Backward Masking

Backward masking is a perceptual phenomenon in which the detection or identification of a briefly presented stimulus, called the target, is impaired because a second stimulus, called the mask, is presented immediately afterward in the same location, even though the mask comes after the target rather than before it. In vision research, a common demonstration presents a target letter or shape for a very brief duration followed almost immediately by a masking pattern, and researchers measure how the delay between target and mask, known as the stimulus onset asynchrony, affects how accurately observers can report what the target was, with very short delays producing strong masking and longer delays allowing the target to be seen clearly. Because it can suppress or degrade conscious perception of a stimulus that was nonetheless physically presented for long enough to register on the retina, backward masking has been an important experimental tool for studying the time course over which visual information is processed and consolidated into a stable conscious percept, and for probing the extent to which a masked stimulus can still influence behavior even when it cannot be consciously reported. One influential variant, metacontrast masking, uses a mask whose contours immediately surround rather than overlap the target, and has been particularly important in debates over how and when visual information becomes available to conscious awareness.