Effects of bright light exposure on human fear conditioning, extinction, and associated prefrontal activation

Bright light (BL) not only regulates human emotion and circadian physiology but can also directly modulate emotional memories. Impaired fear extinction and enhanced fear acquisition and consolidation are hallmarks of fear-circuitry disorders; thus, we tested whether BL facilitates fear extinction and inhibits fear acquisition. We randomly exposed 29 healthy humans to high- (9000 lx) or low-intensity light (<500 lx) for 15 min, near the nadir of the phase response to light, in a single-blind manner. Simultaneously with the light exposure, subjects performed fear extinction training and second fear acquisition, where a visual conditioned stimulus (CS), previously paired with an electric shock unconditioned stimulus (US), was presented without the US, while another CS was newly paired with the US. Conditioned responses (CRs) and changes in prefrontal cortex (PFC) activity were determined during encoding and delayed recall sessions. BL-exposed subjects exhibited lower extinction-related PFC activity and marginally higher acquisition-related PFC activity during light exposure than subjects exposed to control light. Twenty-four hours later, BL reduced CRs to both the extinguished and non-extinguished CSs with marginally lower extinction-related PFC activation, suggesting that BL enhanced fear extinction, while suppressing fear acquisition. Further, BL sustained tolerance to fear re-conditioning. Our results demonstrate that a single and brief BL exposure, synchronized with fear extinction and acquisition, instantaneously influences prefrontal hemodynamic responses and alleviates fear expression after 24 h. Although the specificity of BL effects deems further investigation, our findings indicate the clinical relevance of adjunctive BL intervention in exposure-based cognitive-behavioral therapy for fear-circuitry disorders.