Cortical Dynamics Underlying Perceived Visibility: An Event-Related Potential Investigation of Forward Masking.

Forward masking has been a fruitful approach for understanding the temporal organization of perception and the dynamics of visual processing. However, there is still limited knowledge on the neural correlates of forward masking and cortical processes underlying the modulations of perceived visibility under this masking type. Notably, the ERP components (event-related potentials) associated with this paradigm have not been identified yet. Therefore, we designed an ERP study combined with a forward-masking paradigm and recorded cortical activities while observers performed a contour discrimination task on the target. We manipulated stimulus onset asynchrony (SOA) and mask-to-target (M/T) contrast ratio to have different masking conditions. In line with previous research, the perceived visibility of the target significantly depended on SOA, with a strong suppression elicited by the preceding mask at short SOAs. More importantly, the magnitude of this brief inhibition was significantly enhanced by the M/T contrast ratio. ERP analyses revealed nonlinear interactions in both early and late positive components. In particular, we identified nonlinear suppressions in early P1 and late positivity, and the magnitude of these suppressions became larger for the short SOA and high M/T contrast ratio. On the other hand, the nature of interactions in N1 was facilitatory and became dominant in the longer SOA. We discuss these findings in light of previous empirical and modeling studies suggesting a three-process explanation of forward masking. Our findings provide important insights into the ERP correlates of forward masking and highlight the diverse, multiprocess nature of contextual influences on perceptual dynamics.