How many cone spectral classes are described?

How many cone spectral classes are described?

• A. 2
• B. 3
• C. 4
• D. 5

Answer: (B)

In human color vision there are three cone spectral classes: short-, medium-, and long-wavelength cones, commonly called S, M, and L cones. Each type has a peak sensitivity at a different portion of the spectrum—blue for S, green for M, and red for L—and their combined responses allow the brain to discriminate a wide range of colors. Because the standard description of cone photoreceptors in humans identifies exactly these three classes, the correct count is three. Two would imply only two cone types (not typical human color vision), while four or five would reflect additional cone types seen in other species or special cases, not in the usual human retina.

In human color vision there are three cone spectral classes: short-, medium-, and long-wavelength cones, commonly called S, M, and L cones. Each type has a peak sensitivity at a different portion of the spectrum—blue for S, green for M, and red for L—and their combined responses allow the brain to discriminate a wide range of colors. Because the standard description of cone photoreceptors in humans identifies exactly these three classes, the correct count is three. Two would imply only two cone types (not typical human color vision), while four or five would reflect additional cone types seen in other species or special cases, not in the usual human retina.