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An Introduction to Colour

As a living, conscious human being I struggle to make sense of the world. It often feels as if there is so much more than I understand and that I understand only a small part of what I see. Yet survival and the precise course of my life depends on getting this stuff right.

A STARTING POINT

A simple description of the challenge that all human beings face concerns the correspondence between organism and environment. In purely physical terms the boundary between the two follows the contours of our skin and there is a significant difference between the inside and outside until our lives come to an end.

The relationship between the living organism on the inside and the environment on the outside is mediated by the epidermis, where blood and tissue interface with air. Nerves, embedded in every square inch of the surface of our skin, enable tiny changes in the immediate environment to be sensed. Other nerves within the nose and mouth collect their own sensory impressions. Add to this a proprioceptive awareness of our own movements and sense of balance, then, taken together, these sensations provide an essential avenue to assembling our understanding of both ourselves and the world.

Obviously, there are two other organs evident on the surface of our bodies that vastly expand our relationship with the world outside. They are hearing and vision. Of these two it is vision that provides the principal focus on subsequent pages.

To add vision to the picture outlined so far involves our eyes, optic nerves and brain. All three can be counted as component parts of a single organ, a large part of which is safely embedded within our skulls, but with the eye-balls mounted as high up and as far forward as possible from where they provide a panoramic view of the world.

It may already be clear that this idea of human existence as a transactional relationship grounded in being-in and being-of the world in a purely physical sense is too simplistic. Our lives stretch far beyond the reach of our sensory organs. Our immediate circumstances run out into global networks, allowing us to engage in transactions worldwide. We can climb into machines that whisk us off to distant locations that our bodies alone could never reach. Connect the human mind to a microscope and we can see into the infinitesimally small world of neurons and synapses that power vision and conscious perception. When we do an internet search, or access libraries online, we unlock petabytes of knowledge of ourselves and the world accumulated over centuries.

But, in a very real sense, it is visual perception that provides the key to this array of perspectives on our very human condition. Vision brings our experience of the world into sharp relief and fills every corner with colour.

Light of different wavelengths enters the human eye. Visual perception enables us to see colour in the presence of light. (1)

With the thoughts outlined so far in mind, consider the following three points:

  • Colour sensations are always available to us whether we are aware or pay attention to them or not. Colour is what human beings see in the presence of light.
  • Colour is an artefact of human vision, something that only exists for living things like ourselves.
  • Seeing is a sensation produced by light and takes the form of colour. If human beings and related species were all to disappear overnight, the world would still be full of light but there would be no colour.

In the sections that follow, four closely related terms are introduced that help to build on the ideas introduced so far. They are visual perception, colour vision, the perception of colour and sense-making.

Caption. Wavelengths and colours (2).

ATTRIBUTES OF VISUAL PERCEPTION

Attributes of visual perception are the innate abilities and the skills we develop over the course of a lifetime that enable us to make sense of what we see. They are evident in the diverse properties of the world we see around us.

Innate attributes of visual perception associated with the response of the human eye and brain to light include:

  • Colour perception: The ability to see colour in the presence of light including all the greys between black and white.
  • Visual attention: The ability to focus on important visual information and filter out the rest.
  • Sensory processing: Accurate registration, interpretation and coordination of visual information alongside other forms of sensory stimulation.
  • Visual discrimination: The ability to recognise differences or similarities between objects based on size, colour, shape etc.
  • Spatial relationships: The ability to understand the relationships of objects, particularly their position, distance, and direction of movement relative to an observer.
  • Stereo vision: The ability to see the world in three dimensions.
  • Figure-ground: The ability to locate something and treat everything else as a background.
  • Form constancy: The ability to know that a form or shape is the same, even if it becomes larger, smaller or its orientation changes.
  • Visual closure: The ability to recognise a form or object when part of it is hidden or missing.
  • Visual memory: The ability to recall the outline and details of a view or object.
  • Visual sequential memory: The ability to recall a sequence of experiences in the correct order.


Caption (3)

COLOUR VISION

In terms of human experience, colour vision is the ability to distinguish objects according to the wavelengths and intensities of light they absorb, emit, reflect or transmit etc. The human eye and brain together translate light into colour.

