CP5-Centered Parietal HD-tACS Is Associated with Improved Performance in a Smartphone-Based Shopping Task in Older Adults: A Behavioral and EEG Investigation

Background/Objectives: Older adults often experience difficulties in smartphone use, especially when digital tasks require goal maintenance, visual search, sequential action, and response verification. Working memory and parietal theta-band activity may support these cognitively demanding operations, but it remains unclear whether a single session of theta-frequency high-definition transcranial alternating current stimulation (HD-tACS), centered over CP5 as a parietal scalp location intended to approximate the left inferior parietal region, is associated with short-term changes in smartphone-task performance in aging. Methods: This study examined performance in a controlled smartphone-based shopping task and exploratory post-stimulation EEG correlates. In Experiment 1, 40 older adults were randomly assigned to active HD-tACS or sham stimulation. In Experiment 2, 28 older adults completed a reduced-trial EEG extension of the same task with electroencephalography (EEG) recording before and after stimulation. Results: Active stimulation improved smartphone-task performance, including faster completion under high cognitive load, higher target selection accuracy, and reduced difficulty–time slope. Working-memory performance on a two-back task was also improved, and individual differences in working-memory gains were associated with improvements in smartphone-task efficiency. Active HD-tACS most strongly improved target selection accuracy, and exploratory post-stimulation theta-power changes in posterior/parietal regions may have accompanied high-demand target-selection-accuracy improvement. These neural findings should be interpreted cautiously because the omnibus EEG effects were trend-level, EEG–behavior correlations were based on a small active-stimulation subgroup, data-quality sensitivity analyses indicated artifact-related instability in theta-power estimates, and the full exploratory EEG–behavior correlation matrix did not survive FDR correction. Conclusions: These findings provide short-term behavioral evidence that CP5-centered parietal HD-tACS may support performance in a cognitively demanding smartphone-based task and motivate further work at the intersection of neuromodulation, cognitive aging, and human–technology interaction.