Subliminal Perception

Definition and Overview

The term subliminal perception refers to one's ability to perceive stimulation below the limen. The limen refers to the amount of intensity at which the stimulus can be noticed half the time. That is, our sensory systems are not capable of detecting all the stimulation present in the environment. The stimulation has to reach some intensity before it can be noticed. Subliminal stimulation refers to situations in which a stimulus is presented at an intensity below the limen; hence, the stimulus is seldom, if ever, perceived with awareness.

The American Heritage Dictionary provides two definitions for perceive. The first is “to become aware of directly through any of the senses, especially sight or hearing.” The second is “to achieve understanding of; apprehend.” The implication is that when sensory information is perceived, one may become aware of the stimulus provoking perception (i.e., supraliminal perception), or one may gain some understanding of the stimulus even in the absence of awareness (i.e., subliminal perception). This latter possibility has intrigued researchers for over a century: Is it possible for one to perceive information that somehow alters our understanding of the world without our awareness of said perception occurring?

Brief History

This question of whether stimuli presented in a subliminal manner can be perceived despite the absence of awareness was the focus of the first published article from a psychology laboratory in North America. In 1884, Charles Pierce and Joseph Jastrow asked participants to guess whether cards contained letters or digits after first establishing a presentation distance wherein participants claimed to be unaware of what was on the cards. Guessing performance was above chance, which the authors attributed to some entity other than the “primary waking self,” perception in the absence of awareness.

This issue became of interest to the general public in 1957 when an unemployed market researcher named James Vicary claimed to have subliminally presented the words “Drink Coca Cola,” and “Eat popcorn,” during a movie, resulting in an increase of product sales. Later, Vicary admitted his study was a marketing stunt with the intent of increasing the number of movie goers. However, the report spawned the worry that it might be possible to influence human behavior without their awareness, a worry reinforced by subsequent books such as Wilson Bryan Key's Subliminal Seduction; Ad Media's Manipulation of a Not So Innocent America.

The Scientific Debate

Given the public interest, psychological researchers began attempts to scientifically document, or refute, the existence of subliminal perception. Some argued there was clear evidence for subliminal perception, but critics countered that the methodologies employed were insufficiently rigorous to sustain such claims. The crux of these disagreements centered on the procedure that was used to document subliminal perception.

To demonstrate perception of a stimulus presented at energy levels low enough to preclude awareness, one must first devise a scientific way of measuring awareness and then find an energy level at which this measure indicates null sensitivity. Once established, if some other measure indicates the stimulus is being processed, then subliminal perception has been demonstrated. This methodological approach is termed the dissociation paradigm, as the goal is to dissociate some general measure of perception from a more specific measure of perception leading to awareness. The study by Pierce and Jastrow highlighted previously provides such an example in the sense that participants’ claims were used to indicate awareness, and their guessing performance provided the more general measure of perception. When guessing performance remained above chance despite claimed unawareness, the general measure of perception was dissociated from the specific measure of perception resulting in awareness.

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