Mask contrast and size do not alter suppression depth in the tracking continuous flash suppression paradigm

Continuous flash suppression (CFS) is a variant of interocular conflict that occurs when one eye views a dynamic high-contrast mask that increases the duration of target suppression. A variant of CFS known as tracking continuous flash suppression (tCFS) was developed, allowing the depth of interocular suppression to be measured. Although previous research has measured how the duration of suppression may be modulated by the contrast and size of the masking stimulus, no study has assessed how mask features impact suppression depth. In our first study, we manipulated mask contrast to measure the consequent impact on suppression depth as measured by the tCFS procedure. We observed that high mask contrast increased the threshold required for a target to break into awareness. Critically, the decrease in contrast required to re-suppress each target was proportionately the same across all conditions so that suppression depth-the ratio of the two thresholds-remained constant. In the second experiment, we manipulated the size of the masking stimulus and found no change in breakthrough/suppression thresholds or suppression depth (i.e., the difference between the thresholds when using log-contrast). These findings clarify that, although changes in mask contrast may alter the threshold to enter awareness, there is no overall change in suppression depth as the changes in breakthrough threshold are reflected by proportionately equivalent changes in suppression threshold. This result matches findings obtained with binocular rivalry showing that suppression depth is constant despite changes in stimulus contrast. Differing levels of mask contrast and size, therefore, can be used by researchers in CFS without altering the strength of suppression, consistent with the perspective that interocular suppression operates in small local spatial zones determined by receptive field size in the primary visual cortex.