Chromatic vision

17/01/2019

We can define color vision as normal color recognition. Those responsible for this are the cones, which are photosensitive cells found in the retina.

Color is the impression made on the retina by rays of light reflected and absorbed by an object according to the wavelength of these rays.The human eye can perceive eleven million shades of colors, the perception of these colors will depend on the characteristics of the objects that will absorb one type of wavelength or another, on the absorption of the photoreceptors that will pick up the wavelengths not absorbed by the object and the characteristics of the visual pathway that will interpret said wavelengths captured by the receivers.

The retina is the innermost layer of the eye and is the photoreceptor tissue. It is made up of ten layers and contains numerous types of cells responsible for receiving the different aspects of vision.In this membrane there are two main types of photoreceptor cells that make vision possible, receive stimuli from light and transmit them to the brain. These receptors are the rods and cones. The rods are responsible for night vision and the cones for color vision.

There are three types of cones, each one receives the light corresponding to a certain wavelength: those that perceive long wavelengths correspond to the color red, those that perceive medium lengths correspond to the color green and those that perceive short wavelengths correspond to the color blue.The cones, when stimulated, emit signals separately that go to the brain and, there, these signals are integrated in such a way that we can perceive color information.

The alteration of color vision is called dyschromatopsia and consists of partial color blindness. It can be due to hereditary and congenital or acquired disorders.

The congenital defects of these cells can be classified according to which pigment the alteration affects and are called:

• Protanopia: absence of red pigment.
• Deuteranopia: absence of the green pigment.
• Tritanopia: absence of blue pigment.

In the event that the pigment is present but is deficient, they are called protanomaly, deuteronomaly or tritanomaly.

When you have total color blindness, it is called achromatopsia.

The acquired chromatic effects can be triggered by a series of pathologies such as:

• Alterations of the lens.
• Macular degeneration.
• Pathologies of the optic nerve.
• Neurological injuries.
• Choroidal pathologies.

When it is suspected that any of these alterations may exist, a series of tests can be carried out to determine it.
The tests for the evaluation of color vision can be classified into four types:

1. Confusion test: like the Ishihara test and Lanthony's Tritan Album.
2. Classification test: such as the Farnsworth D15 and the 100-tone Farnsworth-Munsell.
3. “Matching test”: like the Nagel Anomaloscope.
4. Naming tests: like the Farnsworth Lantern.

These tests are performed in a monocular way and with visual correction if necessary.

Olga Riera

Visual Function Area, Department of Biometrics