Subliminal emotion perception in brain imaging: findings, issues, and recommendations

Many theories of emotion propose that emotional input is processed preferentially due to its relevance for the organism. Further, because consciousness has limited capacity, these considerations imply that emotional input ought to be processed even if participants are perceptually unaware of the input (subliminal perception). Although brain imaging has studied effects of unattended, suppressed (in binocular rivalry), and visually masked emotional pictures, conclusions regarding subliminal perception have been mixed. The reason is that subliminal perception demands a concept of an awareness threshold or limen, but there is no agreement on how to define and measure this threshold. Although different threshold concepts can be identified in psychophysics (signal detection theory), none maps directly onto perceptual awareness. Whereas it may be tempting to equate unawareness with the complete absence of objective discrimination ability (d'=0), this approach is incompatible with lessons from blindsight and denies the subjective nature of consciousness. This review argues that perceptual awareness is better viewed as a continuum of sensory states than a binary state. When levels of awareness are characterized carefully in terms of objective discrimination and subjective experience, findings can be informative regarding the relative independence of effects from awareness and the potentially moderating role of awareness in processing emotional input. Thus, because the issue of a threshold concept may never be resolved completely, the emphasis is to not prove subliminal perception but to compare effects at various levels of awareness.