Abstract
In recent years the results from studies employing Continuous Flash Suppression (CFS) have been used as evidence that stimuli that do not reach consciousness are still processed to some degree by the visual system. However, little is known about whether a given type of CFS mask works equally well for all kinds of targets, or if different targets may be suppressed to different degrees. To investigate this, we employed a b-CFS paradigm with photographic face/house stimuli as targets. Faces are well known to break CFS faster than houses, in particular when using traditional Mondrian masks. These are very rectangular in their content, emphasizing cardinal orientations, and are thus hypothesized to match houses better than faces, possibly giving an “unfair” advantage to the face stimuli. To counter this, we designed random noise masks to match the spatial frequency and orientation contents of each individual target image (target-matched), thus ensuring fairness across target categories. As a more general alternative, we also generated masks from the average spatial frequency and orientation contents across both target categories. Data from 24 participants replicated the face advantage with Mondrian masks (13.5% faster, t(23)=3.60, p=0.002). Even with the target-matched masks, faces still broke suppression faster than houses (4.3% faster, t(23)=3.07, p=0.005); however the difference was significantly smaller (t(23)=2.45, p=0.022). Using the average masks, the face advantage remained as well (11.1% faster t(23)=5.47, p<0.001), however the difference was larger than with the target-matched masks (t(23)=2.83, p=0.009). We conclude that, while faces are privileged in the visual system even before reaching awareness, a portion of that advantage (here more than 60%) may actually come from unbalanced masking or unintentional mask-target interaction rather than the target stimuli alone. CFS masks should be carefully designed to optimally match the desired target material, otherwise results may suffer from potentially fatal bias.