The ability to learn which stimuli in the environment pose a threat is critical for adaptive functioning. Visual stimuli that are associated with threat when they are consciously perceived can evoke physiological [1] and neural [2] responses consistent with fear arousal even when they are later suppressed from awareness. It remains unclear, however, whether a specific new fear association can be acquired for stimuli that are never consciously seen [3], and whether such acquisition develops differently from conscious learning. It has recently been suggested [4] that, rather than simply affording a degraded version of conscious experience, processing of emotional stimuli without awareness may differ qualitatively from conscious perception, evoking different patterns of neural activity across the brain or differences in the time-course of behavioral and physiological responses. Here, we investigated nonconscious fear acquisition and how it may differ from conscious learning using classical fear conditioning, and found that conscious and unconscious fear acquisition both occur, but evolve differently over time.
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