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Psychophysics

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Psychophysics is the branch of psychology that measures the relationship between stimuli in the outside world (physical events) and the way a person perceives and experiences them (psychological events).[1] It was one of the first areas of psychology to use measurement and mathematics, and it helped to show that psychology could be an experimental science.[2][3]

Physical and psychological

The physical world and the world as we experience it are not the same. A pound of lead and a pound of feathers weigh the same, but held in the hands the lead feels much heavier. Similarly, about 24 still pictures shown each second in a film are seen as smooth movement.[1] Pairs of physical and psychological measures include wavelength and colour, light intensity and brightness, frequency and pitch, and sound intensity and loudness.[3]

Background

The philosopher Immanuel Kant argued that mental processes could not be described with mathematics, so psychology could never be a true science. Later thinkers took this as a challenge.[2] Johann Friedrich Herbart (1776–1841) suggested giving numbers to the strength of mental experiences, but he had no way of linking these numbers to an objective standard.[2]

Ernst Weber

Ernst Weber (1795–1878), professor of physiology at Leipzig, studied the sense of touch.[1] He touched blindfolded people with the two points of a compass-like device, moving them gradually apart until the person felt two points instead of one. This distance, the two-point threshold, is small on sensitive areas such as the thumb and larger on less sensitive areas such as the upper arm.[4][1]

Weber also asked people to compare a weight with a standard weight. He found that the smallest difference a person could notice, called the just noticeable difference (jnd), was not a fixed amount but depended on the size of the standard.[5] For example, a person who could just tell 30 grams from 33 grams could also just tell 60 grams from 66 grams, as both pairs differ by the same proportion.[4] This rule is now known as Weber's law. It works well for middle-sized stimuli but not for very small or very large ones.[5]

Gustav Fechner

Gustav Fechner (1801–1889) trained as a doctor but became professor of physics at Leipzig; he thought of himself as a philosopher and wanted to solve the problem of how mind and body are related.[6] He wrote that on the morning of 22 October 1850 he suddenly saw that mind and body could be linked by measuring sensations and the stimuli that cause them. He called this new field psychophysics.[7]

Fechner assumed that every jnd feels like the same amount of difference. From this he worked out that sensation increases with the logarithm of the stimulus, a rule written S = k log R, known as Fechner's law or the Weber–Fechner law.[5] The assumption was challenged almost at once and holds only in limited cases.[7] To gather data, Fechner is said to have lifted weights in each hand more than 67,000 times.[8]

Methods

Fechner's work led to a decade of research on the absolute threshold, the smallest stimulus that can be detected, and the difference threshold, the smallest difference that can be detected.[9] In the method of limits, for example, a tone is made gradually louder until the listener hears it, and gradually quieter until they no longer can; the results of 20 or more such series are averaged.[9] Fechner also invented the method of adjustment.[9] These methods are still in use and are his most lasting legacy.[7]

Influence

Fechner's book Elements of Psychophysics (1860) convinced early psychologists such as Wilhelm Wundt and Hermann Ebbinghaus that an experimental science of psychology was possible.[3] It is often considered the first book of experimental psychology.[7]

See also

== References ==

  1. ↑ 1.0 1.1 1.2 1.3 Benjamin, Ludy T., Jr. (2019). A Brief History of Modern Psychology (3rd ed.). Hoboken, NJ: John Wiley & Sons, Inc. p. 29. ISBN 978-1-119-49324-2. Retrieved 2026-10-23.{{cite book}}: CS1 maint: multiple names: authors list (link)
  2. ↑ 2.0 2.1 2.2 Pickren, Wade E.; Rutherford, Alexandra (2010). A History of Modern Psychology in Context. Hoboken, New Jersey: John Wiley & Sons. p. 45. ISBN 978-0-470-27609-9. Retrieved 2026-10-23.
  3. ↑ 3.0 3.1 3.2 Benjamin, Ludy T., Jr. (2019). A Brief History of Modern Psychology (3rd ed.). Hoboken, NJ: John Wiley & Sons, Inc. p. 32. ISBN 978-1-119-49324-2. Retrieved 2026-10-23.{{cite book}}: CS1 maint: multiple names: authors list (link)
  4. ↑ 4.0 4.1 Goodwin, C. James (2015). A History of Modern Psychology (5th ed.). Hoboken, NJ: John Wiley & Sons, Inc. p. 85. ISBN 978-1-118-83375-9. Retrieved 2026-10-23.
  5. ↑ 5.0 5.1 5.2 Benjamin, Ludy T., Jr. (2019). A Brief History of Modern Psychology (3rd ed.). Hoboken, NJ: John Wiley & Sons, Inc. p. 30. ISBN 978-1-119-49324-2. Retrieved 2026-10-23.{{cite book}}: CS1 maint: multiple names: authors list (link)
  6. ↑ Goodwin, C. James (2015). A History of Modern Psychology (5th ed.). Hoboken, NJ: John Wiley & Sons, Inc. p. 86. ISBN 978-1-118-83375-9. Retrieved 2026-10-23.
  7. ↑ 7.0 7.1 7.2 7.3 Goodwin, C. James (2015). A History of Modern Psychology (5th ed.). Hoboken, NJ: John Wiley & Sons, Inc. p. 87. ISBN 978-1-118-83375-9. Retrieved 2026-10-23.
  8. ↑ Pickren, Wade E.; Rutherford, Alexandra (2010). A History of Modern Psychology in Context. Hoboken, New Jersey: John Wiley & Sons. p. 47. ISBN 978-0-470-27609-9. Retrieved 2026-10-23.
  9. ↑ 9.0 9.1 9.2 Benjamin, Ludy T., Jr. (2019). A Brief History of Modern Psychology (3rd ed.). Hoboken, NJ: John Wiley & Sons, Inc. p. 31. ISBN 978-1-119-49324-2. Retrieved 2026-10-23.{{cite book}}: CS1 maint: multiple names: authors list (link)
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