Psychophysics is a subdiscipline of psychology dealing with the relationship between physical stimuli and their subjective correlates, or percepts. Psychophysics has been described variously as “the scientific study of the relation between stimulus and sensation”[1] or, more completely, as “the analysis of perceptual processes by studying the effect on a subject’s experience or behaviour of systematically varying the properties of a stimulus along one or more physical dimensions.”[2] It is a general-purpose theory that can be applied to any sensory system. The techniques of “classical” or Fechnerian psychophysics are still widely used, although the theoretical background is heavily influenced by Signal detection theory.[3]
Many of the classical techniques and theory of psychophysics were formulated in 1860 when Gustav Theodor Fechner published Elemente der Psychophysik.[4]. He coined the term “psychophysics”, described research relating physical stimuli with how they are perceived, and set out the philosophical foundations of the field. Fechner wanted to develop a theory that could relate matter to the mind, by describing the relationship between the world and the way it is perceived. He was influenced by the work of German physiologist Ernst Heinrich Weber [5][6] Fechner’s work formed the basis of psychology as a science. Wilhelm Wundt, the founder of the first laboratory for psychological research, built upon Fechner’s work.
One author[7] has argued that the medieval scientist Alhazen should be considered the “founder of psychophysics.” Although al-Haytham made many subjective reports regarding vision, there is no evidence that he used quantitative psychophysical techniques and such claims have been rebuffed.[8]
Psychophysicists usually employ experimental stimuli that can be objectively measured, such as pure tones varying in intensity, or lights varying in luminance. All the senses have been studied: vision, hearing, touch (including skin and enteric perception), taste, smell, and the sense of time.
Regardless of the sensory domain, there are three main topics in the psychophysical classification scheme: absolute thresholds, discrimination thresholds, and scaling.
The most common use of psychophysics is in producing scales of human experience of various aspects of physical stimuli. Take for an example the physical stimulus of frequency of sound. Frequency of a sound is measured in hertz, cycles per second. But human experience of the frequencies of sound is not the same as the frequencies. For one thing, there is a frequency below which no sounds can be heard, no matter how intense they are (around 20 Hz depending on the individual) and there is a frequency above which no sounds can be heard, no matter how intense they are (around 20,000 Hz, again depending on the individual). For another, doubling the frequency of a sound (e.g., from 100 Hz to 200 Hz) does not lead to a doubling of experience. The perceptual experience of the frequency of sound is called pitch, and it is measured by psychophysicists in mels.
More analytical approaches allow the use of psychophysical methods to study neurophysiological properties and sensory processing mechanisms. This is of particular importance in human research, where other (more invasive) methods are not used due to ethical reasons.
Areas of investigation include sensory thresholds, methods of measurement of sensitivity, and signal detection theory.
