Sahereh Varastegan¹, Weiwei Zhang²; ¹Ph.D. Student at University of California, Riverside, ²Professor of Psychology at University of California, Riverside

Converging evidence suggests that alpha-band activities within the fronto-parietal networks are linked to the limited capacity of Visual short-term memory (VSTM). To provide causal evidence supporting the roles of fronto-parietal alpha synchronization for VSTM, the present study tested whether the phase relationship between frontal and parietal alpha oscillations affect VSTM capacity. Healthy adults performed a lateralized delayed-estimation task with a continuous color-wheel report while receiving bifocal alpha-frequency transcranial alternating current stimulation (tACS) over fronto-parietal electrode pairs. On each trial, stimulation within the left or right hemisphere was delivered either in-phase or anti-phase across frontal and parietal sites. Recall errors were analyzed with the mixture model to dissociate capacity and mnemonic precision. The findings from 26 participants showed that in-phase fronto-parietal tACS reliably increased VSTM capacity relative to anti-phase stimulation, and this enhancement was selective for items presented contralateral to the stimulated hemisphere. In contrast, mnemonic precision was comparable across conditions. These findings provide causal evidence that phase-aligned fronto-parietal alpha activity selectively increases the number of items that can be stored in VSTM, without substantially altering mnemonic precision of those representations.