A: Two symbols (“+” and “ × ”) used for target detection and fear conditioning. During fear conditioning, CSs were masked (top) and unmasked (bottom), with opponent-color stimuli or same-color stimuli presented to each eye, respectively. In the former case, masking produces a percept of uniform yellow. B: Schematic of stimulus presentation, with a stimulus pair being flashed five times (ON for 50 ms, OFF for 150 ms). During fear conditioning, the shock was delivered at the final termination of the unmasked CS+.