Backward masking has several meanings:
The original meaning of the term, in psychoacoustics, refers to temporal masking of quiet sounds that occur moments before a louder sound.
A similar meaning, in use in cognitive psychology, refers to a phenomenon wherein presenting one visual stimulus (a "mask" or "masking stimulus") immediately after another brief (≤ 50 ms) "target" visual stimulus leads to a failure to consciously perceive the first stimulus.[1] It is unknown how a later stimulus is able to block an earlier one. However one theory for this phenomenon known as the dual channel interaction theory, proposes that a fast signal created by the second stimulus is able to catch up to and overcome a slower signal sent from the first impulse.[2] A similar phenomenon can occur when a masking stimulus precedes a target stimulus rather than following it: this is known as forward masking.[1] While not consciously perceived, the masked stimulus can nevertheless still have an effect on cognitive processes such as context interpretation. It has been shown that visually masked stimuli can elicit motor responses in simple reaction-time tasks (e.g., Response Priming) independent of their conscious visibility.[3] It is a widespread belief that masked stimuli can be used for psychological manipulation (see subliminal messages, psychorama). However, the empirical evidence for subliminal persuasion is limited.
In popular music, "backward masking" incorrectly refers to backmasking, or hiding messages in sound recordings that are audible when played backward.