Na Zhao^1,2 (), LiNan Shi¹, Sheng He^1,2; ¹State Key Laboratory of Brain and Cognitive Science, Institute of Biophysics, Chinese Academy of Sciences, ²University of Chinese Academy of Sciences, Beijing

Brain oscillatory activities are strongly linked with cognitive functions. For example, gamma band oscillations are associated with higher-order cognitive processes. Recent reports suggest that the 40 Hz gamma band rhythm can stimulate microglial activation and facilitate the clearance of amyloid-β, thereby potentially improving cognitive function. However, such a suggestion was questioned by other studies in mice showing that sensory stimulation at this frequency does not consistently induce oscillations in brain regions such as the hippocampus. This study explores the efficacy of flicker stimulation in entraining deeper brain structures, including the hippocampus. We employed three types of stimuli (checkerboard patterns, famous faces, and landscapes) flickering at varying frequencies (20, 30, 40, and 48 Hz). Functional Magnetic Resonance Imaging (fMRI) revealed that high-level stimuli (faces and landscapes) triggered broader activation in the temporal lobe compared to low-level stimuli (checkerboard patterns). Compared with high-level stimuli, the activation of lower-level stimuli diminished more rapidly in the temporal cortex as the flicker frequency increased. Additionally, using magnetoencephalography (MEG), we examined if the frequency-specific activation signals in the temporal cortex. The power spectral analysis revealed a decrease in signal power with increasing flicker frequency across all conditions, with high-level stimuli showing a slightly more gradual decline in signal power. However, there was no significant advantage of high-level over low-level stimuli at a 40Hz frequency. Collectively, our findings suggest that high-level stimuli may be more effective in inducing gamma band entrainment in deep brain structures compared to low-level stimuli.