Qualitatively, the phototransduction of cones resembles that of rods. Light-activated cone opsins initiate an enzymatic cascade that hydrolyzes cGMP and closes cone-specific cGMP–gated cationic channels on the outer-segment membrane. Cone phototransduction is comparatively insensitive but fast and capable of adapting significantly to ambient levels of illumination. The greater the ambient light level is, the faster and more temporally accurate is the response of a cone. Speed and temporal fidelity are important for all aspects of cone vision. This is one reason visual acuity improves progressively with increased illumination. Because of their ability to adapt, cones are indispensable to good vision. A person without cones loses the ability to read and see colors and can be legally blind. In comparison, lost rod function is a less severe visual problem, except under scotopic conditions.
Several factors contribute to light adaptation. For example, higher levels of illumination bleach away photopigments, making the outer segment less sensitive to light. As light levels increase, so does the noise level, which reduces sensitivity. Biochemical and neural feedback speed up the cone response. This feedback must be increased as light intensity increases and the cone absorbs more and more light. All the processes that turn off the rod response are probably stronger in cones.
Cones also show neurally mediated negative feedback. Horizontal cells of the inner nuclear layer synapse antagonistically back onto cones, releasing γ-aminobutyric acid (GABA), an inhibitory neurotransmitter. When light hyperpolarizes a cone, the cone hyperpolarizes neighboring horizontal cells. This effect inhibits the horizontal cells, stopping the release of GABA, which depolarizes (disinhibits) the cone by a recurrent synapse. This depolarization antagonizes the hyperpolarization produced by light and restores the cone to its resting state. Depolarization occurs with a synaptic delay so that its main effect is on the later response of the cone. Horizontal-cell feedback occurs with strong stimuli, preventing the cone from being overloaded. The feedback also turns off the cone response more quickly, enabling the cone to respond rapidly to a new stimulus. Flicker fusion threshold is the frequency of a repetitive stimulus at which it appears to be a completely steady light stimulus. This threshold is much higher in cones (approximately 100 Hz) than in rods (approximately 30 Hz).
Trivariant color vision
To see colors, mammals must have at least 2 different spectral classes of cones. Most humans with normal vision have 3 types of cones and consequently a 3-variable color vision (3 cone opsins) system:
short-wavelength-sensitive cones (termed S cones), which detect only color by comparing their signals with those of the M cones; this mechanism creates blue-yellow color vision
middle-wavelength-sensitive cones (termed M cones), which detect high-resolution achromatic (black and white) contrast
long-wavelength-sensitive cones (termed L cones), which evolved in primates to enhance color vision; this mechanism creates red-green color vision
Both L and M cones contribute to achromatic and chromatic contrast. Therefore, both are more numerous than S cones in the human retina.
Most color vision defects involve red-green discrimination and the genes coding for the L- and M-cone opsins. These genes are in tandem on the X chromosome. There is 1 copy of the L-cone opsin gene at the centromeric end of the X chromosome, and there are 1–6 copies of the M-cone gene arranged in a head-to-tail tandem array. Normally, only the most proximal of these 2 genes is expressed. Most color vision abnormalities are caused by unequal crossing over between the L- and M-cone opsin genes. This inequality creates hybrid opsins that have different spectral absorption functions, which are usually less ideal than those of normal opsins. Some males have a serine-to-alanine substitution at amino acid 108 on the cone opsin gene, which allows more sensitivity to red light. Potentially, females with both the serine-containing and the alanine-containing opsins could have tetravariant color vision.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.