Historical Figures in Psychophysics: Weber, Von Helmholtz, & Fechner

Ernst Heinrich Weber (1795- 1878) was born in Wittenberg Germany on Jun 24^th 1795. Weber Studied medicine at Wittenberg University. He then became a professor of physiology and anatomy at the University of Leipzig, and is now known as the father of psychophysics. Along with his brother, who discovered the inhibitory power of the vagus nerve, Weber studied acoustics and wave motion[1]. He was also a specialist in the cutaneous and aural senses. He also managed to get elected as a member of the Saxonian parliament as a representative from the university. He worked alongside Gustav Fechner in 1860 to set a foundation for psychophysics[2]. Weber then attempted to find a way to physically measure stimulus perception through a mathematical formula known as Weber’s law of Just Noticeable Difference. Weber’s law states, “ The difference threshold (or “Just noticeable Difference”) is the minimum amount by which stimulus intensity may be changed in order to produce a noticeable variation in sensory experience.”[3] This is represented mathematically with the following formula:

∆S/S = K

Delta S represents “the change in the stimulus which is just noticeable or perceived by the subject 50 percent of the time.” S is the actual value of the stimulus (i.e. hertz, decibels, etc.) and K is a constant term.[4] What this law says is that if for example you hear a tone there is a point at which you can tell that the properties, normally the intensity, of the tone have changed that is just noticeable difference. This law of noticeable difference can be applied to several modalities or sensory properties such as loudness, length, brightness, weight, and mass. Although Weber is most well known for his just noticeable difference he also developed the theory of signs, which formed a foundation for Wundt’s psychophysical psychology.[5] Ernst Weber died on January 26^th 1878.

Gustav Theodor Fechner was born on April 19^th, 1801 in Gross-Sarchen. His father was a minister, who liked to take chances and challenge the social norm, which he did by not wearing a wig while preaching. Gustav’s mother was a minister’s daughter and his uncle was also a minister. His father died when Gustav was five years old. At sixteen he began to study medicine at Leipzig University. On the side for fun Fechner took up writing for extra money. Once out of school he also took to translating texts since the student paid docent’s, they did not receive a salary. He soon became a professor and then head of the physics department at Leipzig University. In 1833 Gustav married Clara Marie Volkmann. When he was 33 Gustav was in the center of the intellectual elite circles of society. But because of his defense of certain theories in physics that soon changed. In 1839 he became sick and resigned as the chair of the physics department. He did not return to teaching after recovering from his illness, which began as photophobia and developed into an anorexia, which nearly killed him. Gustav did, however, continue to lecture on topics that he was fond of. In 1861 Fechner defined psychophysics as “ The functionally dependent relations of body and soul, or more generally, of the material and the mental, of the physical and psychological worlds.” [6]

While contemplating the mind body connection Gustav looked for a way to explain the psychological with math and science. He wanted some physical evidence of sense perception. As Kimble, Wertheimer, and White explain:

“ The first idea occurred to him on the morning of October 22, 1850 while he was still in bed…. The essential step was to provide a means of measurement of the mental side of the relationship. To accomplish this he proposed an indirect approach. Because physical measurement was well established, and because mental intensity was proportional, it was possible to measure one side of the equation.”[7]

Two years after this epiphany Gustav was drafted into the German army, but by the 50’s he was working towards supporting his theories by testing them.

