Colour vision

Human colour vision is a complex process that enables us to distinguish millions of shades of colour. It is based on the interaction of light with specialised cells in the eye and the processing of these signals in the brain.

1. Basics of colour vision

Colour vision is created by the perception of light of different wavelengths. Light is electromagnetic radiation and the visible range of the light spectrum extends from around 380 nm (violet) to 740 nm (red).

2. Structure of the human eye

The human eye has several structures that are crucial for colour vision:

• Cornea: Light first enters through the cornea, which refracts it and transmits it to the lens.
• Lens: The lens focusses the light onto the retina.
• Retina: The retina contains photoreceptors that convert light into electrical signals. There are two main types of photoreceptors: rods and cones.

3. Photoreceptors

• Rods: Rods are responsible for seeing in dim light (twilight vision) and are very sensitive to light, but do not contribute to colour vision.
• Cones: Cones are responsible for colour vision. There are three types of cones, each of which is sensitive to different wavelengths:
  • S-cone (short-wavelength): Sensitive to short-wavelength light (blue).
  • M-cone (Medium-wavelength): Sensitive to medium-wavelength light (green).
  • L-cone (long-wavelength): Sensitive to long-wavelength light (red).

4. Colour perception

The perception of a colour is based on the relative excitation of the three cone types. If all three cone types are equally excited, we perceive white light. Different combinations and intensities of excitation lead to the perception of different colours.

5. Colour space and colour models

• RGB model (red, green, blue): An additive colour model that describes how different colours of light are combined to produce other colours. Used in digital screens.
• CMYK model (cyan, magenta, yellow, key/black): A subtractive colour model used in printing processes to create colours by mixing dyes or pigments.

6. Colour perception phenomena

• Colour adaptation: The ability of the eye to adapt to different lighting conditions and to perceive colours consistently.
• Colour constancy: The ability to perceive colours of an object as being the same under different lighting conditions.
• Metamerism: Phenomenon in which two colour mixtures appear the same under certain lighting conditions but different under other lighting conditions.

7. Colour vision and the brain

The electrical signals generated by the cones in the retina are transmitted to the brain via the optic nerve. These signals are processed and interpreted in the visual cortex of the brain, which leads to the conscious perception of colours.

8. Colour vision disorders

• Colour blindness: A common colour vision disorder in which one or more types of cones are missing or do not function properly. The most common form is red-green colour blindness.
• Achromatopsia: A rare disorder in which all cones are missing or malfunctioning, resulting in complete colour blindness.

9. Colour vision in animals

Not all animals see colours like humans. Many mammals have only two types of cones and therefore see fewer colours. Birds and some fish, on the other hand, have four or more cone types and can perceive a broader colour spectrum, including ultraviolet light.

In summary, colour vision is a highly complex process based on the interaction of light with the photoreceptors in the eye and the subsequent processing of the signals in the brain. It enables us to perceive the world in a variety of colours, which plays an important role in our daily lives.

Colour vision of women and men

Current research shows interesting differences in colour perception between men and women.

Cone types and colour perception

1. Cone types in humans:

   • Most people have three main types of cones: S (short-wave), M (medium-wave) and L (long-wave). These are sensitive to blue, green and red.
   • These cone types are localised on the X chromosomes, with males having one copy on their X chromosome (XY), while females have two copies (XX).

2. Differences in colour perception:

   • Research suggests that, on average, women may have a finer discrimination between colour tones than men. This could be due to the fact that women may have more differentiated cones or a higher sensitivity of the cones.
   • There are a few studies that suggest that an additional variant of the L-cone may exist in a very small number of women, which could lead to an extended colour perception in the yellow-orange range.
   • Men generally have the same three cone types (S, M, L), unless there is a genetic deviation that leads to colour vision disorders such as red-green vision deficiency.
   • Men do not generally have less colour perception than women, but there are differences in the specific characteristics of colour perception, which can be influenced by genetic and possibly also hormonal factors.
   • Men could therefore tend to perceive fewer nuances in certain colour tones, which could be partly explained by the different gene expression of the cone types.

3. Genetic and hormonal influences:

   • Differences in colour perception between the sexes could be due to genetic factors as well as hormonal and epigenetic influences.
   • Studies suggest that these differences are not universal, but are based on average statistical differences, and that there can be considerable variation within each gender.

The idea that women generally have four cone types and men only three does not seem to be correct at present. Instead, the variation in colour perception is more likely to be in the way these cone types are developed in women and men or how they react to certain stimuli. The exact nature of these differences continues to be researched in order to better understand the mechanisms behind colour perception.

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