Once upon a time, on a clear night, a family paused to gaze in wonder at the sky.
The little ones were transfixed by the moon’s bright face. Meanwhile their parents could just make out the blue aura of what we call ‘Rigel’, the orange-tinged ‘Aldebaran’, the yellowed gleam of ‘Pollux’, and red Mars on its seasonal journey across the night sky.
Eons later, astronomers in the high Andes pause to gaze in similar wonder at the coloured trails of the stars…
The human eye may see, but it is also a social tool.
Baby’s first task is to identify and bond with mother. Within moments of birth, a child’s eyes are open and scanning the room for a face to imprint as familiar. We are highly sensitive to patterns, and code meaning into them easily. In particular the ability to recognise faces is so strongly wired into our neurology that we ‘see’ them everywhere, even in the shadows on the moon.
Newborn children see only black and white. They learn to distinguish basic colours during their first 6-7 months of life. During this time the infants’ attention becomes more focussed as they gain control of their eye muscles and start to direct their gaze. Similarly babies learn to discern signals from ‘noise’ across all of their senses.
Vision has evolved repeatedly in numerous groups of animals, including those lacking a complex nervous system. The pigment proteins that in our eyes detect light are found in very similar forms across all kingdoms. The simplest ‘eyes’ have no colour capacity, so only see in black and white. They contain a protein known as rhodopsin. This acts as a pigment which is sensitive to light. Such eye spots may be a relatively simple system, but sudden shifts between light and shade convey warnings and perhaps the need for action.
Cells of the retina at the back of our eyes include ‘rods’ which contain rhodopsin and ‘cones’, carrying similar proteins, the photopsin pigments. Rods respond to light at even low intensities, whilst cones detect coloured light at three different wavelengths.
Our cone cells harvest light of ‘short’ (S), ‘medium’ (M) and ‘long’ (L) wavelengths using our three types of photopsin. These channels correspond to what we understand as blue, green and red light.
Fix your eyes on something nearby, and hold your gaze steady. As you do this, you will see the fixated object in sharp detail, and be aware of an out-of-focus zone around it. This is your ‘peripheral vision’.
Our three types of cone cell cover a region of the retina known as the fovea. Our eye muscles compress the lens to focus an image here, so that cone cells can capture details from our view. Their photopsin pigments allow them to take in substantial amounts of information over a narrow range of wavelengths.
Cones are however relatively insensitive to low intensities of light. If we strain to see the different colours of the stars, the photopsins are close to their perceptive limits. We see starlight thanks largely to the rod cells around the fovea. These cells contribute to our peripheral vision.
Birds, most reptiles, and ‘primitive’ vertebrates such as the sturgeon, have four types of photopsin. Most mammals have only two, so giving them ‘dichromatic’ colour vision. The genes for the other pigments may have been lost during a nocturnal stage early in mammal evolution, favouring eyes with a higher number of rhodopsin-rich ‘rod’ cells that are more effective in low light conditions, when perhaps dinosaurs were less of a threat.
Like other mammals, all primates carry the ‘S’ photopsin gene. Subsequently our primate ancestors re-evolved a third photopsin, thus giving us a more detailed (‘trichromatic’) colour vision. This extra pigment arose through a duplication and subsequent modification of the ‘L’ gene which was carried on our ancestors’ X-chromosome.
This has an important consequence because it means that two of our photopsins, the ‘M’ and ‘L’ forms, are ‘sex-linked’. Men have only one X-chromosome and hence only one copy of each of these genes. If one of them is defective, the result is colour-blindness.
Apes and ‘Old World’ monkeys, along with howler monkeys in the New World, share our arrangement of ‘M’ and ‘L’ genes on the X-chromosome. Most New-World monkeys have instead only one X-carried photopsin. This can be either the ‘M’ or ‘L’ gene. A male monkey will inherit only one of these two gene forms (alleles). Accordingly their vision corresponds to one or the other forms of human X-linked colour-blindness. Females are also colour-blind if the alleles on both X chromosomes are the same, or have our trichromatic colour vision if they inherit different alleles.
