Color Vision

Color is the feature of visual perception achieved by a complex process that starts in the eye and ends in an observer's brain. The visual perception is derived from the stimulation of photoreceptor cells by the photons (from the visible region of the electromagnetic radiation with a certain wavelength) on the retina and interpreted in the visual-related cortex region of the observer's brain.

Vision is the state of being able to see. Together, color vision is the ability to identify and distinguish between photons with different wavelengths from the visible region of electromagnetic radiation. Since light can behave as a wave and a particle, color vision uses several light photons and the wavelength of the absorbed photons received at the retina to create the perception of visual color in the brain.

How does it work?

The light that falls on the retina is passed on to light-sensitive photoreceptor cells called rods and cones. The names of these cells refect their approximate shape. The light-sensing structure in both photoreceptors has two parts, the retinal that absorbs the photon and the retinal molecule embedded in an opsin protein. The retinal is the same for both types of cells, but the retinal molecule determines which photoreceptors are sensitive to which color of light.

The rod cells are used to see in low light conditions because many hundred rod cells are attached to a single ganglion cell, which creates the nerve impulse passed on to the brain. Therefore, rods can amplify the signal in low lighting conditions. Since rods are the only type of cells that function under low light conditions, only two kinds of signals are passed on to the optic nerve, light (white) or no-light (black). Therefore, it allows the observer to see only grey shades for a different proportion of mixture from black and white.

On the other hand, the cone cells are receptors that work in bright light that facilitate color vision. Only a few cone cells are connected to one ganglion cells, and therefore, have lower sensitivity but high accuracy in good light conditions. The cone cells are of three main types categorised as L cones (or red cones), M cones (or green cones) and S cones (or blue cones) corresponding to Long, Medium and Short wavelengths of photons, respectively.

When these photoreceptors receive the photons, the three types of cone cells react in different amounts to different wavelengths of photons leading to varying spectral sensitivity in each type of cone cells (because they contain different forms of opsin – a photopigment protein). The mixture of these three signals in varying amounts can produce the whole spectrum of rainbow colors.

Those photoreceptors emit the resulting signal that propagates towards the retinal ganglion cells, and along with the nerve response outputs, are sent along the optic nerve, then to optic chiasma, and then to the lateral geniculate nucleus in the thalamus while propagating through many magnocellular neuron layers and parvocellular neuron layers (which are responsible for color detection), and finally reaching to the primary visual cortex in the brain announcing the arrival of light. The primary visual cortex is connected to the secondary visual cortex and visual association cortex via dorsal and ventral streams, where the color-related visual analysis occurs.

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