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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, the same situation,  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. Cross section of the human eye

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 predictions about what is happening in the world being generated at the top end of the visual system whilst it 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 early 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

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The Visual Pathway

“Quote”

Colour is something we see every moment of our lives if we are conscious and exposed to light. Some people have limited colour vision and so rely more heavily on other senses – touch, hearing, taste and smell.

Colour is always there whether we are aware and pay attention to it or not. Colour is what human beings experience in the presence of light. It is important to be clear about this. Unless light strikes something, whether it is air, a substance like water, a physical object or the retina at the back of our eyes, light, as it travels through space, is invisible and so has no colour whatsoever. As suggested in the previous section, colour is an artefact of human vision, something that only exists for living things like ourselves. Seeing is a sensation that makes us aware of light and takes the form of colour.

The experience of colour is unmediated. This means that it is simply what we see and how the world appears. In normal circumstances, we feel little or nothing of what is going on as light enters our eyes. We have no awareness whatsoever of the chemical processes going on within photosensitive neurons or of electrical signals on their way to the brain. We know nothing of what goes on within our visual cortex when we register a yellow ball or a red house. The reality is, we rarely even notice when we blink! In terms of immediate present perception, colour is simply something that is here and now, it is an aspect of the world we see as life unfolds before us and is augmented by our other senses, as well as by words, thoughts and feelings etc.

It takes about 0.15 seconds from the moment light enters the human eye to conscious recognition of basic objects. What happens during this time is related to the visual pathway that can be traced from the inner surface of the eyeball to the brain and then into conscious experience. The route is formed from cellular tissue including chains of neurons some of which are photosensitive, with others tuned to fulfil related functions.

So, let’s start at the beginning!

Before light enters the eye and stimulates the visual system of a human observer it is often reflected off the surfaces of objects within the field of view. When this happens, unless the surface is mirror-like, it scatters in all directions and so only a small proportion travels directly towards the eyes. Some of the scattered light may illuminate the body or face of the observer or miss them completely. Some is reflected off the iris and enables us to see the colour of a person’s eyes. A little more is reflected off the retina – think of red-eye in flash photography.

Caption [Light rays from light source to retina showing refraction]

Caption [Eyeball with labels]

If we think of light in terms of rays, then some rays will be in line with the eyes of our observer as they look at an object. Rays that strike the outer surface of the eyeball directly in front of the pupil encounter various transparent media including the cornea, then the lens followed by vitreous humour, the gel that fills the eyeball. Then, they arrive at the retina.

Along an axis corresponding with the central line of vision, light enters perpendicular to the curvature of the cornea and travels straight towards the retina striking the fovea centralis at the centre of the macula where the sharpest image is formed. All the rays of light around this central line of vision change direction slightly because of refraction. The lens also affects their direction of travel as it adjusts in shape to ensure that as many rays as possible are focused exactly onto the retinal surface.

Visual Design – Colour Theory
http://www.dsource.in/course/visual-design-colour-theory/colour-perception-and-human-responses

The retina

Human beings see the world in colour because of the way their visual system processes light.  The retina contains light-sensitive receptors, rod and cone cells, that respond to light stimuli. It is the variety of wavelengths and intensities of light entering the eyes that produces the impression of colour.

The retina is the innermost, light-sensitive layer of tissue inside our eyes. It forms a sheet of tissue barely 200 micrometres (μm) thick, but its neural networks carry out almost unimaginably complicated feats of image processing.

The physiology of the eye results in a tiny, focused, two-dimensional image of the visual world being projected onto the retina’s surface. Because of the optics of lenses, it appears upside down and the wrong way around. But no worry, sorting that out is child’s play for the human brain! The real challenge is that the photosensitive receptors in the retina must produce precise chemical responses to light and translate every minute detail of the image into electrical impulse ready to be sent to the brain where they produce visual impressions of the world. In a very limited sense, the retina serves a similar function to a photosensitive chip in a camera.

As research continues to reveal ever-increasing amounts of detail about these signalling processes across and beyond the retina, it required new thinking, not only of the retina’s function but also of the mechanisms within the brain that shape these signals into behaviourally useful perceptions.

The retina consists of 60-plus distinct neuron-types, each of which plays a specialized role in turning variations in the patterns of wavelengths and intensities of light into visual information. Neurons are electrically excitable nerve cells that collect, process and transmit vast amounts of this information through both chemical and electrical signals. Retinal neurons work together to convert the signals produced by a hundred and twenty million rods and cones and send them along around one million fibres within the optic nerve of each eye to connections with higher brain functions. In this process rods and cones are first responders whilst ganglion cells are the final port of call before information leaves the retina.

