According to the Young-Helmholtz Theory, color perception is due to the activity of how many photoreceptors?

The Young-Helmholtz Theory, also known as the trichromatic theory of color vision, posits that color perception arises from the combined activity of three types of photoreceptors in the retina. These three types of cones are sensitive to different wavelengths of light corresponding to blue, green, and red. When light enters the eye, it stimulates these cones to varying degrees depending on the light's wavelength, resulting in the perception of a wide range of colors through the interplay of these three signals.

This theory was revolutionary in explaining how humans are capable of perceiving such diverse colors from just three types of receptors, laying the foundation for our understanding of color vision. The fact that color perception relies on three types of cones emphasizes the importance of trichromatic processing in the visual system, which is why the number three is integral to this theory.