For decades, psychologists have sought to determine the degree to which invisible stimuli are processed without conscious awareness. Recently, a promising new approach has been introduced for examining preserved high-level processing of perceptually suppressed stimuli. This approach makes use of continuous flash suppression (CFS), a variant of binocular rivalry to render stimuli presented to one eye invisible. Unconscious processing is inferred from the time that initially invisible stimuli need to overcome such interocular suppression and become visible. This breaking CFS paradigm has received increasing popularity, and many studies have interpreted shorter suppression durations for familiar, meaningful or emotional stimuli as evidence for enhanced unconscious processing of such stimuli. In the present study, we asked if the breaking CFS paradigm indeed reflects unconscious stimulus processing or if other factors such as lower detection thresholds or criteria can account for reduced suppression durations. In a series of six experiments we compared the time it took upright and inverted faces to become visible during CFS and in perceptually similar control conditions not involving interocular suppression. We found that not only during CFS, but also in appropriate control conditions upright faces were detected faster and more accurately than inverted faces. The breaking CFS paradigm may thus simply reflect different thresholds or criteria for visual stimulus detection, rather than unconscious processing. We conclude that, unless proven otherwise, the breaking CFS paradigm is not capable of providing unequivocal evidence for unconscious visual processing. Instead, we propose an alternative approach for measuring the processing of high-level stimulus attributes during interocular suppression. We developed an indirect probe detection method that circumvents the influence of detection thresholds and criteria and hence provides an accurate and unbiased estimate of suppression durations during CFS.
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