There are three principal forms of processing that take place within the retina itself. The first organises the outputs of the rod and cone photoreceptors and begins to compose them into around 12 parallel information streams as they travel through bipolar cells. The second connects these streams to specific types of retinal ganglion cells. The third modulates the information using feedback from horizontal and amacrine cells to create the diverse encodings of the visual world that the retina transmits towards the brain.

As mentioned above, the image of the outside world focused on the retina is upside down and the wrong way around. But the human retina is also inverted in the sense that the light-sensitive rod and cone cells are not located on the surface where the image forms, but instead are embedded inside, where the retina attaches to the fabric of the eyeball. As a result, light striking the retina, passes through layers of other neurons (ganglion, bipolar cells etc.) and blood-carrying capillaries, before reaching the photoreceptors.

The overlying neural fibres do not significantly degrade vision in the inverted retina. The neurons are relatively transparent and accompanying Müller cells act as fibre-optic channels to transport photons directly to photoreceptors. However, some estimates suggest that overall, around 15% of all the light entering the eye is lost en-route to the retina. To counter this, the fovea centralis, at the centre of our field of vision, is free of rods and there are no blood vessels running through it, so optimising the level of detail where we need it most.

Caption [Retina close-up]

Retina
https://en.wikipedia.org/wiki/Retina

From retinal input to cortical processing and perception

Visual input is initially encoded in the retina as a two-dimensional distribution of light intensity, expressed as a function of position, wavelength and time in each of the two eyes. This retinal image is transferred to the visual cortex where primary sensory cues and, later, inferred attributes, are eventually computed (see figure). Parallel processing strategies are employed from the outset to overcome the constraints of the individual ganglion cell’s limited bandwidth and the anatomical bottleneck of the optic nerve.

Caption [Fovea centralis close-up]

Parallel Processing Strategies of the Primate Visual System.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2771435/
Adapted from DeYoe and Van Essen (1988).

Rods and cones

Both the photosensitive rods and cones form a regularly spaced mosaic of cells across the entirety of the retina – bar the absence of rods in the fovea centralis. Because there are 100 million rod receptors and 20 million cone receptors in each eye, rods are packed more densely per unit area. The synaptic connections of both rods and cones vary in function in different locations across the retina, reflecting the specialisations of different regions. This, for example, allows the eyes to deal with daylight and darkness and with what we see at the centre and periphery of our field of view.

Rods and cones are easily distinguished by their shape, from which they derive their names, the type of photopigment they contain and by distinct patterns of synaptic connections with the other neurons around them.

Neurons (nerve cells) are present throughout the human central and peripheral nervous systems and fall into three main categories: sensory, motor and interneurons. Rods and cones are both sensory neurons. Rods don’t produce as sharp an image as cone cells because they share more connections with other types of neurons. But a rod cell is believed to be sensitive enough to respond to a single photon of light whilst cone cells require tens to hundreds of photons to be activated.

The principal task of rod and cone cells alike is phototransduction. This refers to the type of sensory transduction that takes place in the visual system. It is the process of phototransduction that enables pigmented chemicals in the rods and cones to sense light and convert it into electrical signals. Many other types of sensory transduction occur elsewhere within the body enabling touch and hearing for example.

Caption

Functional Specialization of the Rod and Cone Systems
https://www.ncbi.nlm.nih.gov/books/NBK10850/

 

Trichromatic colour vision (Trichromacy)

Phototransduction by cone cell receptors is the physiological basis for trichromatic colour vision in humans. The fact that we see colour is, in the first instance, the result of interactions among the three types of cones, each of which responds with a bias towards its favoured wavelength within the visible spectrum. The result is that the L, M and S cone types respond best to light with long wavelengths (biased towards 560 nm), medium wavelengths (biased towards 530 nm), and short wavelengths (biased towards 420 nm) respectively.

Caption

Trivariance

The term trivariance is used to refer to this first stage of the trichromatic process. It refers to both the phototransductive response of the cone cells themselves and to the three separate channels used to convey their colour information forward to subsequent levels of neural processing.

Each channel conveys information about the response of one cone-type to both the wavelength of the incoming light it is tuned to and to its intensity. In both physiological and neurological terms this process is exclusively concerned with trivariance – three discernible differences in the overall composition of light entering the eye.

