A′. A nonparametric index of signal detectability.
Az. A measure of signal detectability consisting of the proportion of the area of the entire ROC graph that lies beneath the ROC curve. This proportion is calculated by first fitting a z-score ROC curve to the data and then computing z(A) from Δm and the slope, S,. Az is determined by converting z(A) from a z-score to a proportion.
Absolute Magnitude Estimation. A scaling procedure in which observers are required to make magnitude estimations of stimuli such that their subjective impression of the size of a number assigned to a stimulus matches their subjective impression of the stimulus.
Absolute Threshold. Traditionally defined as the smallest amount of stimulus energy necessary to produce a sensation. In more modem and less subjective terms, the absolute threshold is defined as the smallest amount of stimulus energy necessary to achieve some criterion level of performance, such as correct responses 75% of the time, in a stimulus detection task.
Action Spectrum. Combinations of wavelength and light intensity that produce the same response. Action spectra have been measured for photochemical, neurophysiological, and psychophysical responses. In each case, the results are plotted as a graph in which the intensity of light needed to produce some specified response is plotted as a function of wavelength.
Adaptation-Level Theory. Helson's theory that a stimulus is perceived and judged within a frame of reference determined by the values of other stimuli in the situation and by the recent stimulus history of the observer. The frame of reference establishes the adaptation level which, in turn, determines the stimulus that is