The word “subliminal” is derived from a construct of the “limen of consciousness”, which is a threshold that separates conscious from unconscious. This concept quite literally dates back to the very start of modern psychology, and the writings of Johan Friedrich Herbart. Herbart argued that perceptions and thoughts differed from one another in strength, and further, that they compete to achieve enough strength to rise above the “limen of consciousness”, and thus become experienced consciously. Ideas below that threshold would still exist, and therefore would be able to influence which other ideas, including themselves, were subsequently experienced consciously. In this way, perceptions obtained only subliminally (i.e. below the “limen of consciousness”) could still affect what we experience consciously (i.e. what we think about) and how we behave.

Now, this idea, or some variation of it, is what influences most popular discussions of subliminal persuasion or perception. Although, except for Freudian Psychology (which isn’t really a credible scientific theory of perception or cognition), very few models of perception and cognition actually take such ideas seriously.

First, contrary to popular ideas about Psychology in the media, no modern theory actually puts forward “an unconscious”; that is to say, a mind-entity separate from consciousness which ideas, beliefs, perceptions and desires can slip through or exert influence from one side to the other. Instead, perceptual and cognitive processes can, and often do, occur without our awareness, and without us needing to be able to consciously control them.

Consider riding a bicycle, playing a sport, or even reading this article. Not only do they happen “automatically” (assuming that you’ve already become competent at them) and without awareness of how you are doing them; but any attempts to gain conscious control of them will most likely result in the process coming to a sudden halt (i.e. you fall off your bicycle). The important point here is that these processes occur unconsciously — that is to say, without all the internal dialogue that normally accompanies what we would term a “conscious” process. However, there are no reasons to suggest that they would therefore occur in a special mind-place named “the unconscious”, in the same way that you couldn’t claim that the internal mechanisms of your computer, DVD player or your car happened in an “unconscious”; although they most definitely occur unconsciously (i.e. without awareness).

Secondly, there are very few current cognitive or perception theories that hold to the idea of an absolute sensory / information threshold, dividing the events we are aware of from those that we are not aware of. Instead, both perception and cognition are viewed as a continuum along which the amount of information or sensation that we need for any given event, before we are prepared to say that we are aware of it, varies as a function of the event itself, task demands, payoffs and the context etc.

This form of decision making about sensory and other kinds of events happens because the event itself always occurs in the context of both internal and external “noise” i.e. a background of half-formed, fleeting thoughts, desires, sensations, impressions and beliefs of varying and unpredictable intensity. It is against this background that we must detect the event in question; sometimes, for example, this background “noise” is low enough that even relatively weak signals can confidently be detected; at other times, the “noise” completely overwhelms even the most intense of signals, so that a confident decision is impossible.

Unfortunately, we are rarely completely sure which state we are in; sometimes mistaking noise for signal and other times mistaking signal for noise, so we constantly need to balance the costs of deciding that an event happened, when in fact it didn’t, against the costs of failing to detect the event when it did. Thus, when you say that you are “aware” of a given event, you are claiming that in your opinion, for the demands of the specific task in question, you have gathered enough sensory or other evidence to conclude that the event actually happened.

From this perspective, experimental Psychologists have been able to define two different “thresholds” of awareness. The subjective threshold, which refers to the level of information or stimulus intensity at which the person claims to be “just guessing” (or responding at a level equal to chance) which just “feels” as if they have no information to decide one way or another about some event. Note that this threshold corresponds to a testimonial; subliminal events in this respect of being below the subjective threshold are the events whose presence individuals do not report¹.

In contrast, the objective threshold, refers to the lower level of information or stimulus intensity, at which the individual is no longer able (even by guessing) to discriminate between events; for example, detecting the presence or absence of a signal, at a level above chance. Subliminal events in this sense are events whose presence the observer can’t detect¹. The results of research using events below the objective threshold are definite: there is no compelling evidence for unconscious or subliminal perception of them, and no evidence that the meaning or content of such events could affect people’s behaviour². Put more simply, if an event is below an individual’s objective threshold, they cannot detect it, and thus it has no effect on them at all.

The subjective threshold, however, is different; here we can demonstrate an effect on the observer. Consider the results of what was one of the first pieces of Psychological research. In this experiment, Pierce and Jastrow³ investigated their own abilities to discriminate small differences in pressure with their fingertips, and discovered that the accuracy of their decisions was still well above chance (i.e., above what Psychologists call the “objective threshold”,) even when they were convinced they were just guessing. In a similar experiment reported a few years later, Sidis⁴ found that subjects shown cards marked with a single character, at a distance where the subjects claimed to see nothing more than a blur or a spot (and therefore were convinced that they were just guessing), were still able to name the characters at a level above chance. Numerous experiments of this type continued right up to the present time, and virtually all produced the same result: by adjusting the intensity or duration of the target stimulus, people brought to the point of being convinced that they can no longer detect differences between events, for example in a picture or an audio recording with and without an embedded message, still obtain enough information (in their view “unconsciously”) to perform better than chance when forced to guess.

So, this is subliminal perception in the sense of dissociation between awareness and the acquisition of information. However, two important points do still need to be noted about it:

• It is a long-established, unsurprising phenomenon in Psychology, and no special devices or sophisticated processes are needed to produce it. Indeed, many of the techniques claiming to exploit this phenomenon, such as the famous ‘Eat popcorn / Drink Coke’ experiment (a message was allegedly superimposed on a cinema screen for 0.0003 seconds, which was claimed to increase popcorn/cola sales) or the messages contained on self-help audio recordings, are well-below people’s objective thresholds, and therefore are incapable of producing any effect.

• There is no convincing evidence that the effect, when it occurs at all, extends much beyond improving the accuracy of people’s guesses in forced-choice tasks. That is, there is no good evidence that the meaning or content of such events can affect people’s behaviour.

Given the extreme difficulty of ensuring that the stimuli used are simultaneously above the objective threshold, but below the subjective threshold, many cognitive scientists seriously doubt whether there has been a convincing demonstration of the effect even on forced-choice guessing - just because people claim to be “just guessing” doesn’t mean that they had no awareness of the event - possibly they were just being overly careful in attributing awareness, and were saying that they were guessing, unless they were absolutely sure. Doing this on just a few trials would be enough to establish the effect^{1, 2}

References:

1. Moore, T. E., ‘Subliminal Perception: Facts and Fallacies‘, Skeptical Inquirer (1992) 16:3
2. Holender, D., ‘Semantic activation without conscious identification in dichotic listening, parafoveal vision, and visual masking: A survey and appraisal’, Behavior and Brain Sciences (1986) 9:1-66;Greenwald, A. G., ‘New look 3: Unconscious cognition reclaimed’, American Psychologist (1992) 47, 766-779
3. Pierce, C. S., & J. Jastrow, ‘On small differences in sensation’, Memoirs of the National Academy of Science (1884) 3, 75-83
4. Sidis, B., ‘The Psychology of Suggestion‘ (1898) New York: D. Appleton and Company

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Posted by Jonathan in Psychology