Advancing Research on Unconscious Priming: When can Scientists Claim an Indirect Task Advantage?

Current literature holds that many cognitive functions can be performed outside consciousness. Evidence for this view comes from unconscious priming. In a typical experiment, visual stimuli are masked, such that participants are close to chance when directly asked to which of two categories the stimuli belong. This close-to-zero sensitivity is seen as evidence that participants cannot consciously report the category of the masked stimuli. Nevertheless, the category of the masked stimuli can indirectly affect responses to other stimuli (e.g., reaction times or brain activity). Priming is therefore seen as evidence that there is still some (albeit unconscious) sensitivity to the stimulus categories, thereby indicating processing outside consciousness. Although this "standard reasoning of unconscious priming" has been used in many studies, we show that it is flawed: Sensitivities are not calculated appropriately, hereby creating the wrong impression that priming indicated better sensitivity than the close-to-zero sensitivity of the direct discrimination. We describe the appropriate way to determine sensitivities, replicate the behavioral part of a landmark study, develop a method to estimate sensitivities for published studies from reported summary statistics, and use this method to reanalyze 15 highly influential studies. Results show that the interpretations of many studies need to be changed and that a community effort to reassess the vast literature on unconscious priming is needed. This process will allow scientists to learn more about the true boundary conditions of unconscious priming, thereby advancing the scientific understanding of consciousness.