According to the principle of univariance, how can we differentiate wavelengths?

• A. Change the color of the background
• B. Increase light intensity until same response
• C. Compare emission spectra of pigments
• D. Compare number of quanta absorbed by at least 2 types of cones

Answer: (D)

The principle of univariance says a photoreceptor’s output depends only on how many photons it absorbs, not their wavelength. A single cone type can’t tell wavelengths from its signal because different wavelengths can produce the same response if the photon count is the same. To distinguish wavelengths, you need information from multiple cone types that have different spectral sensitivities. By comparing how many quanta are absorbed by at least two kinds of cones, the brain can infer which wavelengths produced the pattern of activations. This inter-cone comparison creates spectral information that a single cone cannot provide. The other options don’t yield wavelength information in the same way: changing the background color doesn’t reveal wavelength through the receptor’s absorption pattern, simply increasing intensity until the same response isn’t reliably tied to a specific wavelength, and emission spectra of pigments aren’t how the retina encodes color signals.