Question

Which of the following is the best explanation for why rod-based vision does not support the perception of different hues (or colours)?

A. Dark Current: Rods only generate action potentials in the dark, so really code the absence of light. This makes colour vision impossible using these photoreceptors.

B. The Principle of Univariance: Variations in both the wavelength and intensity of light result in variation of response in the rod-based system. Because the response from ganglion cells is univariate, both are perceived as changes in brightness.

C. Coarse Coding: Rods are broadly tuned to wavelength, so allow only a coarse sense of the colour of light, making true colour perception impossible.

D. Frequency Coding: Rod-based vision only produces a single frequency of action potentials in the retinal ganglion cells. Colour vision requires the comparison between two or (better) three different frequencies to support the perception of hue.

Answer 1

The spectral sensitivity of photoreceptors expressed is the key to color vision. In human vision rod cells do not contribute to color vision, because they are highly sensitive to intensity and thus they are mostly saturated in their response (does not induce firing of downstream bipolar cells) during normal daylight conditions. Rod cells specialize for night vision (scotopic conditions) which is crucial for survival and under this condition the cone cells are pretty much useless.

The answer is B

B. The Principle of Univariance: Variations in both the wavelength and intensity of light result in variation of response in the rod-based system. Because the response from ganglion cells is univariate, both are perceived as changes in brightness.