Philip M. Merikle
Department of Psychology
University of Waterloo
Waterloo, Ontario, Canada N2L 3G1
Subliminal perception occurs whenever stimuli presented below the threshold or limen for awareness are found to influence thoughts, feelings, or actions. 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.
The concept of subliminal perception is of considerable interest because it suggests that peoples' thoughts, feelings and actions are influenced by stimuli that are perceived without any awareness of perceiving. This interest was reflected in some of the earliest psychological studies conducted during the late 1800s and early 1900s. In these early studies, people were simply asked whether or not they were aware of perceiving. For example, visual stimuli such as letters, digits, or geometric figures were presented at such a distance from observers that they claimed either not to see anything at all or to see nothing more than blurred dots. Likewise, auditory stimuli such as the names of letters were whispered so faintly that observers claimed that they were unable to hear any sound whatsoever. To test whether these visual or auditory stimuli may have been perceived despite the statements to the contrary, the observers were asked to make guesses regarding the stimuli. For example, if half the stimuli were letters and half the stimuli were digits, the observers may have been asked to guess whether a letter or a digit had been presented. The consistent result found in these early studies was that the observers' guesses regarding the stimuli were more correct than would be expected on the basis of chance guessing. In other words, despite the observers' statements indicating that they were unaware of perceiving the stimuli, their guesses indicated that they did in fact perceive sufficient information to make accurate guesses regarding the stimuli. Over the years, there have been literally hundreds of studies following a similar format. Taken together, these studies show that considerable information capable of informing decisions and guiding actions is perceived even when observers do not experience any awareness of perceiving.
Another way in which subliminal perception has been demonstrated in controlled laboratory studies is by showing that stimuli can be perceived even when they are presented under conditions that make it difficult if not impossible to distinguish one stimulus from another stimulus. The classic studies were conducted in the 1970s by the British psychologist Anthony Marcel. These experiments were based on previous findings indicating that a decision regarding a stimulus is facilitated or primed when the stimulus follows a related stimulus. For example, if an observer is asked to classify a letter string as either a word (e.g., doctor, bread) or a nonword (e.g., tocdor, dreab), a letter string such as the word doctor will be classified as a word faster when it follows a semantically related word (e.g., nurse) than when it follows a semantically non-related word (e.g., butter). Marcel found that words facilitated or primed subsequent word/nonword decisions to letter strings even when the words were presented under conditions that made it difficult if not impossible for the observers to distinguish when the words were present from when the words were absent. Since the time of Marcel's original experiments, there have been many other studies that have used similar methods. Not only have these studies confirmed Marcel's original findings, but they have shown that other stimuli such as pictures, faces, and spoken words can also facilitate subsequent decisions when they are presented under conditions that make it difficult to discriminate one stimulus from another stimulus. Although questions have been raised regarding whether the observers in these studies were completely unable to discriminate one stimulus from another stimulus, the one firm conclusion that can be made on the basis of these studies is that considerable information is perceived even when observers experience little or no awareness of perceiving as indicated by their difficulty in discriminating one stimulus from another stimulus.
Examples of subliminal perception are found in studies of patients with neurological damage. A striking characteristic of a number of neurological syndromes is that patients claim not to see particular stimuli but nevertheless respond on the basis of information conveyed by these stimuli. One example is a syndrome called blindsight. Patients with blindsight have damage to the primary visual cortex. As a result of this damage, they are often unaware of perceiving stimuli within a restricted area of their visual field. For example, if the visual field is thought of as consisting of four quadrants, a blindsight patient may have normal vision for stimuli presented in three of the quadrants but be completely unaware of stimuli presented in the fourth quadrant. However, even though these patients may claim not to see stimuli located within the "blind" quadrant, they are still able to guess the size, shape or orientation of the stimuli that they claim not to see. Another neurological syndrome in which subliminal perception occurs is prosopagnosia or face agnosia. Patients with prosopagnosia are unable to recognize familiar faces. Although they may be aware that they are looking at a person's face, they are unable to say who the person may be. Thus, prosapagnosics have no awareness of perceiving any information regarding whose face they may be viewing. However, despite this absence of awareness, some patients with prosapagnosia are able to choose which of two names goes with each familiar face that they claim not to be able to recognize.
Perception without an awareness of perceiving can also occur in surgical patients undergoing general anesthesia. One goal of general anesthesia is to ensure that surgical patients are completely unaware of all events that occur during anesthesia. This goal is satisfied in the vast majority of cases because when patients are asked following surgery to report anything they remember that happened during surgery, just about every patient claims not to remember anything. However, when memory is assessed by more indirect methods, there appears to be some memory for events during anesthesia. For example, during surgery, patients may wear earphones and a tape recording of a number of repetitions of a series of words may be played to the patients. If following surgery, these patients are presented word stems such as gui _ _ or pro _ _ and asked to complete these stems to produce a common English word, there are numerous possible completions (e.g., guilt, guild, guile; prove, prowl, probe). However, if the words guide and proud had been presented during anesthesia, then the patients may be more likely to complete the stems gui _ _ and pro _ _ with letters that reproduce guide and proud than with letters that produce other possible words. Given that patients undergoing general anesthesia are unaware of events in the external environment, memory for specific stimuli presented during anesthesia shows that information is at times perceived without any awareness of perceiving during general anesthesia.