  • Colour vision allows a human observer to distinguish objects by their colour.
  • Colours can be measured and quantified, but an observer’s perception of colour is first and foremost a subjective experience whereby the visual system responds to stimuli produced when incoming light reacts with chemicals inside the photosensitive rod and cone cells of the retina at the back of the eyeball.
  • In normal conditions, light is rarely of a single wavelength, so an observer is often exposed to a range of wavelengths in one area of the spectrum or a mixture of wavelengths from different areas of the spectrum.
  • In everyday life, colour vision includes chromatic and achromatic content. This means that an observer can distinguish between stimuli that appear coloured (chromatic) and others that appear to be without colour (achromatic) and so appear black, grey or white.
  • Different people may see the same object or light source in different ways. Factors that affect what we see include: where we are standing relative to an object, differences in eyesight (eg. colour blindness), previous experiences, expectations or interests.

Caption. (4)

PERCEIVED COLOUR

The perceived colour of an object, surface or area within the field of vision results from colour perception – an attribute of visual perception. First and foremost, perceived colour refers to what an observer sees in any given situation and so is a subjective experience.

  • It is the human ability to perceive and distinguish between colours that provides an important basis for the way that we sense and make sense of the world.
  • A distinction can be made between the physical properties of things in the world around us and how they appear to a human observer. So a small rock in a garden can be described in terms of physical properties but these don’t explain why a child sees a cat moving in the shadows.
  • When thinking about perceived colour, a distinction can be made between:
    • The properties of light.
    • The properties of objects.
    • What an observer perceives as a result of the attributes of visual perception.

Human beings can distinguish between stimuli that appear coloured (chromatic) and others that appear to be without colour (achromatic)

  • Perceived colour can be described by chromatic colour names such as pink, orange, brown, green, blue, purple, etc., or by achromatic colour names such as black, grey and white etc. Colour names can be qualified by adjectives such as dark, dim, light, bright etc.
  • Perceived colours consist of any combination of chromatic and achromatic content.
  • Perceived colour depends on the spectral distribution of a colour stimulus – the range and mixture of wavelengths and intensities of light that enter the eye.
  • Perceived colour depends on factors such as the size, shape and structure of all the objects in view, the composition and texture of their surfaces, their position and orientation in relation to one another, their location within the field of view of an observer and the direction of incident light.
  • Colour perception can be affected by the state of adaptation of an observer’s visual system. An example of this is when the photosensitive cells embedded in the retina become fatigued from long exposure to a strong colour and then produce an afterimage when we look away.
  • Perceived colour is influenced by factors such as an observer’s expectations, priorities, current activities, recollections and previous experience.
  • Perceived colour is defined in the International Lighting Vocabulary of the CIE (The International Commission on Illumination) as a characteristic of visual perception that can be described by attributes of hue, brightness (or lightness) and colourfulness (saturation or chroma) (CIE, 2011, 17-198).

SENSE MAKING

An important factor when considering visual perception is that as light enters our eyes it does not have any properties that allow it to carry information about the world of objects and the other things, we so easily recognise around us. The only type of information carried by light that our eyes can register is related to properties such as wavelength, frequency and intensity etc. Therefore, the sense-making process gathers nothing more from photosensitive cells in the retina other than flickering patterns of light.

Detail of the retina at the back of the eyeball. Notice that light must pass through other types of neurons before it reaches the rods and cones. (6)

  1. Ganglion cell. 2. Bipolar cell. 3. Amacrine cell. 4. Horizontal cell. 5. Cone cell. 6. Rod cell. 7. Pigmented cells.

But if this is the case then how do we make sense of the world? Let’s look at the basics of sense-making in more detail!

Most people are familiar with the idea that colours do not have an external objective existence. This understanding has a grounding in physics. Light is composed of energy at different wavelengths and our eyes respond to one small band of those wavelengths within the electromagnetic spectrum. Anatomical studies have in turn revealed the existence and function of the light receptors in the retina that respond to light.

So, there is no red out there in the world. What we call red is our visual system’s interpretation of what is out there. Our visual system constructs the experience of red from the data provided by our eyes. Despite all this, when I see a car, the fact that it is red is an indisputably accurate description of my observation. Somehow redness and car appear as one.

