Notes

This principle states that one and the same visual receptor cell can be excited by different combinations of wavelength and intensity, so that the brain cannot know the color of a certain point of the retinal image. One individual photoreceptor type can therefore not differentiate between a change in wavelength and a change in intensity. Thus, the wavelength information can be extracted only by comparing the responses across different types of receptors. E.g., a rod could be equally excited by a low-intensity, well-tuned wavelength stimuli and by a high-intensity, badly-tuned wavelength stimuli. This also explains why rods (associated with vision in dark situation) tend to make us see everything in grey (but more or less bright) because cones are not active in such a situation, which instead allow us to disambiguate between the wavelength of the light stimuli.

There are cases of monochromats humans that have only one type of cones, which implies the loss of color perception. In such cases, humans can only perceive changes in light intensity. On the other side, there are women that are tetrachromats, which means they have 4 types of cones that allow the perception of "more" colors than a normal trichromats person.