Our eyes are preferentially sensitive to red. Babies usually learn to recognise this colour first, and begin to associate changes of skin tone with social meanings such as anger, calmness or fright. The bias in our neural circuitry for perceiving this information suggests that it has been important for our survival.
How have we evolved to see colour as we do, and why do we need to see in this way?
How do our eyes see ‘red’?
Colours do not exist.
Instead we create them in our mind by coding the relative signal strength between our three colour-perceptive channels.
The differences between L and M channels allow us to discriminate red from green. Blue-yellow distinctions are made by contrasting S cone signals with those from the M and L channels. In contrast, the two photopsins of non-primate mammals allow them to compare only L and S cone signals. As a result they see a more limited spectrum of colours.
The image on your computer screen is comprised of tiny phosphorescent dots emitting red, green or blue light. These emissions correspond to our three photopsins. Different signal intensities in these three channels allow us to see the colours of the rainbow, plus some additions. Magenta, for instance, is not part of the spectrum of white light. Instead we have ‘invented’ it in our mind. This shows us that the colours we ‘see’ are not ‘out there’. When we see red, we perceive signals we have learned to associate with this colour.
The simplest eyes work by looking for contrasts, but so too do our eyes. Our photoreceptor cells are ‘on’ in the dark, and light activation then switches them ‘off’. This change of signal is transmitted across the eye to the optic nerve through a neural network of cells making both direct and lateral connections.
♦ Without light, these cells are in a resting state, relaying their own direct ‘on’ signal whilst sending an ‘off’ signal to their neighbours. This lateral feedback maintains the eye’s neurons in a state of low activation.
♦ When a photoreceptor perceives light and switches its relay to ‘off’, the damping signal to neighbouring cells also shuts down. This enhances their ‘on’ status. Boosting their signal reinforces the contrast between them and the activated photoreceptor relay.
Increasing the contrast between ‘off’ and ‘on’ in adjacent cells sharpens and enhances the image. This neural processing happens before any information is relayed to the brain. It makes reds look more red, and also emphasizes the contrasts in patterns and textures.
Our eyes are often described as being ‘camera-like’. A camera however simply focuses a three dimensional view into a two dimensional film surface. In contrast, our unconscious mind directs where our eyes focus.
We automatically give more attention to certain details from the scene, such as other people’s faces and hands. Subsequently the eye processes the image, raising the ‘gain’ and reducing the ‘noise’. Eyes therefore process what we receive, and extract specific types of information from our vision.
What signals have we evolved to see?
Not surprisingly our brains and sensory apparatus are very like that of chimpanzees. This suggests that our common ancestors were selected for a similar specialization of the visual system to notice red colours, patterns, and faces.
Red has many meanings. Bright colourings, for example of some poisonous frogs, indicates danger, or in the case of ladybirds, unpalatability. It is also an indicator of the ripeness of some fruits. In primates colour also has a role in sexual selection: some species have genital regions or facial skin tones which change colour when they are ready to mate.
We easily detect colour contrasts (particularly reds) and iridescent sheens. Simulations of primate foraging using humans to compare the different types of primate vision show that our trichromatic sight enables us to easily find red, yellow and green fruits, and accurately assess their ripeness by colour. We are less well adapted however to judge the quality of dark (purple or black) fruits by their colour. This suggests that our ancestors may have specialized in foraging particularly for high-sugar red fruits.
Our trichromatic colour vision enables us to better see brightness and detail than dichromats (including colour blind people and most other mammals). This contributes to our ability to distinguish edges and textures. It is our neurology, combined with ancestry, however, that makes us so effective at picking out rhythmical visual patterns. Our perceptive system is particularly sensitive to diamond-shaped patterns and elongated objects – much like the scales and bodies of snakes.
In forest habitats, tree-dwelling snakes are not only frequently encountered but are major predators of wild primates. Anthropologist Lynne Isbell proposes that predation from snakes is a common hazard when foraging for high-sugar fruits in dense foliage. This may have selected our ancestors for their ability to recognize the scale patterns and movements of these reptiles.
This ability is, however, not just dependent on our eyes. Many of us have never experienced a snake in the wild, yet most have a strong fear of snakes. Studies in producing conditioned responses have found that it is much easier to train humans to fear snakes and spiders than other types of animal. Our clear predisposition for fearing these predators suggests that to do so may have been advantageous for the survival of our ancestors.