Over the years, some extraordinary claims have been made concerning the power of subliminal perception. Perhaps the most widely known claim was made in 1957 by James Vicary, a market researcher. He claimed that over a six-week period, 45,699 patrons at a movie theater in Fort Lee, New Jersey were shown two advertising messages, Eat Popcorn and Drink Coca-Cola, while they watched the film Picnic. According to Vicary, a message was flashed for 3/1000 of a second once every five seconds. The duration of the messages was so short that they were never consciously perceived. Despite the fact that the customers were not aware of perceiving the messages, Vicary claimed that over the six-week period the sales of popcorn rose 57.7% and the sales of Coca-Cola rose 18.1%. Vicary's claims are often accepted as established facts. However, Vicary never released a detailed description of his study and there has never been any independent evidence to support his claims. Also, in an interview with Advertising Age in 1962, Vicary stated that the original study was a fabrication. The weight of the evidence suggests that it was indeed a fabrication.
Other claims regarding the extraordinary efficacy of subliminal perception also lack substance. In the 1970s, Wilson Bryan Key wrote such books as Subliminal Seduction and Media Sexploitation in which he claimed subliminal sexual symbols or objects are often used to entice consumers to buy and use various products and services. One of Key's most famous claims is that the word sex was often embedded in products and advertisements. For example, he claimed that the word sex was printed on Ritz crackers and was embedded in the ice cubes of the drink shown in a well-known ad for Gilbey's Gin. According to Key, despite the fact the embedded words are not consciously perceived, they are unconsciously perceived and can elicit sexual arousal which in turn makes the products more attractive to consumers. Although Key's claims are widely known, there is no independent evidence indicating that embedded subliminal words, symbols, or objects are used to sell products. Furthermore, even if such embedded subliminal stimuli were used, there is no evidence to suggest this would be an effective method for influencing the choices that consumers make.
Belief in the power of subliminal perception to induce changes in the way people feel and act is so widespread that a number of companies have been able to exploit this belief by marketing subliminal self-help audio and video tapes. The companies that market these tapes claim that regular use of the tapes can cure a variety of problems and aid in the development of many skills. Each company markets a number of different tapes. Presumably, what distinguishes the different tapes marketed by each company are the embedded subliminal messages that can be neither consciously seen or heard. Some of the more popular tapes are claimed to help individuals stop smoking, lose weight, or reduce stress; other tapes are claimed to help people increase their reading speed, improve their memory, or develop their skills at tennis (or golf or baseball, etc.). Given the extraordinary nature of these claims, there have been a number of controlled studies designed specifically to test of the efficacy of the tapes. All of these studies have failed to find any evidence consistent with the claims of the companies that market these tapes. There is simply no evidence that regular listening to subliminal audio self-help tapes or regular viewing of subliminal video self-help tapes is an effective method for overcoming problems or improving skills. In fact, there is even evidence to suggest that many subliminal self-help tapes do not even contain subliminal messages that could possibly be perceived under any circumstances by a human observer.
A common theme that links all extraordinary claims regarding subliminal perception is that perception in the absence of an awareness of perceiving is somehow more powerful or influential than perception that is accompanied by an awareness of perceiving. This idea is not supported by the results of controlled laboratory investigations of subliminal perception. Rather, the findings from controlled studies indicate that subliminal perception, when it occurs, reflects a person's usual interpretations of stimuli. Furthermore, there is no evidence to suggest that people initiate actions on the basis of subliminal perception. The weight of the evidence indicates that people must be aware of perceiving stimuli before they initiate actions or change their habitual reactions to these stimuli. Thus, although subliminal perception may allow us to make accurate guesses regarding the characteristics of stimuli, subliminal perception cannot lead a person to drink Coca-Cola or to eat Ritz Crackers, and it cannot be used effectively to improve a person's tennis skills or to cure a person's bad habits.
Dixon, N. F. (1971). Subliminal perception: The nature of a controversy. New York: McGraw-Hill.
Comprehensive review of all research findings prior to 1970.
Greenwald, A. W. (1992). New look 3: Unconscious cognition reclaimed. American Psychologist, 47, 766-779.
A review and discussion of recent research findings.
McConnell, J. V., Cutler, R. L., & McNeil, E. B. (1958). Subliminal stimulation: An overview. American Psychologist, 13, 229-242.
This paper was published shortly after the original claims regarding the effectiveness of embedded messages such as "Eat Popcorn" and "Drink Coca-Cola" became widely known. It provides an in-depth evaluation of these claims.
Merikle, P. M., & Daneman, M. (1996). Memory for unconsciously perceived events: Evidence from anesthetized patients. Consciousness and Cognition, 5, 525-541.
Presents and discusses the aggregate results of all studies investigating memory for events during general anesthesia.
Merikle, P. M., & Daneman, M. (1998). Psychological investigations of unconscious perception. Journal of Consciousness Studies, 5, 5-18.
Overview of scientific approaches to the study of subliminal perception. Includes some speculations regarding the consequences of subliminal perception.
Pratkanis, A. R. (1992). The cargo-cult science of subliminal persuasion. Skeptical Inquirer, 16, 260-272.
Evaluates many of the extraordinary claims regarding subliminal perception.
Vokey, J. R., & Read, J. D. (1985). Subliminal messages: Between the devil and the media. American Psychologist, 40, 1231-1239.
A review and evaluation of the claim that some rock music contains subliminal backward messages.
Weiskrantz, L. (1986). Blindsight: A case study and implications. New York: Oxford University Press.
This is the classic case study of a patient with blindsight.
Young, A. W. (1994). Covert recognition. In M. J. Farah & G. Ratcliff (Eds.), The neuropsychology of high-level vision (pp. 331-358). Hillsdale, NJ: Erlbaum.
Detailed summary of many of the studies of prosapagnosic patients.