Search Abstracts | Symposia | Slide Sessions | Poster Sessions
Theta tACS Over Left DLPFC Modulates Resting-State Oscillations and ERP Indices of Novel Word Learning in Middle-Aged Adults
Poster Session C, Thursday, October 1, 10:45 am - 12:45 pm, Wangari Maathai
Joonwoo Kim1, Juyong Park1, Kichun Nam1; 1Korea University
Theta-frequency transcranial alternating current stimulation (tACS) is hypothesized to enhance memory encoding by entraining neural oscillations supporting hippocampal–prefrontal communication. While theta tACS effects have been examined in young adults, evidence in middle-aged adults, a population vulnerable to early memory decline, remains limited. The temporal relationship between tACS-induced changes in resting oscillations and stimulation-related modulations of task-evoked ERPs during learning has also not been jointly characterized. The present study examined whether 6 Hz theta tACS over the left dorsolateral prefrontal cortex (DLPFC) modulates resting-state oscillatory power, ERP signatures of recognition memory, and ERP indices of lexical-semantic access for newly learned non-words. Eleven middle-aged adults (50–60 years) completed a within-subject sham–active design with sessions one week apart and stimulation order counterbalanced. During each 20-minute stimulation period, participants studied novel non-word–word pairs (42 total, 21 per session) through passive reading, 4AFC, and cued recall. Word relatedness (orthographic, phonological, unrelated) was manipulated, but the present analyses were collapsed across relatedness due to limited per-cell trial count. Following stimulation, participants completed a recognition test and a lexical decision task (LDT). EEG was recorded continuously from 64 channels, including pre/post resting-state segments. Behavioral data were analyzed with linear mixed-effects models. Resting-state power changes were evaluated using cluster-based permutation tests over frequency × channel, whereas ERP contrasts (Old vs. New; Learned vs. Novel) were evaluated over time × channel, separately within sham and active conditions. In the resting-state EEG, active stimulation was associated with a pre-to-post reduction in delta–theta power (2.5–6.0 Hz) across 52 of 64 channels (p = .008), overlapping the stimulation frequency range. Sham stimulation showed only a beta-band reduction, consistent with nonspecific time-on-task effects. In the recognition test, accuracy was higher following active than sham stimulation (β = 0.886, p = .008), without a learning-status × stimulation interaction. Active-condition ERPs revealed two positive clusters: a centro-parieto-occipital LPC-like effect (Old > New; 456–500 ms, 21 channels, p = .012) and a fronto-central old/new effect in the FN400 time range (384–426 ms, 19 channels, p = .044). Neither effect reached significance under sham. In the LDT, no stimulation effect emerged on accuracy or RT, although learned non-words showed lower rejection accuracy than novel non-words across conditions (p < .001). Under active stimulation, learned items elicited reduced centro-parieto-occipital N400-like negativity relative to novel items (Learned > Novel; 448–552 ms, p = .001), accompanied by a fronto-central negativity (426–548 ms, p = .005). Sham stimulation showed no significant ERP clusters. These preliminary findings suggest that 6 Hz theta tACS over left DLPFC may modulate resting-state oscillatory dynamics and task-evoked ERP indices of novel word learning in middle-aged adults. Active stimulation was associated with improved recognition performance and enhanced ERP sensitivity to newly learned lexical forms, even in the absence of stimulation-related behavioral effects in lexical decision. Together, these results provide preliminary converging evidence that theta stimulation over left DLPFC may influence lexical-semantic processes involved in novel lexical learning. An expanded sample (target N = 25) is currently being collected to further assess effect robustness and individual differences.
Topic Areas: Language Development/Acquisition, Meaning: Lexical Semantics