It is the separation of the signals produced on each channel that accounts for the ability of our eyes to respond to stimuli produced by additive mixtures of wavelengths corresponding with red, green and blue primary colours. But more of that later!

By way of summary, the rod and trivariant cone systems are composed of photoreceptors with connections to other cell types within the retina. Both specialize in different aspects of vision. The rod system is extremely sensitive to light but has a low spatial resolution. Conversely, the cone system is designed to function in stronger light. As a result, cones are relatively insensitive compared with rods but have a very high spatial resolution. It is this specialisation that results in the extraordinary detail, resolution and clarity of human vision.

Rod SystemCone System
High sensitivity, specialized for night visionLower sensitivity specialized for day vision
Saturate in daylightSaturate only in intense light
AchromaticChromatic, mediate colour vision
Low acuityHigh acuity
Not present in the central foveaConcentrated in the central fovea
Present in larger number than conesPresent in smaller number than rods

Caption

Retinal image

It is the cornea-lens system that determines where light falls on the surface of the retina which results in discernible images.

The images are inverted and obviously very small compared with the world outside that they resolve. The inversion poses no problem. Our brains are very flexible and even when tricked by prisms will always turn the world right-side-up given time. The reduction in size is part of the process by which the fit of the image on the retina determines our field of view.

The images are real in the sense that they are formed by the actual convergence of light rays onto the curved plane of the retina. Only real images of this kind provide the necessary stimulation of rod and cone cells necessary for human perception.

Caption

Fovea centralis

The entire surface of the retina contains nerve cells, but there is a small portion with a diameter of approximately 0.25 mm at the centre of the macula called the fovea centralis where the concentration of cones is greatest. This region is the optimal location for the formation of image detail. The eyes constantly rotate in their sockets to focus images of objects of interest as precisely as possible at this location.

Caption

Accommodation

The distance between the retina (the detector) and the cornea (the refractor) is fixed in the human eyeball. The eye must be able to alter the focal length of the lens in order to accurately focus images of both nearby and far away objects on the retinal surface. This is achieved by small muscles that alter the shape of the lens. The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance remains constant.

The ability of the eye to adjust its focal length is known as accommodation. The eye accommodates by assuming a lens shape that has a shorter focal length for nearby objects in which case the ciliary muscles squeeze the lens into a more convex shape. For distant objects, the ciliary muscles relax, and the lens adopts a flatter form with a longer focal length.

Caption

Bipolar cells

Bipolar cells, a type of neuron found in the retina of the human eye connect with other types of nerve cells via synapses. They act, directly or indirectly, as conduits through which to transmit signals from photoreceptors (rods and cones) to ganglion cells.

There are around 12 types of bipolar cells and each one functions as an integrating centre for a different parsing of information extracted from the photoreceptors. So, each type transmits a different analysis and interpretation of the information it has gathered.

The output of bipolar cells onto ganglion cells includes both the direct response of the bipolar cell to signals derived from phototransduction but also responses to those signals received indirectly from information provided by nearby amacrine cells that are also wired into the circuitry.

We might imagine one type of bipolar cell connecting directly from a cone to a ganglion cell that simply compares signals based on differences in wavelength. The ganglion cell might then use the information to determine whether a certain point is a scene is red or green.

Not all bipolar cells synapse directly with a single ganglion cell. Some channel information that is sampled by different sets of ganglion cells. Others terminate elsewhere within the complex lattices of interconnections within the retina so enabling them to carry packets of information to an array of different locations and cell types.

Caption

Amacrine cells

Amacrine cells interact with bipolar cells and/or ganglion cells. They are a type of interneuron that monitor and augment the stream of data through bipolar cells and also control and refine the response of ganglion cells and their subtypes.

Amacrine cells are in a central but inaccessible region of the retinal circuitry. Most are without tale-like axons. Whilst they clearly have multiple connections to other neurons around them, their precise inputs and outputs are difficult to trace. They are driven by and send feedback to the bipolar cells but also synapse on ganglion cells, and with each other.

Amacrine cells are known to serve narrowly task-specific visual functions including:

  • Efficient transmission of high-fidelity visual information with a good signal-to-noise ratio.
  • Maintaining the circadian rhythm, so keeping our lives tuned to the cycles of day and night and helping to govern our lives throughout the year.
  • Measuring the difference between the response of specific photoreceptors compared with surrounding cells (centre-surround antagonism) which enables edge detection and contrast enhancement.
  • Object motion detection which provides an ability to distinguish between the true motion of an object across the field of view and the motion of our eyes.

