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Ψ  Perception
- Perceptual Thresholds -

Ψ  Gustav Fechner defined the absolute threshold as the lowest level of stimulation that a person can detect. This was redefined to the person having a 50% chance of detecting it.

Ψ  Examples of absolute thresholds:
 
•  Hearing - tick of a watch at 20 feet.
•  Vision - candle flame at 30 miles on a clear dark night.
•  Taste - one teaspoon of sugar in two gallons of water.
•  Touch - wing of a fly falling onto your cheek from a height of 1 centimeter.
•  Smell - one drop of perfume diffused throughout a three-room apartment.

It's a Mickey Mouse World , isn't it?

Ψ  A subliminal stimulus is one that a person has less than a 50% chance of detecting.
 
Ψ  Just-Noticeable Difference (JND) - the difference between two stimuli that (under properly controlled experimental conditions) is detected as often as it is undetected.
 
Ψ  Weber's law - the concept that a Just-Noticeable Difference (JND) in a stimulus is proportional to the magnitude of the original stimulus.

General Psychology
Robert C. Gates