Shrey Grover¹, Feiyang Cao¹, Phillip (Xin) Cheng², Robert M. G. Reinhart¹; ¹Boston University, ²Princeton University

Spatial attention samples the environment rhythmically, waxing and waning at a theta frequency (2–8 Hz). However, the neural mechanisms giving rise to this phenomenon remain under investigation. One possibility is that interhemispheric synchrony drives attentional sampling, given that bilateral, particularly parietal, networks coordinate dynamically during attentional allocation. Here, we tested this hypothesis by manipulating synchronization between bilateral parietal cortices using inphase or antiphase high-definition transcranial alternating current stimulation (HD-tACS). Prior work shows that inphase HD-tACS enhances interregional synchrony, whereas antiphase stimulation disrupts it. We designed optimized montages for inphase and antiphase stimulation that targeted bilateral parietal regions at comparable voltage gradients (0.19 V/m). Attentional sampling was measured with a probabilistic cueing task in which participants detected targets embedded in dynamic distractors across varying stimulus-onset asynchronies (SOAs), with blocks before, during, and after stimulation. We delivered HD-tACS at 4 Hz, matching the intrinsic frequency of attentional sampling reported in such tasks, and quantified changes in rhythmicity as decibel-normalized spectral-power changes during and after stimulation relative to baseline using the reaction time-SOA function. We found that inphase HD-tACS (N = 28) modulated rhythmicity as a function of the difference between the stimulated frequency (4 Hz) and participants’ intrinsic sampling frequency. Individuals with larger mismatches showed increased power at the stimulated frequency and decreased power at their intrinsic frequency during stimulation, and the increased contribution of the stimulated frequency persisted after stimulation. By contrast, antiphase HD-tACS (N = 29) did not show graded effects but produced a general reduction in low-frequency rhythmicity post-stimulation. These results demonstrate that noninvasive stimulation can causally modify attentional sampling, suggest the possibility of multiple rhythmic oscillators which can be independently entrained through extrinsic stimulation, and show that bilateral parietal dynamics directly contribute to the emergence and strength of behavioral rhythmicity.