Perception

Sensation occurs when sensory areas in the cerebral cortex receive nerve impulses, usually when body sensors such as the touch receptors of the skin are stimulated. Sensation must be distinguished from perception, which is based on the interpretation of patterns of sensation. Perceptions are what your brain makes of those sensory patterns. Under some conditions more than one reasonable interpretation of the same sensory pattern is possible, and in those cases each possible interpretation may give rise to a different perception. For example, in the necker cube, a single pattern of lines gives rise to two alternate perceptions, depending on which surface of the cube the brain "decides" is closer.

For sensation and perception:

We'll start by reviewing some of what we know about sensation. In particular, we'll be focusing on sensory thresholds.

Sensory Thresholds

The first systematic studies of sensory thresholds were conducted by physiologist Ernst Weber at the University of Leipsig in Leipsig, Germany, the same university where Wilhelm Wundt would later transform psychology into an experimental science. Weber's experiments were designed to determine sensory thresholds, of which there are two types:

• Absolute threshold -- the minumum intensity of a stimulus that one can detect

• Difference threshold -- the minimum difference in intensity between two stimuli that one can detect.

The Absolute Threshold

If you've ever had a standard hearing test, you've experienced the testing for absolute thresholds. Typically this involves listening to various pitches of tone through earphones. You are given a button to hold and are told to press the button until you hear a tone, then release the button until the tone fades away, then press the button until you hear it again, and so on. The intensity at which you "lose" and regain the tone is your absolute threshold for that particular tone.

Weber defined the absolute threshold as the intensity at which the stimulus was detected on 50% of trials.  A stimulus detected on, say, 20% of trials is by this definition below the absolute threshold for detection, although of course it is sometimes detected nevertheless.  Such a stimulus is termed subliminal (below threshold; the German word for threshold is limen).

The Difference Threshold

As with the absolute threshold, Weber defined the difference threshold statistically.  Starting with a standard stimulus intensity, one can increase or decrease the intensity until one can just barely tell that changed (comparison) stimulus is either more intense or less intense than the standard.  The lower difference threshold is reached when the comparison stimulus is judged to be more intense than the standard on 25% of trials.  The upper difference threshold is reached when the comparison stimulus is judged to be more intense than the standard on 75% of trials.  The difference threshold is the average of the two differences between the comparison stimuli and the standard.

Weber noticed that the difference threshold is a constant proportion of the initial stimulus intensity.  He expressed this relationship in a formula now called Weber's Law:

delta I

-----

I = k,

where "delta I" is the difference threshold, "I" is the initial intensity before the change, and "k" is the Weber fraction or Weber constant.  For lifted weight, the Weber fraction is about 1/50 or 2%, meaning that the stimulus intensity must be changed by only one part in 50 or 2% of its initial value before you can tell that it is different.  (Note that the smaller the number, the better able you are to discriminate small differences, or in other words, the more sensitive you are to a change in intensity.

Subliminal perception:

Perception without awareness is not the same thing as "subliminal perception." Subliminal perception is supposed to occur when a stimulus is too weak to be perceived yet a person is influenced by it.

How can an undetectable stimulus be created?

As discussed in the section on psychophysics, the word limen was used in the 19th Century to refer to the absolute threshold, which was defined as the point at which a stimulus could be detected 50% of the time. By that definition, a stimulus detected 49% of the time would be subliminal. Obviously this is not what people mean when they discuss subliminal effects on human beings. They are usually referring to stimuli too weak or distorted to be detected through conscious effort. Such stimuli can be created by flashing a visual stimulus then quickly masking it with another stimulus, or by presenting a sound so weak that nobody can detect it without amplification, or by transforming a stimulus electronically until it cannot be recognized.

What was the true story behind the famous "Drink Coca-Cola" study?

Subliminal perception was given a huge publicity boost by a famous 1950s experiment in which the message "DRINK COCA-COLA" was supposedly flashed to an audience in a movie theater, resulting in a dramatic increase of Coke sales. Many people have heard about this study. But the whole thing was a hoax. The "researcher," James Vicary , revealed in a 1962 interview in Advertising Age that "the original study was a fabrication intended to increase customers for his failing marketing business." (Pratkanis, 1992) Vicary's discussion of the issue is reprinted on a web page titled "Vicary Tells All" at this URL:

One type of subliminal perception is easy to replicate. In a procedure called priming, a stimulus is flashed for a split second then quickly masked with another stimulus. Then a target word is shown and the subject is asked to identify it as quickly as possible. If the priming stimulus bears a close relationship to the target word, the subject can respond slightly faster. This is a genuine subliminal effect, because the priming stimulus cannot be seen consciously, and subjects cannot report what it is. However, the effect lasts only a tenth of a second (Greenwald, Draine, & Abrams, 1996). Priming is a robust (easily replicated) effect useful to language researchers for investigating relationships between word meanings.