Breakthrough thresholds in continuous flash suppression are tuned to mask temporal frequency but suppression depth is constant

Continuous flash suppression (CFS) is a popular method for suppressing visual stimuli from awareness for extended periods. It involves a dynamic, high-contrast masking stimulus presented one eye which suppresses a target stimulus presented to the other. The strength of suppression is usually inferred from how long it takes for the target to breakthrough from suppression into awareness (the bCFS threshold). A new variant known as ‘tracking CFS’ (tCFS) directly measures the strength of suppression by measuring both breakthrough and suppression thresholds. Here, we employ the tCFS paradigm while varying the temporal frequency of the masking stimulus. Our data reveal two clear results: (i) CFS exhibits a clear temporal frequency tuning, with bCFS thresholds peaking for masks modulating at ~1 Hz; (ii) suppression depth (the difference between breakthrough and suppression thresholds) remains constant despite changes in bCFS. The first result confirms an earlier finding that peak bCFS occurs for very low temporal frequencies. The second result provides a valuable insight in showing that bCFS changes occur completely independently of suppression strength, which remains constant. In this study, suppression averaged 13 dB, around 2–3 times stronger than suppression reported in binocular rivalry studies.