Centre-surround antagonism refers to the way retinal neurons organize their receptive fields.  The centre component is primed to measure the sum-total of signals received from a small number of cones directly connected to a bipolar cell. The surround component is primed to measure the sum of signals received from a much larger number of cones around the centre point. The two signals are then compared to find the degree to which they agree or disagree.

Caption

Horizontal cells

Horizontal cells are connected to rod and cone cells by synapses and are classed as laterally interconnecting neurons.

Horizontal cells help to integrate and regulate photoreceptor cells, cleaning up and globally adjusting signals passing through bipolar cells towards the regions containing ganglion cells.

An important function of horizontal cells is enabling the eye to adjust to both bright and dim light conditions. They achieve this by providing feedback to rod and cone photoreceptors about the average level of illumination falling onto specific regions of the retina.

If a scene contains objects that are much brighter than others, then horizontal cells are believed to prevent signals representing the brightest objects from dazzling the retina and degrading the overall quality of information.

Caption

The Neuronal Organization of the Retina Richard H. Masland
https://www.cell.com/neuron/fulltext/S0896-6273(12)00883-5?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0896627312008835%3Fshowall%3Dtrue

 

Ganglion cells

Retinal ganglion cells are located near the boundary between the retina and the central chamber containing vitreous humour.  They collect and process all the visual information gathered directly or indirectly from the forty-something types of rod, cone, bipolar, horizontal and amacrine cells and, once finished, transmit it via their axons towards higher visual centres within the brain.

The axons of ganglion cells form into the fibres of the optic nerve that synapse at the other end on the lateral geniculate nucleus. Axons take the form of long slender fibre-like projections of the cell body and typically conduct electrical impulses, often called action potentials, away from a neuron.

A single ganglion cell communicates with as few as five photoreceptors in the fovea at the centre of the macula. This produces images containing the maximum possible resolution of detail. At the extreme periphery of the retina, a single ganglion cell receives information from many thousands of photoreceptors.

Around twenty distinguishable functional types of ganglion cells resolve the information received from 120 million rods and cones into one million parallel streams of information about the world surveyed by a human observer in real-time throughout every day of their lives. They function to complete the construction of the foundations of visual experience by the retina, ordering the eyes response to light into the fundamental building blocks of vision.  Ganglion cells do the groundwork that enables retinal encodings to ultimately converge into a unified representation of the visual world.

Ganglion cells not only deal with colour information streaming in from rod and cone cells but also with the deductions, inferences, anticipatory functions and modifications suggested by bipolar, amacrine and horizontal cells. Their challenge, therefore, is to enable all this data to converge and to assemble it into high fidelity, redundancy-free, compressed and coded form that can continue to be handled within the available bandwidth and so the data-carrying capacity of the optic nerve.

It is not hard to imagine the kind of challenges they must deal with:

  • Information must feed into and support the distinct attributes of visual perception and be available to be resolved within the composition of our immediately present visual impressions whenever needed.
  • Information must correspond with the outstanding discriminatory capacities that enable the visual system to operate a palette that can include millions of perceivable variations in colour.
  • Information about the outside world must be able to be automatically cross-referenced, highly detailed, specifically relevant, spatial and temporally sequenced and available on demand.
  • Information must be subjectively orientated in a way that it is locked at an impeccable level of accurate detail to even our most insane intentions as we leap from rock to rock across a swollen river or dive from an aircraft wearing only a wingsuit and negotiate the topography of a mountainous landscape speeding past at 260km per hour.

It is now known that efficient transmission of colour information is achieved by a transformation of the initial three trivariant colour mechanisms of rods and cones into one achromatic and two chromatic channels. But another processing stage has now been recognised that dynamically readjusts the eye’s trivariant responses to meet criteria of efficient colour information management and to provide us with all the necessary contextualising details as we survey the world around us. Discussion of opponent-processing is dealt with in the next article!

Caption [Humans differentiate between 200 hues in the visible spectrum]

Müller cells

Müller glia, or Müller cells, are a type of retinal cell that serve as support cells for neurons, as other types of glial cells do.

An important role of Müller cells is to funnel light to the rod and cone photoreceptors from the outer surface of the retina to where the photoreceptors are located.

Other functions include maintaining the structural and functional stability of retinal cells. They regulate the extracellular environment, remove debris, provide electrical insulation of the photoreceptors and other neurons, and mechanical support for the fabric of the retina.

  • All glial cells (or simply glia), are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system.
  • Müller cells are the most common type of glial cell found in the retina. While their cell bodies are located in the inner nuclear layer of the retina, they span the entire retina.

