Many people have heard of "subliminal" messages that are not consciously perceived by a subject but nonetheless influence his or her behavior. The concept first became publicized in the 1950s, when the advertiser James Vicary claimed that flashing the words "Drink Popcorn" and "Drink Coke" between frames in a movie theater increased Coke sales by 18.1% and popcorn sales by 57.7%. This caused a storm of controversy, although Vicary later admitted that this "study" was a hoax.(1)Many studies carried out in laboratory conditions do show that "subliminal" inputs can be perceived by the nervous system without the awareness of the conscious "I-function". Inputs perceived "subliminally" have also been shown to influence behavior to some extent, in some instances more than supraliminal inputs. These effects, however, are for the most part limited in magnitude and duration.
The literal meaning of the word "subliminal" is rather misleading since it implies that there is an absolute "limen", or threshold, above which inputs are detected by the "I-function" and below which they are not.(2) However, while there are some "supraliminal" inputs of high intensity where perception is always self-reported by experimental subjects, as well as inputs of low intensity that are never reported, there is also a range of input intensities where subjects report only a fraction of the time.(3) The actual experimental definition of the "liminal" or threshold value of an input is the value where conscious perception occurs 50% of the time. The "limen" can also fluctuate with different prevailing conditions.(4) Therefore, an input could be classified as "subliminal" and still be accessible to the I-function.
Some experiments that indicate a conflict between "self-report" of the perception of an input by a subject and other evidence of perception are "forced-choice" experiments, in which subjects who self-report that they cannot perceive an input are directed to guess what it is from a number of presented choices. The accuracy rate of such "forced choices" usually exceed chance.(2) An early experiment conducted in 1898 involved flash cards imprinted with characters that were presented to subjects at such a distance that they reported only "a dim, blurred, spot or dot". However, if the subjects were forced to guess what the character was, their accuracy was greater than chance.(5)
Other experiments utilized conditioning and the Galvanic Skin Response, changes in skin conductance to electricity that can be measured by a polygraph, to create a response other than self-report that could be indicate perception of an input. Subjects were first conditioned to increase their GSR upon presentation of a series of nonsense syllables like ZUM and TOV by pairing them with a mild electric shock. The syllables, mixed with new ones, were then presented at a short time exposure providing no self-report of perception. However, the subliminally presented...