Information is relayed from the eyes to the visual cortex via the thalamus and limbic system; the centres of our emotional processing circuitry. Our thalamus and those of other primates has specialized cells called pulvinar neurons which respond selectively to visual images of snakes. These circuits elicit changes in our emotional state even fasthan our responses to faces and hands.
Our ability to spot small, nuanced movements means that we are also well equipped to notice subtle changes in skin tone and facial expressions. Such ‘facial gestures’ are important, along with vocal calls, for communicating emotion and other information between tribe members in chimpanzees and other primate species.
Early forms of human language may have arisen as an alternative means for our ancestors to bond and connect with each other using sound. We perceive far more information from speech sounds than can be discerned from the sound trace alone. Our ability to do this reveals that we recognize speech sounds as patterns of movement, and make sense of these patterns in the same way we ‘tune in’ to patterns in visual information.
Is your ‘red’ the same as mine?
We first learn what a particular colour ‘is like’ as young children. We learn to recognise ‘red’ as a set of relative strengths of signal arriving through our ‘S’, ‘M’ and ‘L’ channels. We later recognise red colours as being ‘like our prior experience of red’.
The visual signals we each receive may vary substantially. Colour blindness affects some ten percent of our population. In addition, inherited DNA variations in our photopsin genes can make subtle changes in how these pigments respond to light.
Our perceptions can change in different contexts. Various artists including Johannes Itten and Joseph Albers have experimented with how we see adjacent colours. Their work emphasizes the way our eyes increase the contrasts in what we see. For instance the same hue can be perceived as a quite different colour against a different background.
Our unconscious processes direct our gaze to focus more intently on the things that matter to us, and so the way we understand something affects how we look at it. The way in which our culture and language refers to colours therefore affects how we see them.
Words for colours are unlike most other adjectives, because they put artificial boundaries onto the natural spectrum of visible light. The placing of these boundaries varies between cultures.
For example, the Himba tribe of Namibia, whose language has only four colour words, use the same word for green and blue. They are slower to recognise contrasts between these hues than people whose language imposes separate categories onto this part of the spectrum.
All perceptions elicit changes in our emotional state, which alters our physiology. Colours affect our mood, and even our perceptions of time. We have gender-based differences as well as distinctly personal responses in how we see certain colours and hues. Since our individual learnings are unique, even if we share a similar cultural heritage, the way we look at what we see will differ in at least subtle ways. These factors ensure that no two people will see ‘red’ in quite the same way.
“If one says ‘red’ – the name of color – and there are fifty people listening, it can be expected that there will be fifty reds in their minds. And one can be sure that all these reds will be very different.”
- Our first use of seeing is social; as newborns we recognise and bond with our parents and social group.
- Our ability to recognise patterns makes us surprisingly effective at spotting snakes in dense undergrowth. It also assists us in decoding the subtle emotional cues readable in other’s faces and body language.
- Seeing colour is a social, emotional and perceptively biased process which enables us to respond to each other and to our world.
- We recognize faces so easily we even ‘invent’ them, for instance from otherwise unrelated lines and shapes. Face recognition is part of a general ability to recognize visual rhythm patterns of lines or movements.
- Colour vision, like other types of seeing, is learned during childhood. We learn to perceive contrasts. This allows us to understand details in what we see.
- Our colour vision, like that of other primates, is ‘trichromatic, detecting distinct wavelengths of light using three types of photopsin pigment. This level of detail in our colour awareness is not present in most other mammals, which see with a two-pigment ‘dichromatic’ colour vision. This is much like what is seen by a colour-blind human.
- Photopsins reside in ‘cone’ cells in our eyes. Neurons in the eye enhance contrasts, ‘sharpening’ the image we see before this information is relayed to the brain.
- Our visual processing of colour in the mind is learned, and varies between individuals and even between cultures.
- Human three-channel colour vision may have arisen through selection of our ancestors for the ability to assess red, yellow and green fruits for their ripeness and consequently sugar content.
Text copyright © 2015 Mags Leighton. All rights reserved.
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