Caption

Pigment epithelium

Pigment epithelium is a layer of cells at the boundary between the retina and the eyeball that nourish neurons within the retina. It is firmly attached to the underlying choroid is the connective tissue that forms the eyeball on one side but less firmly connected to retinal visual cells on the other.

Caption

Optic nerve

The optic nerve is the cable–like grouping of nerve fibres formed from the axons of ganglion cells that transmit visual information towards the lateral geniculate nucleus.

The optic nerve contains around a million fibres and transports the continuous stream of data that arrives from rods, cones and interneurons (bipolar, amacrine cells). The optic nerve is a parallel communication cable that enables every fibre to represent distinct information about the presence of light in each region of the visual field.

Caption

Optic chiasm

The optic chiasm is the part of the brain where the optic nerves partially cross. It is located at the bottom of the brain immediately below the hypothalamus.

The cross-over of optic nerve fibres at the optic chiasm allows the visual cortex to receive the same hemispheric visual field from both eyes. Superimposing and processing these monocular visual signals allows the visual cortex to generate binocular and stereoscopic vision.

So, the right visual cortex receives the temporal visual field of the left eye, and the nasal visual field of the right eye, which results in the right visual cortex producing a binocular image of the left hemispheric visual field. The net result of optic nerves crossing over at the optic chiasm is for the right cerebral hemisphere to sense and process left-hemispheric vision, and for the left cerebral hemisphere to sense and process right-hemispheric vision.

Caption [Hemispheric visual field diagram]

Lateral geniculate nucleus

The lateral geniculate nucleus is a relay centre on the visual pathway from the eyeball to the brain. It receives sensory input from the retina via the axons of ganglion cells.

The thalamus which houses the lateral geniculate nucleus is a small structure within the brain, located just above the brain stem between the cerebral cortex and the midbrain with extensive nerve connections to both.

The lateral geniculate nucleus is the central connection for the optic nerve to the occipital lobe of the brain, particularly the primary visual cortex.

Both the left and right hemispheres of the brain have a lateral geniculate nucleus.

There are three major cell types in the lateral geniculate nucleus which connect to three distinct types of ganglion cells:

  • P ganglion cells send axons to the parvocellular layer of the lateral geniculate nucleus.
  • M ganglion cells send axons to the magnocellular layer.
  • K ganglion cells send axons to a koniocellular layer.

The lateral geniculate nucleus specialises in calculations based on the information it receives from both the eyes and from the brain. Calculations include resolving temporal and spatial correlations between different inputs. This means that things can be organised in terms of the sequence of events over time and the spatial relationship of things within the overall field of view.

Some of the correlations deal with signals received from one eye but not the other. Some deal with the left and right semi-fields of view captured by both eyes. As a result, they help to produce a three-dimensional representation of the field of view of an observer.

  • The outputs of the lateral geniculate nucleus serve several functions. Some are directed towards the eyes, others are directed towards the brain.
  • A signal is provided to control the vergence of the two eyes so they converge at the principal plane of interest in object-space at any particular moment.
  • Computations within the lateral geniculate nucleus determine the position of every major element in object-space relative to the observer. The motion of the eyes enables a larger stereoscopic mapping of the visual field to be achieved.
  • A tag is provided for each major element in the central field of view of object-space. The accumulated tags are attached to the features in the merged visual fields and are forwarded to the primary visual cortex.
  • Another tag is provided for each major element in the visual field describing the velocity of the major elements based on changes in position over time. The velocity tags (particularly those associated with the peripheral field of view) are also used to determine the direction the organism is moving relative to object-space.

Caption

Optic radiation

The optic radiations are tracts formed from the axons of neurons located in the lateral geniculate nucleus and lead to areas within the primary visual cortex. There is an optic radiation on each side of the brain. They carry visual information through lower and upper divisions to their corresponding cerebral hemisphere.

Caption

Primary visual cortex

The visual cortex of the brain is part of the cerebral cortex and processes visual information. It is in the occipital lobe at the back of the head.

Visual information coming from the eyes goes through the lateral geniculate nucleus within the thalamus and then continues towards the point where it enters the brain. The point where the visual cortex receives sensory inputs is also the point where there is a vast expansion in the number of neurons.

Both cerebral hemispheres contain a visual cortex. The visual cortex in the left hemisphere receives signals from the right visual field, and the visual cortex in the right hemisphere receives signals from the left visual field.

Caption [Cerebral hmispheres, occipital lobes, primary visual cortex, optical radiations]