Mehak Riaz
Doctor of Optometry, Class of 2024, University of Waterloo
Life Sciences, Class of 2020, McMaster University
riazm6@mcmaster.ca
Most people learn about colours in kindergarten. Blue and yellow makes green, red and blue makes purple! Despite this, even today, much is unknown about the science of colour perception. However, a good understanding of current theories is crucial to help advance our knowledge of how colour is perceived.
A Brief History of Colour Perception
The first known theory of colour vision was developed by Aristotle, and was accepted for over 2,000 years1. Aristotle believed that colour was essentially mixtures of two shades, black and white, and was related to the 4 elements; air, fire, water and earth1.
Aristotle’s theory was not seriously challenged until Isaac Newton’s infamous series of prism experiments, where he demonstrated that white sunlight could be split into 7 distinct colours with the use of a basic prism1.
Colour perception entered the picture around 1810, following the publication of Johann Wolfgang von Goethe’s book, Theory of Colour1. Goethe argued against the purely scientific composition of colour, and instead proposed that colour is subjective, and perceived differently by each individual1.
Since then, the world of colour perception has only grown alongside advancements in neuroscience, perhaps significantly with the first description of the back of the eye, the retina, by Santiago Ramón y Cajal1. Cajal demonstrated that the retina is made up of multiple layers, one of which contains specialized receptor cells, called photoreceptors, which feed into retinal neurons1. These neurons feed into the brain, and specifically synapse in areas proven to be associated with things like memory, form or shape determination, movement and facial recognition 1.
These discoveries were, and will continue to be, crucial in the future direction of colour perception. They have helped us understand that the physical properties of colour, the biological processes of viewing colour, and the subjectivity of colour perception are not separate entities, but rather a complicated web integrated in the brain.
The Visual Pathway
Vision begins with the visual pathway, which describes how light from our surroundings is converted into chemical energy, to electrical energy, and finally processed in the brain to give us visual perception2.
The visual pathway starts at the level of the retina, which, interestingly, is inverted in humans2. This means that light must pass through all layers of the retina first to be picked up by special cells called photoreceptors at the back of the retina2. These photoreceptors turn the light into chemical, and then electrical energy in the form of an action potential2.
The electrical visual signal is sent forward through a series of fibers of the retina, eventually forming the optic nerve2. The stimulus then passes through the optic chiasm, a crossover of the optic nerves in front of the hypothalamus2. Visual information then passes down the optic tract into the lateral geniculate body, the visual relay center of the thalamus2. From here the visual information is relayed to primary and secondary visual cortices for further processing2.
Neurophysiology of Colour Perception
As mentioned before, photoreceptors are specialized receptor cells which detect photons of light2. Humans have two types of photoreceptors, rods and cones2. Rods are used for low-light vision. They are able to detect general brightness differences but have poor acuity overall2. Cones are used for seeing in higher light levels2. They have high spatial acuity, and detect light based on wavelength2. There are three subsets of cones: L or red cones, which detect long visible wavelengths, M or green cones, which detect medium sized visible wavelengths, and, S or blue cones, which detect short visible wavelengths2.
While cones preferentially absorb certain wavelengths, it is important to remember that once the action potential is created, all physical information of the light, like its wavelength and thus colour, is lost2. This is called the Principle of Univariance2. Cones do not code for colour directly, but rather it is the magnitude of the response of each cone for a certain wavelength which makes distinguishing between colours possible2. Cones generally respond to a range of wavelengths, however, respond maximally to a certain wavelength within that range, this is seen in figure 12:
Colour processing and perception occurs at the level of the visual cortices2. The primary visual cortex, or V1, is where colour processing begins and is integrated with some shape and motion information2. The secondary visual cortex, or V2, not only continues with this initial processing, but also acts as a relay center to send information forward to visual areas 3 (V3), 4 V4), 5 (V5) and 7 (V7)2. It is in these areas where the brain primarily integrates visual and other information to give sense of perception2.
V4 specifically is important for the integration of colour, shape and attention information2. From V4, colour information is then relayed to the posterior inferior temporal cortex (PIT)2. Specific regions in the PIT are colour tuned, meaning that they are not only selective for receiving information on certain colours, but nearby cells respond to the same colour as well, displaying organization of colour perception at higher levels of processing2. What is most interesting is that these colour-tuned regions are closely associated with other regions in the temporal cortex involved with facial recognition, object recognition and scene perception2. Studies which use visually evoked potentials to map out cortical functions have shown that tasks involving facial recognition also light up areas responsible for colour processing in the temporal cortex2. It is the integration of these processes which is believed to give us the ability to perceive colour2.
Differences in Colour Perception – A Grey Area
Despite advancements and research in recent decades, the subjective perception of color is still a mystery. For example, there are significant cultural differences in the naming of colours.
One group demonstrated this by studying native Greek speakers, who have two words distinguishing light and dark blue, versus native English speakers, who have just one word for the colour blue3. The Greek-speaking individuals were not only more precise, but also faster in discriminating between light and dark shades of blue when compared to the English-speaking subjects. This alludes to a link between colour perception and language3.
Another study researched which colours are distinguished, and which are not, in languages of different parts of the world4. Their results are shown in figure 2 below:
Figure 2: Semantic colour identities4
They found that the majority of societies in Europe have semantic colour identities in their primary-colour naming system4. This means that their languages distinguish colours like black, blue, green and yellow4. However, some societies (shown in figure 2), mostly in the equatorial regions, do not distinguish these colours in their languages4. For example, the squares on the map indicate regions where the native language refers to both green and blue by the same word4. Further research on how this affects their perception of blue and green would be interesting, and perhaps allude to an evolutionary link for such geographical distribution.
Overall, the use of neuroscience and physiology has been crucial to the understanding of colour perception. Future studies should look more into the neurophysiological processes of colour vision, and also link colour perception to sociological factors, such as language.
- The science of color [Internet]. 1970 [cited 2021 Mar27]. Available from: https://library.si.edu/exhibition/color-in-a-new-light/science#:~:text=Aristotle%20developed%20the%20first%20known,air%2C%20earth%2C%20and%20fire
- Schwartz SH. Visual perception: a clinical orientation. 4th ed. New York, NY: McGraw-Hill Education; 2017.
- Thierry G, Athanasopoulos P, Wiggett A, Dering B, Kuipers J-R. Unconscious effects of language-specific terminology on preattentive color perception. Proceedings of the National Academy of Sciences. 2009;106(11):4567–70.
- Bornstein MH. Color vision and color naming: A psychophysiological hypothesis of cultural difference. Psychological Bulletin. 1973;80(4):257–85.
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