cones

Meaning: Cones are photoreceptors in the retina that convert light into neural signals. They function best in bright light and provide color vision and sharp detail. Most humans have three cone types whose pigments respond best to short (blue), medium (green), or long (red) wavelengths. Cones are densest in the fovea and sparse in the periphery.

On the MCAT: Expect a rod versus cone comparison. Cones: fovea, color, high acuity, bright light, need more light to respond. Rods: periphery, dim light, black-and-white, more numerous, more sensitive to light. Damage to the fovea impairs fine detail and color, and a faint star is easier to see slightly off-center, where rods dominate. Cones also anchor two color theories: trichromatic theory (three cone types, with color coded by comparing their outputs) and opponent-process theory (red-green and blue-yellow pairs, at later stages such as ganglion cells and the thalamus). Both are accepted and describe different stages of processing.

Common confusion: Students assume the cells that give sharp detail are also the most sensitive. Cones need brighter light, and rods are more sensitive. Also, a "blue" cone is not the only cell that responds to blue light. Each type responds across a range of wavelengths, and color comes from comparing the outputs of the types.