Jacob Coorey¹, Matthew Davidson², David Alais¹; ¹The University of Sydney, ²University of Technology Sydney

In continuous flash suppression (CFS), a high-contrast dynamic mask in one eye suppresses a small, static, low-contrast target in the other. While mask size and contrast have been shown to affect suppression duration, their impact on suppression depth—a measure of the strength of suppression—remains unexplored. Additionally, the influence of high-order predictive factors, such as expectations of target location and identity, on suppression depth is unknown. We investigate these questions using a new tracking-CFS (tCFS) method. We measured breakthrough thresholds (a weak target increasing in contrast until visible), suppression thresholds (a strong target decreasing in contrast until suppressed), and suppression depth (the ratio of suppression-to-dominance). In our first study, we manipulated participant expectations by having stimuli unexpectedly change location or identity while suppressed. In our second study, we manipulated the size and contrast of the masking stimulus. Our findings reveal three key points: (i) suppression depth is unaffected by target predictability; (ii) suppression depth does not change with mask size; (iii) suppression depth increases with higher mask contrast. The lack of significant effects from mask size suggests an early interocular suppression process, likely occurring in V1, where interocular conflict is first detected in smaller receptive fields. Similarly, the absence of effects from target expectation aligns with our previous findings of consistent suppression depth irrespective of target features. These results suggest that suppression occurs before global target properties are processed. Finally, the change in suppression depth with increased mask contrast indicates that the ask masking strength increases, so to does the required contrast change from the target image. Future research will explore whether the surround of a small mask impacts suppression depth in CFS, as it does in binocular rivalry, to further examine and distinguish between these phenomena.