Psychophysics is the oldest field of the science of psychology. It stems from attempts in the nineteenth century to measure and quantify sensation. It attempts to quantify the relationship between a stimulus and the sensation it evokes, usually for the purpose of understanding the process of perception. Historically, psychophysics has centred around three general approaches. The first involves measuring the smallest value of some stimulus that a listener can detect – a measure of sensitivity known as a threshold. The second is discrimination, where the subject is presented with two or more stimuli (e.g. two tones of different frequency) and then asked whether the stimuli are different. The third approach involves directly asking the listener about the stimulus. These are usually called scaling procedures.
The stimuli used in psychophysical tests can be varied along a number of dimensions. For example if the stimulus is light, it could vary in wavelength, size or shape; if it is sound it could vary in intensity, frequency or duration, etc. The response to the stimulus may be a verbal report (‘yes I see it’, ‘no I don't see it’, ‘these two appear the same’) or a mechanical response, such as pressing a button.
To estimate absolute thresholds and discrimination abilities, two basic classical psychophysical methods are used. The first of these is the method of limits and the second, the method of constant stimuli.