Understanding Colour and Photoreceptors

What is Colour

• Visible light is a small part of the electromagnetic spectrum that humans can see, ranging from 360 to 780 nm
• Colour is not intrinsic to light; instead, different wavelengths induce specific perceptions
• White light contains all wavelengths and changes in refractive index cause surface reflectance
• The wavelength reflected into the eyes determines the perceived colour
• Colour is a subjective experience, not an object property
• Colour exists as a perceptual response to light from self-luminous or reflected sources
• Colour vision is the ability to distinguish light/objects based on spectral properties

Photoreceptors

• Approximately 6 million cone cells are in each eye for chromatic vision
• Cone cells are densely located at the fovea and are sensitive to bright light
• There are three types of cone cells in a trichromatic arrangement
• These cone types evolved from a split of a yellow cone approximately 30-40 million years ago
• Each cone cell contains photopigments (photopsins) that are sensitive to a specific light wavelength
• Photopsins are transmembrane proteins in the cell membrane invaginations, which form membranous disks in the outer segment
• Unstable chemicals react with light, triggering action potential and transducing electromagnetic radiation into electrical neural signals
• Long-wavelength (L) "red" cones detect ~560 nm and contain "red-sensitive" photopsin
• Middle-wavelength (M) "green" cones detect ~530 nm and contain "green-sensitive" photopsin
• Short-wavelength (S) "blue" cones detect ~420 nm and contain "blue-sensitive" photopsin
• Light is converted into an electrical signal through a photochemical reaction
• During the process 11-cis-retinal is converted to all-trans-retinal and hyperpolarizes the cell
• Each cone cell type has a sensitivity function, and the opsin determines the spectral sensitivity of mammalian photoreceptors
• Spectral sensitivities and wavelength tuning curves are not static
• They have different but overlapping spectral sensitivities broad enough that each cone responds to light throughout much of the visible spectrum

Univariance

• Individual cone cells are colour-blind
• A single type of photoreceptor cone responds to any and all light stimulation
• Once a photon is absorbed by a cone, the cone only records the photon count and loses wavelength information
• Light absorption induces changes in the structure of photopigment
• A single photoreceptor cannot distinguish between changes in light wavelength or intensity
• Different wavelength and intensity combinations can cause the same cone system response
• Changes in neural firing can result from lower-intensity wavelengths near peak sensitivity or higher-intensity wavelengths farther from maximal spectral sensitivity
• The visual system compares signal from different cones to compute colour
• Wavelength and intensity combinations produce unique pattern of responses across the three cone types, which is the advantage of trichromacy

Colour Perception

• Colour can only be extracted by comparing the ratio of signals from photoreceptors with different spectral sensitivities
• Multiple cones result in wavelength distinguished by signal ratio and intensity, allowing perception of colours and brightness
• Any species with multiple cone types can distinguish between colours

Chromatic Vision and Colour Blindness

• Single-cone system (monochromatic vision) is present in marine and nocturnal mammals

• They perceive the world in shades of grey ranging from black to white

• The majority of terrestrial mammals are cone dichromats

• The second cone is sparsely inserted into the retina to prevent reduction in the ability to respond to fine detail, meaning it only responds to coarse detail

• Old World monkeys, apes, and humans possess trichromatic colour vision that enables colour perception based on the output of three cone systems

• The trichromatic system arose from the split of the yellow cone approximately 30-40 million years ago, allowing greater distinction

• Blue cones are rare in the retina and absent from the fovea

• Regions where blue cones are inserted cause reduce visual acuity and are primarily present within the periphery

• Tetrachromatic species exceed the red-green-blue vision of humans and extend both ends of the visible light spectrum

• Anomalous trichromacy features three cone types, but include a functional deficiency

• Either M- or L-cone sensitivity is displaced, resulting in an altered absorption pattern-

• This occurs in approximately 8% of males and 0.6% of females with Northern European ancestry

• Protanomalous = mutated L cones cause partial spectral sensitivity shift towards shorter wavelengths, reducing sensitivity to red light

• Deuteranomalous is most common in males and mutated M cones cause a partial spectral sensitivity shift towards longer wavelengths (red light), reducing sensitivity to green light

• Tritanomalous mutated S cones reduce sensitivity to blue light

• Dichromacy results in the lack of one of three cone photoreceptor types, reducing the dimensionality of colour perception

• Protanopia is the lack of L-cone system, resulting in red-green colour-blindness

• Short-wavelength light is perceived as blue with reduced saturation to a neutral point (grey) with longer wavelengths perceived as yellow with increasing saturation

• Deuteranopia is the lack of the M-cone system, resulting in red-green colour-blindness

• Short-wavelength light is perceived as blue with reducing saturation to a neutral point (grey), which differs from protanopia

• longer wavelengths perceived as yellow with increasing saturation

• Tritanopia is the lack of the S-cone system resulting in colour-blindness

• Short-wavelength light is perceived as blue with reduced saturation to a neutral point (grey) and longer wavelengths perceived as red

• Rod monochromacy (achromatopsia) is the lack of any functioning cone cells

• Perception is restricted to shades of grey and high-reflectance objects perceived as white, low-reflectance objects as black

• Cone monochromacy is the presence of only one cone type (typically S-cone system retained) and is an X-chromosome linked trait with greater prevalence in males than females.