The ability to learn, remember, and respond to emotional events is a powerful survival strategy. However, dysregulated behavioral and physiological responses to these memories are maladaptive. To fully understand learned fear and the pathologies that arise during response malfunction we must reveal the environmental variables that influence learned fear responses. Light, a ubiquitous environmental feature, modulates cognition and anxiety. We hypothesized that light modulates responses to learned fear. Using tone-cued fear conditioning, we found that light enhances behavioral responses to learned fear in C57BL/6J mice. Mice in light freeze more in response to a conditioned cue than mice in darkness. The absence of significant freezing during a 2-wk habituation period and during intertrial intervals indicated that light specifically modulates freezing to the learned acoustic cue rather than the context of the experimental chamber. Repeating our assay in two photoreceptor mutant models, Pde6b rd1/rd1 and Opn4 −/− mice, revealed that lightdependent enhancement of conditioned fear is driven primarily by the rods and/or cones. By repeating our protocol with an altered lighting regimen, we found that lighting conditions acutely modulate responses when altered between conditioning and testing. This is manifested either as an enhancement of freezing when light is added during testing or as a depression of freezing when light is removed during testing. Acute enhancement, but not depression, requires both rod/cone-and melanopsin-dependent photoreception. Our results demonstrate a modulation by light of behavioral responses to learned fear.L ight is a pervasive feature of the environment and exerts broad effects on behavior and physiology via two parallel pathways (1). The familiar image-forming visual pathway allows discernment of objects in the environment according to physical qualities: their color, form, texture, and motion. The parallel non-image forming (NIF) pathway enables light to exert numerous effects on physiology and behavior independently of image formation, such as pupil constriction, modulation of heart rate, and the synchronization of circadian rhythms and sleepwake cycles to the daily light-dark cycle (2). In addition to the effects of light on basic physiological functions, light also modulates higher-order cognitive processes, including anxiety, mood, and alertness/awakeness (3, 4). The retina, the sole photosensory organ in mammals, projects directly to brain regions involved in emotional responses. Among these are the amygdala, the bed nucleus of the stria terminalis, and the periaqueductal gray (5). Activity in some of these regions is known to be acutely modulated by light in a wavelength-dependent manner (3, 6), whereas the link between photoreception and function in other retinorecipient emotional processing areas remains to be elucidated.Brain sites involved in emotional processing participate in the critical function of learning and remembering emotionally arousing events. This function enables ...