 259.45 (SD = 51.33) to 274.76 (SD = 42.43) out of 324 FRI cues correctly recalled for participants in the active group, and an average of 259.17 (SD = 94.12) to 286.92 (SD = 43.51) out of 324 FRI cues correctly recalled for participants in the sham group.

These results suggest that the HD-tDCS did not alter participants’ ability to use MST during these five training sessions. The lack of behavioral change is unexpected since participants receiving active HD-tDCS showed widespread increases in fMRI BOLD signal (findings we report elsewhere). Future studies should investigate longer treatment courses using more challenging paradigms given the relatively brief duration and high-level of performance in our current study.  Regardless, we replicated and extended our earlier findings and clearly demonstrate that patients with amnestic MCI are able to independently develop and implement MST, which improves memory test performance.