Such concern came into sharp focus with the development of signal detection theory (SDT Green & Swets, 1966). For instance, subjects may be reluctant to indicate having seen a stimulus when the available evidence is very weak. Historically, an important concern with subjective procedures is that they can be quite sensitive to response bias (Eriksen, 1960). In fact, there is a mounting tension between objective and subjective threshold approaches (Merikle et al., 2001 Snodgrass, Bernat, & Shevrin, 2004). Unfortunately, no consensus exists regarding the “best” approach to measure and characterize awareness. Under such conditions, behavioral effects of unaware stimuli (e.g., faster reaction time for undetected fearful faces), as well as associated physiological or neuroimaging signals, would constitute correlates of unaware perception. Thus, the perception of visual stimuli is objectively unaware when the sensitivity measure of awareness (such as d′) is at the null level-that is, subjects exhibit null sensitivity. According to objective criteria, unaware conditions occur when a subject's performance in a yes/no or forced-choice task is at chance, such as when subjects fail to detect alternative stimulus states (presence vs. On the other hand, objective criteria have been used in studies that have suggested that awareness may be necessary for the processing of emotional faces (Pessoa, Japee, & Ungerleider, 2005 Pessoa, Japee, Sturman, & Ungerleider, 2006). Subjective criteria hold that only the subjects themselves have access to their inner states and that their introspection is a reliable source of information about conscious experiences (Merikle, Smilek, & Eastwood, 2001). According to subjective criteria, unaware perception occurs when subjects report not having seen target stimuli or being unable to perform the task better than chance (independent of their actual performance). On the one hand, behavioral and neuroimaging studies that report unaware perception of emotional faces have often evaluated awareness according to subjective criteria (e.g., Whalen et al., 1998). One factor that may help explain the discrepancy of previous results concerns the use of different criteria to determine whether or not a subject is aware of perceiving a stimulus. The determination of such a dissociation zone may help in understanding the conditions linked to aware and unaware fear perception. Taken together, our findings are consistent with a dissociation of fear perception according to objective and subjective criteria, which could be assessed for each individual participant. The analysis of single-session data suggests that previous experiments may have lacked sufficient statistical power to establish above-chance performance. Reliable subjective sensitivity was also observed for 33-ms fearful-face targets and, for some subjects, even for 17-ms targets. Our findings revealed that nearly all subjects could reliably detect 17-ms fearful-face targets, thus exhibiting above-chance objective perception at this target duration. To evaluate single-participant awareness, we employed a nonparametric receiver operating characteristic (ROC) analysis of the behavioral data, which involved collecting a large number of trials over multiple sessions. Both objective and subjective sensitivity measures were assessed within a common signal detection theory framework. Whereas previous studies of fearful-face perception have probed visual awareness according to either objective or subjective criteria, in the present study, we probed the perception of briefly presented and masked fearful faces by assessing both types of perception within the same task.
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