Fig. 1.

Two hypothetical response functions. Each function corresponds to a different context (e.g., different chromaticity of background) and plots a mechanism's response as a function of stimulus strength (e.g., test spot contrast). Equality of appearance across the context change is predicted by equality of mechanism response, so that the stimuli indicated by the downward arrows would be predicted to match in appearance. Discrimination thresholds (i.e., test increment required to discriminate between a pedestal presented alone and a pedestal plus the test) are predicted to be inversely proportional to response function slope.