Subliminal perception

In 1959, James McDonald Vicary, a market researcher, claimed that he could get moviegoers eat more popcorn with a technique, called “subliminal advertising”. Apparently, he did an experiment with 45 699 naive subjects whereby one group received the subliminal message “eat popcorn” and another “drink coca-cola”. These persons were not aware (or informed) of the message. Subliminal means “below the threshold”. The messages were only visible for 1/3 of a millisecond. The pictures below show you (approximately) flashes of 5 ms, 30 ms and 50 ms.

I don’t know, if sales of Telenet’s digicorder grew, but in the Vicary study it apparently did. In the six weeks of the test period sales of popcorn increased with 57.5% and 18.1% for Coca-Cola. Of course, this attracted a lot of commotion and questions. Allegedly, because of a pending patent, Mr. Vicary could not even answer simple questions like the location of the movie theatre (45,000 visitors in 6 weeks?).

This story is one of the top urban legends in cognitive psychology. Alas, the study was a fabrication. In 1962, Mr. Vicary admitted his “Popcorn Experiment” was a “gimmick” intended to save his failing business; see Pratkanis (1992) and Crandall (2006) for an detailed recount of the events.

But the harm was done. The idea that wicked persons can influence our behavior through subliminal messages was planted in our “collective unconscious” (a bit like in the magnificent movie “Inception“). A compelling story of this paranoia for example is recorded in the documentary Programming the Nation; see also it’s website to get an idea of how far this can go. More light footed are the so called hidden messages in songs. Only by reversing the tape, you can hear these messages. Probably the best known is the “Satan” verse in the megahit Stairway to Heaven by Led Zeppelin.

There have been however several attempts to prove subliminal perception. Early studies were based on introspective measures of awareness. A good example are the investigations of Sidis (1889) at Harvard. Sidis showed his subjects cards containing a single printed digit or letter. The subject was placed at such a distance from the card that the character was far out of his range of vision. He saw nothing but a dim, blurred spot or dot. The subject, however, had to name the character but often complained that “it is nothing but a mere guess” or that  “they might as well shut their eyes and guess” (p. 171). Nevertheless, their responses were more correct than one should expect based on chance. For example, the letter set consisted of the letters B, Z, K, U, H and 2, 4, 5, 7, 9 for the digit set. The chance that a subject could pick the correct category (letters of numbers) is 50% and for an individual character 10% (1 out of 10 possibilities). For one group of subjects, the correct category response was 140/200 or 70%. 68/200 (34%) of the particular characters were correctly guessed (p. 171).

Another classical demonstration was done by the philosopher C.S. Pierce and his graduate student Joseph Jastrow in 1884. Pierce and Jastrow presented themselves with two stimuli (a pressure on the finger) and made thereafter an obligatory choice which of two pressures was the heavier one. Also, they must assign a confidence level (on a 0-3 scale) to their judgments. The stimuli were very close to each other, so that for example Pierce completed 706 out of 1,125 trials at a confidence level of zero (meaning “absence of any preference for one answer over its opposite, so that it seemed nonsensical to answer at all”). On these trials, he was 436 times correct (61.8%); where chance level is of course 50%.  Although, Mr. Pierce didn’t consciously experience the difference between the two stimuli, he was able to differentiate between them above chance. Unfortunately, the authors ended their article with the following conclusion, once again, bringing the subliminal phenomenon in the domain of occultism and parapsychology.

The general fact has highly important practical bearings, since it gives new reason for believing that we gather what is passing in one another’s minds in large measure from sensations so faint that we are not fairly aware of having them, and can give no account of how we reach our conclusions about such matters. The insight of females as well as certain “telepathic” phenomena may be explained in this way. Such faint sensations ought to be fully studied by the psychologist and assiduously cultivated by every man. Pierce and Jastrow, 1884, p. 83

The problem with these early studies was their reliance on introspective reports. When a person says that he didn’t see of hear anything or was not aware of anything, can we believe that?  Therefore, a new operational definition was proposed: the objective threshold for consciousness is defined as a situation where forced-choice discrimination is at chance. In the case of Pierce and Jastrow, this means that we only accept that a subject is unaware of a pressure stimulus when he can not discriminate this stimulus with another.

A very nice experiment that used this behavioural measure was done by Kunst-Wilson and Zajonc (1980). The experiment consisted of an exposure phase and of a test series. In the exposure phase, subjects were shown 10 meaningless, irregular shaped octagons. Because of their high contrast, chance recognition could be obtained only after exposures were reduced to a one millisecond duration. The instructions to subjects at the beginning of the exposure phase were that the experiment consisted of two parts and that during the first part slides would be shown on the screen at durations so brief that one could not really see what was being presented. Nevertheless, the subject was instructed to pay close attention to the flashes, even if nothing could be distinguished, and to acknowledge verbally the occurrence of each flash. The second part of the experiment required subjects to make paired comparisons. Now the slides were presented under adequate viewing conditions (the exposure time was extended to 1 second). For each of the ten pairs, all containing one octagon previously exposed and one new, the subjects had to indicate (i) the one they liked better and (ii) the one they thought had been shown previously. For both judgments, confidence ratings were obtained on a three-point scale: “sure”,  “half-sure”, and “guess”. The results are shown in figure 1. Subjects performed no better than chance (i.e. 50% correct) when they were asked to select the shape in each pair that had been presented previously, but they performed significantly better than chance (i.e. 60% correct) when they were asked to select the shape in each pair that they preferred. The results thus suggest that there may exist a capacity for making affective
discriminations without “extensive participation of the cognitive system”, i.e. awareness.

Figure 2. Masked priming experiment

Another attempt to prove that subliminal perception exists used a technique called masked priming.The technique is illustrated in the figure 2 and consists of a target stimulus (in this case, the word  “salt”) to be recognized under two conditions. In one condition the word “pepper” is displayed before “salt”. In the other condition is the word  “rain” displayed. Because language is organized in meaningful clusters, reading the word “pepper” activates the mental representation of the word “salt”, which is responsible for the shorter reaction time to the word “salt” than to the word “rain”. This is called “priming”. Due to the masking (the series of X for 500 ms) and the very short exposure time of the prime, the prime is not consciously processed. You can get the feeling of this experiment with the this demo of Foster and Davis.

Of course, one can always argue that these experiments are not waterproof. For example, in the masked priming approach, it is possible that the subject was indeed aware of the priming stimulus but has somehow forgotten about is. In the experiment of Kunst-Wison & Zajonc, the subjects who didn’t see “a flash” were excluded. Somehow, most subjects are aware of something. Of course, you can take the approach of Kenneth Foster (see above demo): “We find that there is usually not a lot of discussion about whether subjects were really aware once the display has been viewed” or as Kouider and Dehaene (2007) acknowlegde:

Construction of a convincing empirical demonstration of subliminal processing has constituted a challenging task. Indeed, this topic has faced some of the most complex problems of experimental psychology, not only technically (e.g. How to present stimuli that are invisible but still processed?), but also methodologically (e.g. How to measure non-conscious influences from a stimulus? How to demonstrate an absence of conscious perception?), theoretically (e.g. Should we trust introspective subjective measures or rather rely on objective measures?) and epistemologically (e.g. Why do so many subliminal perception experiments fail to be replicated?). Such difficulties, among others, are the reasons why the topic of perception without awareness has taken so long to achieve respectability. Kouider and Dehaene, 2007, p. 857

The post Subliminal perception appeared first on Cogpsy.info.