Understanding Subliminal Stimuli

11 Jun

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The first major double-blind tests of subliminal “self-help” audio tapes were conducted in 1990 by UW psychology professor Tony Greenwald. The results of the study, says Greenwald, showed that the audio tapes had no effect that could be credited to the subliminal content–that is, to the words or sounds embedded in the audio material not consciously apparent to the listener. Greenwald’s results have established a basis for protecting consumers against false advertising claims made by the marketers of subliminal self-help tapes.

The study, which was funded by the National Institute of Mental Health, represents the largest and most definitive such test ever conducted. Studies of visual subliminal materials carried out at the UW from 1989 to 1992 also have led to some firsts.

Subliminal visual stimuli are words or pictures that are presented so as to be unidentifiable to the viewer’s conscious perception. For example, images may be flashed before the eye too quickly for the conscious mind to apprehend. Such stimuli can nevertheless exert an effect on judgment and behavior.

An infamous example of the phenomenon is the alleged case in 1957 of the drive-in theater in New Jersey, where messages such as “Drink Coke” and “Eat Popcorn” were flashed on the screen. Sales of those refreshments were said to have increased as a result. Greenwald regards the case as something of an urban legend, and notes the alleged effects were not scientifically verifiable. Recent evidence, he explains, suggests the effects of subliminal information are too fleeting to have an influence over periods of time as long as several minutes or more.

Greenwald and colleagues have established rigorous scientific methods for investigating the phenomenon of subliminal activation. They invented a statistical method for establishing that subliminal visual stimuli indeed exert their effect by acting through the subconscious mind. The results of that study were published in early 1995.

Furthermore, the researchers invented a reliable procedure for detecting evidence of the effects of visual subliminal activation. The invention, subject of a patent application on behalf of the UW by the Washington Research Foundation in 1995, involves the concept of a “response window.” Subjects are constrained to respond rapidly within a fixed, brief interval after a pair of stimuli are presented–one subliminal, one visible. The researchers then examine the patterns of errors in the subject’s responses to the visual stimuli that are explainable only by the content of the subliminal stimuli.

The approach provides a ten-fold increase in sensitivity in the measurement of subliminal effects compared to the previous state of the art. The sensitivity increase in turn allows differences between individuals in subliminal responsiveness to be studied. In addition, the method involves a more efficient experimental set-up that allows researchers to complete studies in a fraction of the time previously required to study subliminal effects.

The advances in testing visual subliminal activation have “succeeded in converting subliminal cognition from a controversial and empirically unreliable phenomenon into a result that can be obtained routinely in any laboratory,” says Greenwald. “It seems certain,” he adds, “that research stimulated by availability of this method will lead to a rapid development of theory to explain unconscious operations of mind triggered by subliminal visual stimulation.”

Greenwald anticipates the method may find considerable use in diagnosing psychological conditions and in monitoring therapeutic effectiveness. “As a test of personality characteristics, this is better and more accurate than existing tests,” he says. Unlike the written tests Greenwald refers to, it is unlikely that responses to the subliminal test could be faked, and therefore the test may provide more accurate information about the subject than other methods. Uses may include home self-testing packages, or voluntary self-testing by managers who wish to obtain a realistic appraisal of their own traits, strengths, and prejudices. “This technique may help managers to be sensitive to unconscious prejudices and stereotypes that can unintentionally influence their interactions with others, for example their personnel decisions,” he explains.


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