Fast unconscious fear conditioning

How do conscious and unconscious visual processing differ? Suppressing visual stimuli from awareness has previously revealed quantitative differences, either reducing or eradicating their processing. Finding qualitative differences, however, would further benefit the understanding of systems mediating conscious versus unconscious processing. Here, we show such a difference in the timing of conscious and unconscious fear acquisition, an essential function for adaptive behavior. Threatening stimuli can be detected, and the fear they elicit physiologically expressed, outside awareness; but whether new fear can be learned for stimuli suppressed from awareness is unknown. We used Pavlovian fear conditioning, suppressing long-duration conditioned stimuli (CSs) from awareness with continuous flash suppression (CFS). During CFS, stimuli presented to one eye are rendered invisible by salient dynamic stimulation of the other eye. One suppressed stimulus (CS+) was occasionally paired with a shock, whereas the other (CS−) was not. Importantly, the temporal parameters of CFS enabled measurement of participants' skin conductance responses (SCRs) during acquisition, so the development of learning could be tracked over time. Two participant groups were conditioned with identical CSs, either with CFS (unaware group) or without it (aware group). We found significantly greater SCRs to the CS+ in both groups, but the temporal pattern of learning differed. Robust learning was observed only during early acquisition (1st half of the 36-trial session) for unaware participants, and only during late acquisition (2nd half) for aware participants. Conditioning magnitude was negatively correlated with state anxiety in both groups, but only during the stage in which differential learning occurred. Conscious fear acquisition developed gradually, whereas unconscious fear conditioning was rapid but habituated swiftly. Unconscious learning may therefore involve automatic orienting to threats, probably mediated by amygdala activity (known to habituate quickly); conscious conditioning may employ higher-level cognitive mechanisms (perhaps involving cortical structures) allowing associations to form over time.