Neuroscience is currently trying to explain how this happens. What we know is that our visual system favours fast reaction times and rapid interpretation and there is nothing to be gained from the brain revealing its inner workings in the course of everyday experience. To the contrary, it specialises in providing us with just the information we need and in precisely the form we need it. We receive no information about how our eyes and brain gather or process information. The car just looks red and if we see a tiger then hopefully there is still time to run away as fast as we can!

A naïve view of sense-making

A basic lack of awareness of the act of seeing in favour of the immediate experience of the scenes and objects around us is the basis of naïve realism. From this perspective, perception simply produces a mirror of the world around us, though our attention may swing inward at a moment’s notice if we feel pain or have a disturbing thought. But what we see is not just an internal reflection of an external reality!

Caption. Naïve realism. What we see is just an internal reflection of an external reality

A BOTTOM-UP VIEW OF SENSE-MAKING

Investigations over the last two centuries have revealed a lot about sense-making. So let’s consider a bottom-up perspective first of all, and the idea that what an observer sees and understands about the world starts as light enters the eyes and ends with conscious perception.

The core idea is that light, in the form of waves (sometimes described as particles called photons) bounce off things in front of us and enter our eyes through the pupil. The lens then focuses the light on the retina at the back of the eye-ball where it forms an image. The retina contains photosensitive cells that produce chemical and then electrical signals in response to light. The signals go through further processing stages by other types of neurons including ganglion and bipolar cells. The output is then dispatched along the optic nerve to the visual cortex and related areas of the brain.

This view is supported by anatomical descriptions of the visual system showing connections going towards the eye from the brain controlling things like eye movement, vergence (cross-eyes when looking at objects close-up), focus and blinking but that there are vastly greater numbers of connections going in the other direction towards the brain.

Caption. Eye to visual cortex

Today, sense-making is generally understood to develop stage by stage as signals are transmitted through the visual system. Different facets of perceptions of a recognisable world including colour, shape, depth, stereo vision and movement are all constructed progressively en-route, enabling us to compose pictures which integrate local details and global features of a scene into a comprehensible view of the world.

A TOP-DOWN VIEW OF SENSE-MAKING

Now let’s consider a top-down view of the same sense-making process. This suggests that the chemical and electrical processes resulting from light stimulating the eyes occur simultaneously with other types of neurological activity within the brain. From this perspective, conscious perceptions are as much to do with brain activity as they are to do with raw information gathered by the eyes.

An important consideration here is that in view of the complex of eye-brain connections mentioned above, it is a mistake to think of our eyeballs as a separate organ or functioning independently from the rest of the visual system. Eye-balls are literally extensions of the brain, mounted remotely from the core of the visual system, but directly connected by great ribbons of neurons linking the retina at one end and the visual cortex at the other.

This leads to the notion that perception and sense-making depend not only on information derived from light entering our eyes but also from a complex interplay of processes that originate in our brains. In this case, perception is not just a question of what we see with our eyes but the fact that the brain has its own ideas about what is going on. In this sense, different kinds of perception are like different kinds of hypothesising.

Caption. Eye to visual cortex

The implications are that the activities of the visual system are as much about mental processes at higher levels as about raw visual information coming up the optic nerve. This comes down to the idea that the visual system is trying to imagine what is out there and what is going on. Depending on circumstances, out there might mean in the distance, inside my room, inside my shoe or inside my stomach!

A top-down view, therefore, involves signals carrying predictions about what is happening in the world being generated at the top end of the visual process whilst the brain also tries to make sense of what is causing sensory data at the bottom end. It is a meeting of many types of processing out of which visual experience is constructed. What we see is the result of the visual system’s best guess about what is causing sensory data and its predictions about what will happen next.

AN INTEGRATED VIEW OF SENSE-MAKING

If the bottom-up and top-down perspectives are combined a third option emerges that gets away from an overly physiological or hierarchical ordering of the visual system and opens ways of thinking about sense-making grounded in our bodies as they actively live, learn and act in the world.