Gustav (1801-1887) more or less agreed with Weber’s idea stating “ psycho-physics is an exact doctrine of the relation of function or dependence between body and soul.”[8] Fechner did take it upon himself to modified the Weber’s formula to look like the following:

S = K log R

This was known as the fundamental equation. In this formula S is how the participant experiences the stimuli presented or the psychological experience. R is the stimulus itself like S in Weber’s law; K is once again a constant. Fechner says that how the participant experiences the stimulus is a logarithmic function of the stimulus properties R. This makes Fechner’s law the integral of Weber’s law.[9] This relationship is not a linear relationship. In fact as the actual stimulus properties increases there needs to be a larger as the standard increases for a noticeable difference occur.[10] Fechner’s law also known as the measurement equation is:

S = k log (Â/Âo) · (Âo = threshold stimulus)[11]

Because Fechner saw just noticeable difference in logarithms the equations he developed can be expressed as ratios, since logarithms are a way to express ratios. The verbal expression of this equation is “ equal stimulus ratios produce equal sensation intervals.”[12] This measurement equation produced a scale on which the decibel scale of loudness, the Richter scale, and the magnitude of the brightness of stars are founded.[13]

Gustav Fechner on November 18^th 1887. Another influential psychologist Wilhelm Wundt gave the eulogy at his funeral.

Another important figure in psychophysics is Hermann Ludwig Ferdinand Ludwig Von Helmholtz (1821- 1894). Hermann was born in 1821 in Potsdam, near Berlin. His father was a teacher at a secondary school and encouraged his son to get a broad based education. Hermannn’s mother was a descendant of William Penn. Hermann took a liking to physics and medicine. He soon became a surgeon in the Prussian army. In 1842 he began to publish articles on various theories that he was developing. Such theories that he contributed to science include: a theory on the conservation of energy (1847), young-Helmholtz theory of color[14], and he invented the ophthalmoscope (1851). He also discovered what mechanisms cause the eye to focus and the mechanisms that cause the moving eyes to produce a single image[15].

This Prussia born physicist, philosopher, and physician worked with theory measuring perceptions of visual stimuli. Helmholtz revised a theory first developed by Thomas Young in 1801 only to refute it, and reverse his refutal of the theory in 1858 becoming an advocate for the theory[16]. This theory is known as Young-Helmholtz theory[17] or triple component theory of color vision. According to the theory there are three types of cones in the retina that causes the perception of three colors: red, green, and blue. These cones are: short preferring (blue); middle preferring (greens), and long preferring receptors (reds), indicating the type of wave length the receptor is sensitive to. If there is any loss or damage of these receptors than an individual can loose their color perception of primary colors. Also, the use of these receptors results in the perception of all perceivable colors[18]. Although this is a good theory for describing the mixing of colors it does not explain afterimages, or colorblindness.

References

Fechner, Gustav. Elements of psychophysics. New York: Holt, Rinehart and Winston, inc.,1966.

Kimble, Gregory, A., Wertheimer, Michael, & White, Charlotte. Portraits of pioneers in psychology: volume 2. Washington, DC: Lawrence Erbaum Associates, Inc., 1996. http://www.netlibrary.com/Reader/.

Manning, Sidney A., Rosenstock, Edward, H. Classical psychophysics and scaling. New York: McGraw-Hill Book Company, 1968.

http://www.psybox.com/web_dictionary/young.htm. Retrieved 2/20/04.

http://www.fasthealth.com/dictionary/y/Young-Helmholtz_theory.php. Retrieved 2/20/04

http://www.fordham.edu/halsall/mod/1862helmholtz-conservation.html. Retrieved 2/20/04.

http://9.1911encyclopedia.org/W/WE/WEBER_S_LAW.htm. Retrieved 2/20/04

http://www.uni-leipzig.de/~psy/eng/weber-e.html. Retrieved 2/20/04.

http://www.cartage.org.1b/en/themes/Biographies/MainBiographies/W/WeberE/1.html. Retrieved 2/20/04.

http://www.usd.edu/psyc301/WebersLaw.htm. Retrieved 2/20/04.

[1] http://www.cartage.org.1b/en/themes/Biographies/MainBiographies/W/WeberE/1.html

[2] http://www.uni-leipzig.de/~psy/eng/weber-e.html

[4] Classical psychophysics and scaling. Quote from page 12.

[5] http://www.uni-leipzig.de/~psy/eng/weber-e.html