The effects of stimuli that are so weak the receiver is unaware of their presence.
The term subliminal is derived from the Latin words sub (below) and limen (threshold). The threshold, in this case, is the threshold of conscious awareness. Can we be influenced by stimuli that are so faint or brief that we are unaware of their presence? In other words, can people be affected by invisible stimuli? This controversial notion has intrigued scientists and the public for decades. A public relations stunt in 1957 triggered widespread concern that consumers were being induced to "eat popcorn" and "drink cola" by means of subliminal messages flashed onto a movie screen. Although there was never any good evidence that this procedure actually worked, the possibility of such "mind control" caused considerable alarm.
Careful laboratory research has explored the extent to which subliminal stimulation can affect our behavior. The best evidence for subliminal perception comes from studies on semantic priming. In a priming task, the viewer's task is to decide whether or not a presented letter string (the target) is a word or not. The task is not difficult. If the target is a legitimate word (e.g., DOCTOR), the respondent pushes the "yes" button. If it isn't a word (e.g., TORCOD), he pushes the "no" button. Of special interest is how long it takes the subject to make his or her decision. Reaction times are faster when the target is preceded by a word whose meaning is similar to the target's meaning (e.g., NURSE). Thus, people will identify the target string DOCTOR as a word more quickly if it is primed with NURSE than they would if it were primed with an unrelated word like TRUCK. This priming effect is a well-established phenomenon. Researchers interested in subliminal perception wondered what would happen if the prime was presented so briefly that the viewer could not recognize it. Would the priming effect still occur? The answer is yes, and this finding is interesting because it shows that the prime initiates cognitive activity in the brain , even though the viewer does not feel as if any word recognition took place. Thus, there is a discrepancy between what is perceived and what the viewer is aware of having perceived.
It is important to emphasize that subliminal priming is obtained under extraordinarily artificial conditions, and that the effect is very subtle and brief. It does not show that people's motives, beliefs, or behavior can be significantly altered by secret messages. In fact, two-word subliminal primes do not appear to work. While some researchers have reported subliminal effects of a more profound nature than a mere priming effect, the claims are, at best, controversial and sometimes completely false. For example, in the 1980s subliminal auditory tapes were advertised as being able to produce many desirable effects, including weight loss, memory enhancement, and improvement of sexual function. Extensive testing by researchers has shown that these products have no therapeutic utility.
Timothy E. Moore
Moore, T. E., "Subliminal Perception: Facts and Fallacies" Skeptical Inquirer, 16, 273-281, 1992.