This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

Subliminal Perception

Perception without awareness is not the same thing as "subliminal perception." Subliminal perception is supposed to occur when a stimulus is too weak to be perceived yet a person is influenced by it.

How can an undetectable stimulus be created?

As discussed in the section on psychophysics, the word limen was used in the 19th Century to refer to the absolute threshold, which was defined as the point at which a stimulus could be detected 50% of the time. By that definition, a stimulus detected 49% of the time would be subliminal. Obviously this is not what people mean when they discuss subliminal effects on human beings. They are usually referring to stimuli too weak or distorted to be detected through conscious effort. Such stimuli can be created by flashing a visual stimulus then quickly masking it with another stimulus, or by presenting a sound so weak that nobody can detect it without amplification, or by transforming a stimulus electronically until it cannot be recognized.

What was the true story behind the famous "Drink Coca-Cola" study?

Subliminal perception was given a huge publicity boost by a famous 1950s experiment in which the message "DRINK COCA-COLA" was supposedly flashed to an audience in a movie theater, resulting in a dramatic increase of Coke sales. Many people have heard about this study. But the whole thing was a hoax. The "researcher," James Vicary , revealed in a 1962 interview in Advertising Age that "the original study was a fabrication intended to increase customers for his failing marketing business." (Pratkanis, 1992) Vicary's discussion of the issue is reprinted on a web page titled "Vicary Tells All" at this URL:

What did the Bjork committee determine, in its review of research?

Despite the lack of evidence for genuine effects, subliminal perception became the basis for a multimillion-dollar industry. Cassette courses using "subliminal suggestion" claimed to bolster self-esteem, help people stop smoking, improve memory, and more, all using messages too weak to be consciously detected on audio cassettes. In the late 1980s a group of psychologists headed by Robert Bjork of UCLA was hired by the United States Department of Defense to evaluate subliminal learning techniques. The group issued a 269 page report in 1991 concluding there was "neither theoretical foundation nor experimental evidence" for effectiveness of subliminal self-help tapes (Swets & Bjork, 1990).

Another study (Greenwald, Spangenberg, Pratkanis, and Eskenazi, 1991) found that "neither memory nor self-esteem tapes produced their claimed effects." This study was discussed in Chapter 1 in the section on placebo effects because "more than a third of subjects had the illusion of improvement." Half the tapes were deliberately given the wrong label, and when the "illusion of improvement" occurred, it corresponded to the label, not the actual contents of the tape. This is good evidence that subliminal tapes work through placebo effects and not through some form of perception.

What is "backmasking"? Does it have any detectable effect?

There is no evidence to support the claim, popular a few years ago, that hiding little demon faces in pictures of ice cubes will influence readers, or that hiding sexual symbolism in advertising will influence buyers. Nor is there any evidence that "backmasking" or playing messages backwards in rock songs can influence listeners. Repeated tests showed people could not decipher messages that were played backwards, nor did the messages have any detectable influence on people. For an amusing discussion of this old but seemingly immortal myth, with letters from believers as well as skeptics, see <>.

What is an example of subliminal perception of the first type?

One type of subliminal perception is easy to replicate. In a procedure called priming, a stimulus is flashed for a split second then quickly masked with another stimulus. Then a target word is shown and the subject is asked to identify it as quickly as possible. If the priming stimulus bears a close relationship to the target word, the subject can respond slightly faster. This is a genuine subliminal effect, because the priming stimulus cannot be seen consciously, and subjects cannot report what it is. However, the effect lasts only a tenth of a second (Greenwald, Draine, & Abrams, 1996). Priming is a robust (easily replicated) effect useful to language researchers for investigating relationships between word meanings.

Write to Dr. Dewey at

Don't see what you need? Psych Web has over 1,000 pages, so it may be elsewhere on the site. Do a site-specific Google search using the box below.

Custom Search

Copyright © 2007-2011 Russ Dewey