Modified: 2020-03-18
One of the discoveries is the notion of threshold. A threshold or limen is a psychological limit to perception. The absolute threshold is the lowest amount of sensation detectable by a sense organ. The textbook gives some examples of the amount of sensation necessary. The relative or difference threshold is the lowest difference in sensation detectable. An example of a relative threshold problem might be deciding which of two lights is brighter. Manufacturers of light bulbs have begun to take advantage of the difference threshold lately by selling 95 watt bulbs. Their advertising says that you are getting the light of a 100 watt bulb for less usage of electricity. What they are really saying is that we cannot tell differences in brightness between 95 and 100 watt bulbs. They are not more than one JND apart .
Both the absolute and the relative thresholds are not points on a scale. Rather they can be thought of as a range. The theoretical threshold is a point. When one reaches a certain intensity of stimulation one is able to detect it. However, in actuality, thresholds are ranges. The 50% threshold is the point where subjects can detect a stimulus half of the time. By convention, the 50% threshold is most commonly used. Note that the 100% threshold, or the place on the intensity axis where all subjects can detect a stimulus, corresponds to a much more intense stimulus than does the 50% threshold.
Ernst Weber was the first to describe the difference threshold mathematically. Weber's law can be stated as follows. For any particular sensory system, the ratio of the difference in stimulation divided by the original stimulation is a constant. Different sensory systems have different constants.
Gustav Fechner, working at the same university (Leipzig), but unaware of Weber, discovered the same law, but stated in an equivalent mathematical form. When he discovered that Weber had already discovered the relationship, he named the discoveries Weber's Law. Today, we honor both by calling the relationship the Weber-Fechner law.
Modern psychophysical research has generally abandoned the Weber-Fechner law, and instead uses a Power Law, a log-log relationship between stimulation and perception that yields straight line graphs.