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Absolute threshold

The minimal amount of energy necessary to stimulate the sensory receptors.

The method of testing for the absolute threshold is similar for different sensory systems. Thus, the tester can briefly present a light or a sound (or any other kind of stimulus) at different, low intensities until the observer is unable to detect the presence of the stimulus. In such a task, the person may undergo thousands of trials before the researcher can determine the threshold.

While the absolute threshold is a useful concept, it does not exist in reality. That is, on one occasion, an individual might be unable to detect a certain faint light

EXAMPLES OF ABSOLUTE THRESHOLDS
Sense	Example of threshold
Vision	The amount of light present if someone held up a single candle 30 mi (48 km) away from us, if our eyes were used to the dark. If a person in front of you held up a candle and began backing up at the rate of one foot (30 cm) per second, that person would have to back up for 44 hours before the flame became invisible.
Hearing	The ticking of a watch in a quiet environment at 20 ft (6 m).
Taste	One drop on quinine sulfate (a bitter substance) in 250 gal (946 l) of water. Quinine is one of the components of tonic water.
Smell	One drop of perfume in a six-room house. This value will change depending on the type of sub-stance we are smelling.
Touch	The force exerted by dropping the wing of a bee onto your cheek from a distance of one centimeter (0.5 in). This value will vary considerably depending on the part of the body involved.

but on a subsequent occasion, may detect it. In addition, scientists cannot determine with absolute certainty how much energy is present in a light because of limits to the physics of measurement. As a result, psychologists often define the threshold as the lowest intensity that a person can detect 50 percent of the time.

A number of different factors can influence the absolute threshold, including the observer's motivations and expectations, and whether the person is adapted to the stimulus. Scientists have discovered that cognitive processes can influence the measurement of the threshold and that it is not as simple as once understood. Psychologists have also studied how different two stimuli have to be in order to be noticed as not being the same. Such an approach involves what are called difference thresholds.

Further Reading

Galantner, E. "Contemporary Psychophysics." In New Directions in Psychology, edited by R. Brown. New York: Holt, Rinehart & Winston, 1962.