Gamma tACS Over the Prefrontal and Parietal Cortices Enhances Episodic Memory Performance

Kenta Honma^*Tomonori Nomura

• Niigata University of Health and Welfare, Niigata, Japan

Background: Episodic memory is a critical component of daily functioning and is vulnerable to aging and neurological disorders. Gamma-frequency transcranial alternating current stimulation (tACS) has been proposed as a noninvasive approach to modulate memory-related neural activity. Objective: This randomized, sham-controlled study examined whether gamma-frequency tACS applied to the left prefrontal cortex (PFC) and posterior parietal cortex (PPC) during encoding and retrieval is associated with differences in episodic memory performance in healthy young adults. Methods: A total of 51 right-handed adults with no underlying health issues (mean age = 20.9 ± 1.0 years) were randomly assigned to one of three groups: two-site stimulation over the left PFC and PPC (PFC– PPC, n = 17), single-site stimulation over the left PFC (PFC, n = 17), or sham stimulation (n = 17). Participants completed a verbal recognition task across three sessions (Days 1, 2, and 7). On Days 1 (learning phases) and 2 (recognition phases), 60 Hz tACS (1.5 mA) was delivered. The primary outcome was the discrimination index (d-prime) on Day 7. Accuracy and d-prime were analyzed using two-way repeated-measures analysis of variance and effect sizes (Hedges' g). Results: Significant effects of time and time-by-group interactions were observed for accuracy and d-prime. The PFC–PPC group showed higher d-prime scores than the sham group on Days 2 and 7, with medium-to-large effect sizes. Single-site PFC stimulation was also associated with numerically higher d-prime scores relative to sham on Day 7. Although effect sizes were larger in the PFC–PPC group than in the PFC group, direct comparisons between the two active stimulation conditions did not yield statistically robust differences. Conclusions: These findings provide preliminary behavioral evidence that gamma-frequency tACS delivered to memory-related cortical regions is associated with differences in episodic memory discrimination at delayed time points. However, the results are based solely on behavioral measures, and replication with larger and more diverse samples, as well as studies incorporating neurophysiological recordings, will be necessary to clarify the underlying mechanisms and robustness of these effects.