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What is Subliminal Influence?^*
by Todd I. Stark - 2/1999

The term subliminal is technically archaic, though it is still in common use. The problem is that it is hopelessly bound to the concept of a well-defined sensory threshold, a concept made obsolete by the introduction of signal detection theory into psychology (Green & Swets, 1967).
We continue to use this term, (like the term "trance" in hypnosis), because it is so deeply ingrained in the minds of both scientists and the public. The term itself means "below the threshold," but there is no single clear threshold to sensory perception. There are a vast array of different sensory features detected and processed before being brought to awareness, with the help of attention. This makes for a great deal of flexibility in how and what we integrate when we bring sensory impressions into awareness.
In technical usage, we consider a stimulus subliminal if it is too faint, too brief, or otherwise does not stimulate our senses sufficiently to cause us to notice it, yet influences subsequent tests in some way. However, a more technically accurate term for this would be marginally perceptible.
The popular meaning of the term subliminal is anything that influences us from outside of our awareness.
Interest in subliminal influence in modern times goes back to sensory physiologist Otto Poetzl in the early 1900's, who studied the effect of rapidly flashed pictures on dreams. In the 1950's the invention of the tachistoscope for more rapidly flashing images spurred a renewed research interest in subliminal perception, the fate of stimuli not noticed. Originally a sensory curiosity, it was growing into both a research controversy and a public controversy.
The story of "subliminal persuasion" begins with the report of market research consultant James Vicary that he could influence the buying behavior of movie patrons by flashing invisible messages on the screen. In a climate prepared by the dire view of social critic Vance Packard, this created an amazingly widespead and enduring fear of unseen messages in media. This has evolved into fear of movies, television, magazine pictures, embedded messages in Muzak in stores, in computer software and even fear of backward-played satanic messages in music.
Packard himself seems to have been mostly concerned about the use of psychoanalysts, or "depth men" by advertisers to craft carefully symbolic messages in advertisements. This created still more sources of fear, such as the neverending rumors of hidden sexual symbols in children's movies.
The story has an offshoot, into the lucrative realm of audiotapes that are supposed to help us reprogram our mind effortlessly and unconsciously with self-help messages. Similar subliminal influence is also claimed to help would-be seducers win hearts and minds of their quarry.
The term subliminal is commonly used to mean a number of different kinds of hidden messages.

• Images of carefully contrived social situations and suggestive body language
• Blatant or artistically concealed symbols suggestive of powerful instinctual drives
• Faint or briefly flashed words or images
• Acoustically masked or backward messages in music
• Metaphor and other "hypnotic" language patterns

Of these various kinds of "subliminal" influence, psychologists usually refer only to the briefly flashed words or images, and sometimes the acoustic masking, when they talk about marginal or subliminal perception. Those are the stimuli used in the subliminal priming experiments that produce temporary, weak psychological effects.
In other words, the experimenters most actively involved in research into subliminal priming generally agree that it exists, but that it consists of relatively weak and fleeting effects of primarily theoretical interest. (Merikle, in press) This is certainly true of unconscous semantic priming.
Several widely-read authors, particularly Wilson Bryan Key, brought the term subliminal into wider public awareness. Most of this literature is a warning that advertisers are capable of influencing us through hidden messages, and actively do so, with malicious intent. The warning refers to not only what the psychologists would call subliminal priming, but also a variety of other kinds of sneaky persuasion tactics popularly associated with the term subliminal.
Following the lead of Key, Vance Packard before him, and others in their genre, the term subliminal is more commonly used to refer to any influence on us that we don't notice. In addition to the weak stimuli used by researchers, this also includes things like hidden images in pictures, the crafting of scenes for emotional content, playing sounds backwards, metaphorical or otherwise embedded symbolism in language, the use of visual symbols to invoke instinctual drives, and so on.
Whether any of these things has an "unconscious" effect is a matter of definition, but whether they are effective influence is an empirical matter. Lumping them all together into one category with too faint and too brief messages used in cognitive science experiments makes it virtually impossible to sort out varied things like social psychological influence, suggestion, emotional appeal, associative conditioning, and preconscious processing.
Subliminal semantic priming is the effect of briefly flashed words on categorizing subsequent words in a forced choice test. It only lasts about 100 milliseconds and does not carry over from one trial to the next in experiments. (Greenwald et al, 1996)
Subliminal perceptual priming is the effect of a briefly flashed picture on our preferences in a forced choice test, and is more robust. This is better known as the "mere exposure effect." (Zajonc, 1980). Variations of the mere exposure effect have been demonstrated to activate emotional centers of the brain, particularly the amygdala, without awareness. (Whalen et al, 1998). This probably has some relevance to the "classical conditioning" of emotional memories without awareness. This begins to enter into the realm of something that could be crafted into an effective propaganda tool, especially as a reinforcer.
Subliminal psychodynamic activation is one of the oldest methods, and the most intriguing. Subliminal stimuli can enter into dreams and waking imagery in a transformed way (Shevrin,1986), influence later recall and perception (Shevrin, 1990), and most remarkably even influence our social functioning. (Silverman, 1976, 1978). However, it is also the most difficult to demonstrate, presumably because it is the most vulnerable to the individual differences and the psychological state of the recipient, and the vagueries of subjective interpretation of results.
The most effective techniques in practice involve both conscious and unconscious elements, coordinated to appeal to our emotions and exploit our natural information processing and aesthetic biases, as well as lead our conscious thinking processes in a desired way.
Becoming aware of subliminal stimuli generally negates or reduces their influence, in both the mere exposure and psychodynamic experiments. The combination of conscious and unconscious elements must be carefully coordinated, but not duplictated (unconscious elements should not be made conscious).
The reason why this approach is most effective, compared to one emphasizing or relying on hidden messages, is that hidden messages can influence our thinking and feeling, but not directly cause behavior, at least not by any yet known effect.
The known subliminal effects influence behavior indireectly, if at all, by influencing perception, thinking and feeling. Conscious thinking organizes and triggers most behavior, even though aspects of behavior are unconscious, such as the details of movements and the expression of much of nonverbal communication.
Hypnosis research has shown that effective illusions and compulsions cab be created through suggestion under some conditions with some people, without conscious awareness of the exact source. (Kihlstrom, 1995)
These hypnotic suggestion effects are limited by the expectations of the subject, their relationship with the hypnotist, and the demand characteristics of the situation. (Orne & Evans, 1965). The more the subject expects to be controlled, and the greater their rapport or sense of cooperation with the hypnotist, the more involuntary they perceive their response. (Lynn, Nash, Rhue, et. al., 1984).
It is the degree to which subliminal messages could produce a similar kind of dissociated control that is at the heart of the most virulent potential threat of subliminal persuasion. Without the relationship and expectancy factors that make hypnotic suggestion effective, it is difficult to see how the comparison can be meaningfully made.
Unless they are tailored to the individual, there is no convincing evidence of any more elaborate effect from purely preconscious processing of hidden messages, or even that they are worth persuing as an aid to advertising. (Saegert, 1987).
Achieving this kind of effect through a combination of subliminal messaging and hypnotic techniques remains a theoretical possibility under some conditions. At issue is the problem of creating the cooperative mindset needed for hypnosis, creating the expectancy that we can be controlled, and the differences in the way individuals respond to hypnotic suggestion.
The threat of subliminal influence seems limited at this time to relatively weak reinforcement of conscious messages, but the combination of such effects can be difficult to determine. A message employing subliminal techniques, like any message, can often have unanticipated effects on the listener depending on their own psychological needs and mindset.