It is clear, for example, that during childhood we begin to become familiar with our surroundings, and as that process develops we become more efficient at making sense of it. As time goes on, it involves less effort to recognise features and so the more quickly we apply that familiarity next time around.

How we see objects and extract meaning from a scene may depend on what we are doing with the things before us and whether we are carrying out a familiar task. In another case, faced with something unfamiliar, we may scan an array of barely recognisable objects and ask ourselves questions about what things are and whether they relate to the task at hand. Riding a bicycle might provide a good example in the first case whilst lifting the bonnet of a car for the first time to check the oil could apply in the second.

If we take all this one step further, then sense-making depends heavily on imagining the world we see. Imagination, anticipation, inference and hallucination are all part and parcel of trying to see things. We can’t do the act of seeing without imagining. As a result, we usually get it right, but sometimes we do get it wrong.

Matching mental assumptions about the world with the information simultaneously processed by the retina is clearly something that has evolved over millions of years. Given the benefits of trial and error over that time, we can be reasonably confident that the endurance of our species indicates that the match between the two is often spot on.

It is particularly comforting to see how quickly mistakes like seeing those cats that turn out to be rocks are rectified. At the other end of the scale when paranoia, delusions, fear, conspiracy theories or over-active imaginations prevail, it reflects the degree to which the match can slip out of kilter even for a reasonably well-adjusted personality.

We are constantly checking our immediate needs, our hopes and imaginations against information gathered by the retina. But some people clearly have problems accurately perceiving the world around them. This does not necessarily involve mistaking objects but can take place as moods and emotions intertwine with our objectively perceived view of the world.

Take for example the effect of something as simple as a pain-killer for a headache, a hot drink after a tiresome day, or a substantial meal when physically overtired. The rhythms and shifts that affect every organ in our body impacts on how we see the world.

Then there are situations where we close our eyes whilst listening to music or begin an imaginative activity. By suppressing the generation of information from the eyes we can stimulate a creative process still packed with images that are quite apart from the ordinary features of everyday affairs.

These perspectives fit with contemporary descriptions of the visual system that reject a simple ordering of different components, of processing steps and the idea of narrow areas of specialisation within and around the visual cortex. They suggest instead the idea of myriads of links and relays between neurons throughout the visual system interconnecting the diverse dimensions of what we experience directly as conscious perceptions. This approach recognises the brain’s role as being fluid and adaptable to specific circumstances with a dynamic and synergistic role in constructing our visual experience as a perceptual whole. This, in turn, contributes to what it means to be a conscious living member of humanity embedded in ecosystems which have a 3.8-billion-year history but at the same time needing to accurately resolve whether it is safe to cross the road.

WHO’S EYES ARE THESE ANYWAY?

Then finally, before finishing this section there is the question of self-perception! Who exactly is the person that seemingly lives behind my eyes? Who is it that lives behind any other pairs of eyes I look at during the day? I can talk about myself. I can say that behind each pair of eyes is a separate self. But what is a self?

One point of view within contemporary philosophy suggests that there is always someone having the experience – someone consciously experiencing themselves as directed toward the world, as a self in the act of attending, knowing, desiring, willing, and acting. This view suggests that we have an integrated inner-image of ourselves that is firmly anchored in our feelings, bodily sensations and perceptions, that enable the experience of a point of view. This approach recognises however that there is no little person running things inside our head. (Metzinger, 2010, pp. 7-8)

aption. Eye to visual cortex.

The problem of identifying a self was recognised by another philosopher, David Hume, more than two hundred and fifty years ago in his book A Treatise of Human Nature:

“When I enter most intimately into what I call myself, I always stumble on some particular perception or other, of heat or cold, light or shade, love or hatred, pain or pleasure. I never catch myself at any time without a perception, and never can observe anything but the perception”. (Hume, 2015, p. 254)

The idea of self, along with that of being a subject who can, for example, see objects, is therefore not quite straightforward. What do I mean when I say that I am just being myself. There is obviously more to it than the fact that I can only pretend to be someone else!

https://www.theguardian.com/science/audio/2018/apr/02/a-neuroscientist-explains-where-perception-ends-and-hallucination-begins-podcast