Psychophysics

The study, in numbers, of how the strength of a stimulus relates to the strength of the sensation it produces. Gustav Fechner founded it in his Elemente der Psychophysik of 1860.

The Leipzig physicist Gustav Fechner recorded that an idea came to him on 22 October 1850. The growth of a sensation cannot be measured directly, but the growth of the stimulus needed to produce it can. After experiments on lifted weights, brightness and distances, he published the two volumes of Elemente der Psychophysik in 1860^[2].

His starting point was a finding by Ernst Weber: the smallest difference a person can notice grows in proportion to the size of the stimulus. From this Fechner derived his law, that the strength of a sensation goes with the logarithm of the strength of the stimulus. In his own example, adding 10 to 10 raises the logarithm by a third, while adding 10 to 1,000 raises it by about one seven-hundredth^[1].

Fechner separated outer psychophysics, the relation between stimulus and sensation, from inner psychophysics, the relation between sensation and activity in the brain. He expected the inner relation to follow the same kind of law, but with what was then known about the brain this could not be tested^[1][2].