'Psychometric function' by Nicholas Wade

i-Perception cover based on 'Psychometric function' by Nicholas Wade.

Gustav Theodor Fechner (1801-1887) integrated Weber's fraction to found the discipline of psychophysics. Fechner's insight was that the mental and material worlds could be united mathematically in the domains of sensory and stimulus intensities. He distinguished between an outer and an inner psychophysics; the former was concerned with the sensation and stimulus intensities, and the latter with the relation between brain process and sensations. He realised that experiments in his day would be confined to outer psychophysics, but these were seen as necessary steps towards understanding inner psychophysics. Accepting the validity of Weber's fraction, and naming it Weber's law, Fechner assumed that equal stimulus differences corresponded to equal sensation differences. Sensory magnitude could be assigned values according to the number of just noticeable differences above the absolute threshold. Starting from the absolute threshold the stimulus intensities corresponding to successive difference thresholds could be calculated and plotted. The curve is logarithmic, and it can be expressed mathematically: sensory magnitude is proportional to the logarithm of stimulus magnitude—the lawful relationship that bears Fechner's name. In his eighth decade Fechner applied his quantitative approach to the study of beauty and founded the subject of experimental aesthetics. The curves in which Fechner is just noticeable represent psychometric functions and Fechner's portrait is to be found in the area of uncertainty.