To summarize, vision research provides first-principle explanations for many aspects of Image Theory. The three component limit is due to the way image features are represented in the nervous system and the difficulty of conjunction search. Planar and retinal variables are in fact different because spatial location ties all other attributes together and is hence naturally a better medium for depicting correspondences. The observer views a series of lights differing in intensity and judges apparent brightness of each 5 . When psychophysicists perform such experiments with different physical dimensions, the curves typically fall into one of the three categories shown.
Some curves are linear, meaning that sensation grows in direct proportion with physical intensity.
In these cases, a doubling of physical intensity produces a doubling of sensation, so ratios are maintained. This is required for Bertin's quantitative perception where a doubling of the represented quantity should produce perception of doubled magnitude. Unfortunately, linear curves are rare. Most physical dimensions produce nonlinear scales, with compressive functions being the more common result. In this case, doubling physical intensity does not produce proportional increases in sensation.