Under the principle of univariance, how many cone types are needed to differentiate wavelengths?

• A. One cone type only
• B. Two cones are sometimes enough
• C. Three cone types are needed
• D. Two or more types of cones are needed

Answer: (D)

Universality of color signals rests on the idea that one type of photoreceptor cannot uniquely signal wavelength. This is univariance: a single cone’s response is influenced by both the color (wavelength) and how bright the light is (intensity). Because different combinations of wavelength and intensity can produce the same response in one cone, you can’t tell which wavelength you’re seeing from that alone.

When two or more cone types with different spectral sensitivities are involved, the brain can compare their responses. The pattern or ratio of activity across the cones changes with wavelength in a way that is less affected by intensity, letting you infer the actual color. So, two or more cone types are sufficient to differentiate wavelengths in principle, and in humans there are three cone types, which offers even finer discrimination.

That’s why two or more cone types are the correct answer.