Disorders of vision in children: a guide for teachers and carers
CHAPTER ONE: What is vision and how do we see?
- 1.1 The eye
- 1.2 The seeing nerves
- 1.3 The seeing brain
- 1.4 The brain connections involved
- 1.5 Colour confusion
The fact that 40% of the human brain is devoted to processing visual information shows the complexity and importance of this remarkable sense. Such complexity means that vision is vulnerable to a wide variety of disorders giving rise to a wide range of different effects with many different implications for the child and his or her education. A basic understanding of the structures involved in seeing. from the eye through to the brain is therefore essential to understanding and dealing with the range of problems which children with various visual impairments may experience.
A diagram of the eye is shown in Figure 1.1. The front surface of the eye, the cornea, is a transparent, smoothly curved surface for focusing light into the eye (the cornea provides most of the eye’s focusing power). The surface is kept smooth and in good condition by tears which are spread over the surface by normal blinking. Loss of transparency due to scarring is a common cause of visual impairment in children in developing countries (often being related to measles and malnutrition, Plate 1) but is rare in more affluent countries. A scarred or white cornea can also cause a cosmetic problem for the child. The cornea is continuous with the white of the eye or sclera which makes up the rest of the external wall of the eye (see Figure 1.1).
Plate 1 Cosmetically noticeable and visual impairing corneal scars of
both eyes caused by Vitamin A deficiency
The rest of the eye's focusing power is provided by the lens inside the eye which is also transparent and contributes the variable focus necessary for the eye to be able to see objects both in the distance and close up. This variability is possible because the lens is elastic and is suspended from the ciliary muscle by ligaments or zonules.
Contraction of this muscle allows the lens to become a thicker and rounder shape with the stronger focusing power necessary for seeing near objects. This process is known as accommodation (Figure 1.2). Loss of transparency in the lens can be due to a wide variety of causes and is called cataract. Although it is much more common in older people. it is also a major cause of visual impairment in children worldwide.
Figure 1.1 -Diagram of the eyeball from above
Figure 1.2 Accommodation. The solid lines represent the shape of the lens, iris and ciliary body at rest, and the dotted lines represent the shape during accommodation.
The fluid of the eye
Figure 1.1 shows that between the cornea and the lens is the anterior chamber which is filled with a transparent watery fluid, aqueous humour. This is continually pumped into the eye by the ciliary body and is continually drained out through a channel in the cleft (or angle) between the cornea and iris. The function of the aqueous humour is to provide oxygen and nutrients to the inside of the cornea and to the lens. In most other parts of the body this function is performed by blood but blood is red and the requirement for transparency of the eye has led to the production of this alternative to blood. The balance between the inflow and outflow of aqueous humour is responsible for the pressure of the eye. If the ciliary body fails to produce aqueous humour, the eye has no pressure and collapses, losing its shape (phthisis); if the outflow is blocked then the pressure inside the eye rises, which can cause glaucoma. This can damage vision and in children (whose sclerae are more elastic than adults') can cause enlargement of the eye known as 'buphthalmos' (from the ancient Greek meaning 'the eye of an ox').
The iris and pupil
The iris is the thin layer of muscle situated just in front of the lens and is the visible coloured part of the eye. It contains a central opening, the pupil. which varies in size according to the light level. In dim light the pupil gets bigger to maximise the amount of light entering the eye and in bright conditions it gets smaller to limit light entry. The main function of the pupil however, is to allow some degree of simultaneous focusing on targets at different distances away, in a similar way that reducing the aperture of a camera allows both the foreground and the background of a photograph to appear in focus. There are some conditions in which the iris is either absent (aniridia) or lacking pigment (albinism) and this control mechanism is therefore deficient.
Between the lens and the retina lies a transparent gel, the vitreous, occupying two thirds of the volume of the eye. Its transparency allows focused light to pass through to the retina.
The function of all the structures outlined above is to produce a clear and focused image on the retina, which is the 'screen' of the eye, and is analogous to the film in a camera (see Chapter 2 for more details of this optical arrangement). Plate 2 illustrates the view of the retina seen by an ophthalmologist or optometrist looking into the eye with an ophthalmoscope (the instrument used to examine the retina, consisting of a torch and lens combination). The major landmarks are the optic disc which is the exit of the optic nerve which will be described later, and the macula which is the area about 4mm across and which includes the fovea (the most important point in the retina for detailed central vision) at its centre.
Plate 2 Fundus
The retina is divided into two layers, the inner 'nerve' or neural layer and the outer pigment layer. The neural retina contains the cells (called rods and cones, or photoreceptors) which convert light into electrical signals which are eventually interpreted by the brain as a visual image (Figure 1.3). This process depends on the presence in the rods and cones of substances (called photopigments) which change the shape of their molecules when they absorb light energy. (These substances consist of a protein molecule linked to another molecule closely related to vitamin A (retinene).) The shape change in these molecules results in an electrical signal being sent to the optic nerve and then to the brain for interpretation. Thus there is a conversion of energy forms from the light coming into the eye, through chemical changes in the receptors, to electrical signals generated in the nerve fibres to the brain.
