Rods and Cones
Rod cells and cone cells are the photoreceptive cells in the retina. There are many more rod cells in the retina than there are cone cells.
Rod cells are sensitive to wavelengths of light near 500nm—and provide vision in low levels of light. Rod cells are responsible for night vision and are usually found outside of the fovea, the central part of the macula.
In low light conditions the pupil dilates allowing light to stimulate the rod cells positioned outside of the central area of the macula. Without rod cells night blindness would occur.
Cone cells function best in daytime light—and are responsible for sharp, discerning vision including color perception. Mostly cone cells are located within the fovea. When light comes to a focus on the fovea, cone cells are stimulated by different wavelengths of light.
Cone cells are most sensitive to wavelengths around 550 nm. (Sunlight is approximately 550nm, in the middle of the visible spectrum.) The human eye has evolved to be most sensitive to wavelengths of light emitted from our primary light source—the sun.
Pingelap Atoll: Achromat Island
Pingelap atoll is part of the eastern Caroline Islands of Micronesia. The island is home to the largest population of people with achromatopsia, or colorblindness.
Approximately 10% of the Pingelapese people have rod monochromatism, or complete absence of cone cell function, due to a congenital mutation on the CNGB3 gene on the Chromosome 8. The Pingelapese call the condition “maskun,” which directly translated in Pingelapese means “not see.”
In 1994 Dr. Oliver Sacks wrote the book “Island of the Colorblind” detailing the phenomenon of achromatopsia among the Pingelapese.
—Jenean Carlton, ABOC, NCLC