Causes
In vertebrates there are typically two kinds of receptors: rods, which primarily distinguish between levels of illumination, and cones, which are responsible for perception of color. There are different types of cones; each perceives only a single color. The normal explanation of monochromacy is that the organism's retina contains only a single kind of light receptor cell, or at least that only one kind is active at any particular level of illumination. Monochromacy is caused by either a defect or the complete absence of the retinal cones.
Some individuals possess diseases or injuries that lead to nyctalopia, or night blindness, where rod cells stop responding properly to light.
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