Backward masking is a form of masking in which the target (i.e., a to-be-identified item) temporally precedes that mask stimulus (an image, a tone, or a chemical). The mask can occur immediately after the target or after a delay. The difficulty in one’s ability to perceive the target in this situation is referred to as the masking effect. This effect is influenced by the interval between the presentation of the mask and target, the location of the mask and target relative to each other (e.g., whether two images are superimposed or not), the intensity of the mask and target (e.g., the brightness of a letter), as well as other characteristics (e.g., the direction of the line segments). All else being equal, masking is generally stronger as the interval between the presentation of the mask and that of the target is reduced.