Theta and High-alpha Phase-locking Dynamics Unveil Face Perception in Ambiguous Stimuli

Previous studies emphasize phase synchronization as a fundamental mechanism for integrating local features into coherent percepts. We employed a novel paradigm and dynamic graph analysis based on EEG to track neural dynamics associated with perceiving a face (or not) while keeping the stimuli identical. Thirty participants underwent a pretest to establish perceptual thresholds for detecting faces within images overlaid with visual noise. These thresholds were then applied in an EEG experiment with the same task, focusing on images with 50% and 75% detection probabilities. In the high-alpha band, we observed increased coupling (100–275 msec after stimulus onset) between the left and right occipitotemporal electrodes when an ambiguous stimulus was perceived as a face, indicating conscious face perception. The failure to perceive a face resulted in enhanced theta band phase synchronization between bilateral occipitotemporal electrodes and increased high-alpha-band coupling between the left frontal and right occipitotemporal electrodes, which showed face-selective responses. These synchronizations are likely to reflect the continuous gathering of individual bits of facial information rather than signaling the presence or absence of a face percept. Additionally, we identified frontal–occipitotemporal theta-band couplings related to the stimulus's informational content, irrespective of whether the inherent face was perceived. Our findings reveal distinct temporal dynamics in phase synchronization at specific frequencies, indicating a specialized system for face perception during integrated information processing across the frontal and visual cortex, resolving ambiguities.