Active Bilateral Frontal tDCS During Cognitive Training and 1-Year Everyday Function Outcomes in Healthy Older Adults
DOI: Pending DOI
Synopsis
Transcranial direct current stimulation (tDCS) is often paired with cognitive training (CT) to augment cognitive gains, but its impact on everyday function remains unclear. We examined 1-year Instrumental Activities of Daily Living (IADL) Difficulty through a randomized sham-controlled trial of adults aged 65-89 years without dementia. Participants completed 60 CT sessions over 12 weeks. Active or sham tDCS was delivered during the first 20 minof up to 20 sessions. Active tDCS used 2 mA with an F4 anode/F3 cathode montage. In the adjusted model (N=168), active tDCS was associated with lower 1-year Blom-transformed IADL Difficulty than sham (B=-0.26, SE=0.09, β=-0.17, p=.005). Findings suggest that everyday function may be a useful long-term outcome in stimulation-enhanced CT trials.
Background
Transcranial direct current stimulation (tDCS) is a noninvasive neuromodulation technique that produces polarity-dependent changes in cortical excitability.1,2 By shifting membrane polarization, it alters neuronal firing probability.3 Prefrontal tDCS also influences large-scale network activity and resting-state connectivity.4–6 When delivered during cognitive training, it may interact with task-related neural activity in a state-dependent manner and potentially enhance training-related effects.2,6 Whether these effects translate to everyday function remains unclear, motivating large randomized trials such as Augmenting Cognitive Training in Older Adults (ACT).7 Although meta-analyses report working memory benefits in healthy older adults, effects most often favor trained cognitive outcomes rather than real-world function.8 In well-functioning older adults, high baseline performance and subtle short-term change may limit early detection of functional differences.9–11 Instrumental activities of daily living (iADLs) measure independence in complex everyday tasks.12 Performance relies on executive function, memory, and processing speed, and deficits in these domains predict functional difficulty in aging.13,14 Since iADLs require coordinated cognitive processes in real-world contexts, they may detect delayed intervention effects not evident on short-term cognitive tests.15 We examined whether bilateral frontal tDCS delivered during multidomain computerized cognitive training was associated with 1-year functional outcomes in healthy older adults.7
Methods
This secondary analysis used data from the Augmenting Cognitive Training in Older Adults (ACT) multi-site, randomized, sham-controlled trial (N=379; ages 65–89). Community-dwelling older adults without dementia completed adaptive multidomain computerized cognitive training targeting processing speed, attention, and working memory during daily ~60-minute sessions over a 2-week in-person phase. Participants were randomized 1:1 to receive active or sham transcranial direct current stimulation (tDCS) during those sessions (up to 10 stimulation sessions). Active stimulation consisted of 2 mA direct current delivered for 20 minutes using a bilateral frontal montage (F4 anode, F3 cathode); sham mimicked ramp sensations without sustained current. Cognitive training continued weekly thereafter without additional stimulation. Participants and intervention staff were blinded to stimulation condition. Stimulation adherence was 78.8% (298/378 completed all sessions). Primary analyses followed randomized assignment. Everyday function was measured using the ACTIVE iADL Difficulty Scale (lower scores = less difficulty). Baseline data were available for N=379; observed 1-year data for N=274 (active n=139; sham n=135). The primary outcome was 1-year Blom-transformed iADL Difficulty. Linear regression models estimated the association between stimulation assignment and 1-year outcomes, adjusting for baseline iADL, site, sex, and education. Missing 1-year outcomes were addressed using imputation (N=379).
Results
At baseline, iADL Difficulty was low and comparable between groups (overall mean = 20.14 ± 2.25; sham = 20.11 ± 2.45; active = 20.17 ± 2.04; p = .792), consistent with a high-functioning sample. At 3 months (N = 302), raw means remained similar (overall = 19.86 ± 1.70; sham = 19.95 ± 1.78; active = 19.77 ± 1.62), with no significant group differences (p > .16). At 1 year (N = 274), raw iADL Difficulty was lower overall (19.77 ± 1.75), and the active group remained numerically lower than sham (19.60 ± 1.68 vs 19.95 ± 1.81). The nonparametric comparison at 1 year was significant (Wilcoxon p = .033). In covariate-adjusted mixed modeling of Blom-transformed iADL Difficulty (controlling for sex, site, and education), timepoint-specific active–sham contrasts were not individually significant at baseline (β = −0.106, p = .238), 3 months (β = −0.056, p = .481), or 1 year (β = −0.071, p = .353). However, the overall stimulation × time interaction was significant (F(2,374) = 3.13, p = .0447), supporting differential trajectories over time between active and sham conditions. Site was independently associated with outcome (Arizona vs UF: β = −0.215, 95% CI [−0.359, −0.071], p = .0036), indicating lower reported iADL difficulty at the Arizona site across timepoints. In a sensitivity analysis using imputed 1-year data (N = 379), active tDCS remained significantly associated with lower 1-year iADL Difficulty in a reduced model (β = −0.116, SE = 0.051, p = .024), indicating that findings were not attributable solely to differential attrition. The standardized effect size for the 1-year raw mean difference (active − sham) was small (Cohen’s d = −0.20), consistent with modest but detectable group separation at long-term follow-up.
Discussion
In this randomized trial of healthy older adults undergoing multidomain cognitive training, assignment to active bilateral frontal tDCS was associated with lower self-reported iADL Difficulty at 1-year follow-up. The effect was modest relative to baseline functional stability but remained statistically reliable after covariate adjustment and imputation-based sensitivity analyses. Importantly, this estimate reflects randomized assignment rather than completed stimulation dose. Group differences were minimal at 3 months and more apparent at 1 year, suggesting delayed divergence rather than an immediate post-intervention effect. In well-functioning older adults with low baseline difficulty, longer follow-up may be necessary to detect meaningful functional change. 9,15 This pattern is consistent with prior cognitive intervention research in aging, where functional benefits may emerge over extended intervals rather than acutely.15 Although meta-analytic evidence suggests that tDCS paired with cognitive training can improve proximal working memory outcomes,8 durable far-transfer effects remain uncertain in large randomized trials.7,16 The present findings suggest that everyday functional measures may capture downstream effects not evident in short-term cognitive endpoints. These results support continued evaluation of stimulation-enhanced cognitive training for promoting functional independence in aging while emphasizing careful interpretation of effect size and durability.
Disclosures
None
References
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