ABSTRACT
Early researchers were principally concerned to measure absolute thresholds and differential thresholds. The absolute threshold was defined as the magnitude of the physical stimulus that would just produce detectable sensation by 'lifting it over the threshold of consciousness'. A differential threshold was defined as the physical difference in magnitude between two values of a stimulus that produced a just noticeable difference in sensation. Attempts to measure the absolute threshold for a range of sensory modalities revealed that it was more variable than had originally been expected. Gustav Fechner proposed that a sensory scale could be constructed using just noticeable differences as the unit of sensation, and this is equivalent to stating that sensation increases with the logarithm of stimulus magnitude. The theory of signal detection is based on the idea that all sensory observations take place against a background of noise which varies randomly in intensity over time..
