Psychophysics

[Ready for Fall 2026]

Modified: 2026-07-29 11:34 AM CDST

• One of the places psychology started was in the field of psychophysics.
  • Psychophysics is the attempt to find the physics of the body.
  • It consists of applying a physical stimulus to a subject, and then getting the subject's report of the psychological experience associated with that physical stimulus.

• 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 .
  • Note that the lightbulb market has changed as new technologies have arisen. It is nearly impossible now to find 95 or 100 watt incandescent bulbs.
• 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.