1. Definition

The idea that we perceive information without awareness has a long history. Nearly 300 years age, Leibniz, in his New Essays on Human Understanding, gave a clear statement of the possible role that unconscious perceptual processes may play in directing behavior when he wrote that

“There are hundreds of indications leading us to conclude that at every moment there is in us an infinity of perceptions, unaccompanied by awareness or reflection” and that “the choice we make arises from these insensible stimuli, which, mingled with the actions of objects and our bodily interiors, make us find one direction of movement more comfortable than the other”<9>.

Subliminal perception refers to the perception of stimuli that are below the threshold of conscious awareness. If subliminal means that the individual cannot identify the stimulus, how can it be shown that he or she has indeed perceived it? To accomplish this, researchers typically resort to implicit tests of perception whereby the stimulus is shown to have affected the individual’s judgment <4>.  The term subliminal perception was originally used to describe situations in which weak stimuli were perceived without awareness. In recent years, the term has been applied more generally to describe any situation in which unnoticed stimuli are perceived <13>.

Pratkanis and Greenwald (1998) identify four types of subliminal stimuli: (1) subthreshold stimuli, which are presented at energy levels that are too weak to be detected by the audience (e.g., flashing the words “Eat popcorn” onto a screen so quickly that the audience is not aware of them), (2) masked stimuli, which are hidden from the audience by the presentation of some other, overriding stimuli (e.g., briefly presenting the stimulus immediately followed by a bright flash of light), (3) unattended stimuli, which are presented in such a way that the embedded figure is unlikely to be segregated from its figural context (e.g., hiding the figure of a naked body in the curves and lines of a picture of an ice cube), (4) figurally thransformed stimuli, which are words or pictures blurred or distorted to the point that they are unrecognizable (e.g., commands recorded backward and inserted into popular music) <20>.