The subliminal is below the liminal (the smallest detectable sensation).
Anything truly below the level of detectable sensation could not, by definition, be perceived. However, the subliminal is generally said to be below the threshold of conscious perception. There is a widespread belief, not strongly supported by empirical research, that without being aware of its presence or content, a person's behavior can be significantly affected by subliminal messages. Thus, it is believed that one can influence behavior by surreptitiously appealing to the subconscious mind with words and images.
If this were true, then advertisers could manipulate consumer behavior by hiding subliminal messages in their ads. The government, or Aunt Hilda for that matter, could control our minds and bodies by secretly communicating to us subliminally. Learners could learn while listening to music embedded with subliminal messages. Unfortunately, "...years of research has resulted in the demonstration of some very limited effects of subliminal stimulation" and no support for its efficaciousness in behavior modification (Hines, 312).
The fact that there is almost no empirical support for the usefulness of subliminal messaging has not prevented numerous industries from producing and marketing tapes which allegedly communicate directly with the unconscious mind, encouraging the "listener" not to steal, or coaching the "listener" to have courage or believe in his or her power to accomplish great things. Consumers spend more than $50 million each year on subliminal self-help products (Journal of Advertising Research, reported by Dennis Love, Sacramento Bee, 9-14-2000). A place called Hynotictapes.com offers a wide array of such tapes developed by James H. Schmelter, a hypnotherapist with an MBA and self-proclaimed expertise in synergistic science. If Schmelter's stuff is not to your liking, try Mindwriter Subliminals... A Breakthrough In Human Reprogramming.
It is true that we can perceive things even though we are not conscious of perceiving them. However, for those who put messages in tapes and then record music over the messages so that the messages are drowned out by the music or other sounds, it might be useful to remember that if the messages are drowned out by other sounds, the only perceptions one can have are of the sounds drowning out the messages. There is no evidence of anyone hearing a message which is buried beneath layers of other sounds to the point where the message does not distinctly stand out. Of course, if the message distinctly stood out, it would not be subliminal.
The belief in the power of subliminal messaging to manipulate behavior seems to have originated in 1957 with James Vicary, an advertising promoter who claimed to increase popcorn sales by some 58% and Coke sales by some 18% in a New Jersey movie theater simply by flashing very briefly the messages "Drink Coca-Cola" and "Hungry - Eat Popcorn." Even though the claim has been shown to be a hoax, and even though no one has been able to duplicate the event, belief in the legend lingers. This story and several others were retold by Vance Packard in The Hidden Persuaders (1957), a book that became required reading for a generation of college students.
Belief in subliminal messaging reached a surreal apex in 1980 with the publication of The Clam-Plate Orgy and Other Subliminals the Media Use to Manipulate Your Behavior by Wilson Bryan Key. The book has been reissued under the sexier title: Subliminal Adventures in Erotic Art. Key claims that advertisers use subliminal messaging of a very serious sexual nature in order to manipulate behavior, including imbedding sexy figures and the word 'sex' in images of such things as ice cubes and food. While carefully examining a Howard Johnson's menu, Key saw that the plate of clams pictured on the menu was actually the portrayal of a sexual orgy which included various people and a donkey. Among Key's many unfounded claims is that the unconscious mind processes subliminal messages at the speed of light. Actually, the fastest brain process chugs along at some 40 m.p.h. (Hines).
Despite the fact that there is no body of empirical support for the notion that subliminal advertising is effective, in 1974 the Federal Communications Commission (FCC) issued an order saying that broadcast outlets that knowingly carry subliminal ads are operating "contrary to the public interest." In September 2000, Senators Ron Wyden of Oregon and John Breaux of Louisiana complained to the FCC about a Republican ad that flashed the word ''RATS'' (or "BUREAUCRATS") across the screen for 1/30 of a second. ''We have reason to believe that broadcasters are airing television advertisements that contain subliminal messages in violation of the public interest,'' they said, apparently oblivious to the fact that something which can't be registered by the brain is unlikely to have any effect on viewers and is unlikely to violate anything except the reasonable bounds of credulity.