next up previous contents
Next: Introspectionism Up: Psychology Lecture 1 Previous: Associationism   Contents


Psychophysics dominated early scientific psychology - between late 19th C and early 20th C. Aim is to make objective measurements of contents of mind through physical stimulus followed by subject's report of psychological experience. Today's cognitive science shows strong methodological parallels with psychophysics.

Inspired by Benedict Spinoza (1632-1677) - psychophysical parallelism: Every physical event has a mental counterpart, and vice versa.

Bottom-up assumption: Full perception of objects occurs out of ``elementary sensations'' which reflect simple aspects of the environment: eg. weight, colour, brightness, pitch, loudness. These latter are the building blocks of perception.

Ernst Weber (1795-1878) Sense of touch (two-point threshold for touch: smallest for tongue - 1mm, largest for back - 60mm). Showed that touch is composed of senses of pressure, temperature and pain.

Thresholds for lowest amount of sensation detectable by a sense organ: a threshold is the level of sound, light or touch at which someone reports hearing, seeing or feeling a sensation 50% of the time. (established first by Weber?)

Demonstrated existence of kinaesthesia (the experience of muscle position and movement).

Weber's law: Weber measured perceived intensity of a stimulus (like weight, brightness, loudness, sweetness) by pairing similar stimuli and recording the just noticeable difference in intensity - found that jnd is proportional to the magnitude of the stimulus. (One candle added to 1 or to 100) This law relates physical stimulus with mental experience. 1/40 for weights held in hand.


(Reiz in German is ``stimulus'')

Gustav Fechner (1801-1887) Elements of Psychophysics, 1860, founded an ``... exact science of the functional relationship ... between body and mind.''

Introduced in 1860 the mathematical expression for Webers law:

Let the intensity of stimulus be 7#7 and experienced sensation be 8#8.




Perceived intensity of a stimulus (eg. brightness and loudness) is proportional to the log of its physical intensity (Fechner's law).

Psychological event expressed in terms of measurable physical event!

The Weber-Fechner law, has been replaced in modern psychophysics by Steven's Power Law:



(A log-log relationship - log stimulus-intensity vs log perception gives a straight line.)

Hermann Helmholtz (1850-70) devised a way to measure the speed of transmission of nerve impulses (165-330 feet per second in humans).

Did experiments colour vision, perception (eg. post-effect of wearing distorting lenses; study of restored sight) - ideas of space are not innate.

Developed the idea of unconscious inference - past knowledge affects perception (eg. in optical illusions). Contrast with syllogistic inference.

(Thus Helmholtz used b-u (molecular) as well at t-d (molar) approaches)

F.C. Donders (1868) measured time required for higher mental operations.

Franz Brentano (priest philosopher) 1874 opposed the molecular/ mechanistic approach of the psychophysicists and stressed study of phenomenological aspects like mental acts - judging, sensing, imagining, hearing; so intentions, purpose, goals. This t-d approach was not very popular till it's re-emrgence in the Gestalt and then in the symbolic AI view.

next up previous contents
Next: Introspectionism Up: Psychology Lecture 1 Previous: Associationism   Contents