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. 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). 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). 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). 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). 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. absence of visual targets). 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. 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.