Figure 1.3 Converting light
There are two different types of photoreceptor in the retina, rods and cones. It is important to appreciate the difference between rods and cones not only because they are responsible for quite different aspects of vision but also because some of the commonest causes of visual impairment (eg; retinitis pigmentosa and cone dystrophies) may predominantly affect either the rods or the cones with very different visual implications for the child.
The rods are mainly responsible for vision in dim light and produce images consisting of varying shades of black and white, while the cones work in bright light, detect fine detail and are responsible for colour vision. The density of the rods and cones varies in different parts of the retina. The rods are absent at the fovea, the area about 2mm across responsible for discerning fine detail (Plate 2). The cones on the other hand are most dense at the fovea. Both rods and cones contain photopigments (light-sensitive molecules), but whereas rods all contain the same photopigment, the cones are of three different types, each type containing a different photopigment. One group of cones responds especially to blue light, one group to red and one to green light. Hence the rods cannot respond to colour (we can't see colours in the dark) but the cones do.
Light and dark adaptation:
The eye has the ability to respond to a remarkably large range of different brightnesses. If you spend a considerable time in brightly coloured surroundings and then move to a dimly lit environment, the retinas become slowly more sensitive to dim light as you become accustomed to the dark. This is called dark adaptation and is nearly complete in about twenty minutes. On the other hand when you pass suddenly from a dim to a brightly lit environment, the light seems intensely and even uncomfortably bright until the eyes adapt to the increased illumination. This adaptation occurs over about five minutes and is called light adaptation. The cones and rods undergo dark adaptation separately, the cones adapting more quickly to the dark, but to a lesser degree than the rods which are more important for night vision.
Central and peripheral vision
If a straight line is drawn between the object being looked at and the eye, through the centre of its cornea, it would hit the retina at the fovea. It is the fovea where the image of the object being looked at directly is focused (Figure 1.4).
Figure 1.4 Normal focus
Other objects at increasing distances to the side of the object being looked at are focused onto other areas of the retina at increasing distances from the fovea. This means that the part of the retina with the greatest resolving power for fine detail (the fovea) is directed at the object of interest or fixation point. The maximum resolving power of the eye (that is, the finest detail which can be seen) is an important measurement of visual function and is termed visual acuity. Visual acuity is the most frequently recorded measurement of visual function since the ability to make out or resolve fine detail concerning the object of regard is essential to many of the visual tasks that we perform, particularly in the school environment, such as reading and writing. If you imagine looking at a single object in the middle of a room, there is a visual impression of the rest of the room, though it is not seen in fine detail. This is called our 'peripheral field', or 'field of vision' and is received by the receptors outside the macula. It is very important to our perception of the world around us and enables us to navigate through it. This peripheral vision can be measured by mapping out what is called the visual field, which simply means the total area that can be seen by one or both eyes in any particular position. The outer visual field is very sensitive to movement. When you move through a crowded environment you don't bump into things, even though you do not look directly at them. Similarly, when you run up a flight of stairs you don't usually look at each step directly. The visual field gives us remarkable sensitivity to movement which accounts for the tremendous mobility that some children have despite profound loss of central vision. (When a child is moving through a room, stationary objects are moving in visual terms.) More detailed explanations and implications of the terms visual acuity and visual field, together with some details of their measurement are given in Chapter 2. The outer or pigment layer of the retina consists of a single layer of cells with numerous functions including the absorption of stray light, and the formation (from vitamin A) of the photopigments needed by the rods and cones.
Between the pigment layer of the retina and the sclera is the 'choroid'. This is another pigmented layer which is well supplied with blood vessels (unlike many other parts of the eye which need to be transparent) and which is joined to the iris and ciliary body, all three together being termed the 'uvea' (see Figure 1.1). Its main function is the nourishment of parts of the eye, especially the retina. The retinal blood vessels shown in Plate 2 only nourish the inner part of the retina; the rest is supplied by the choroid. If the retina becomes separated from the choroid, as happens in retinal detachment, then it can suffer permanent damage due to oxygen starvation (similar to the brain damage which results from a stroke). If the macula becomes detached, the central vision will be affected, but if the detachment does not involve the macula, then only an area of peripheral vision, corresponding to the area of detached retina, will be impaired.
The seeing nerves
Electrical signals from the rods and cones are transmitted through other cells in the neural retina to the ganglion cells. These are nerve cells with long fibres which pass from the cell through the optic disc (where all the fibres converge from the other ganglion cells from different positions throughout the retina) and into the brain. Together these fibres form the optic nerve which leaves the eye at the optic disc (see Plate 2). All visual information from the retina leaves the eye through the optic nerve and hence the optic disc is a very important structure. Approximately 1.2 million nerve fibres are found in each optic nerve and when the two eyes are taken in combination this is estimated to contribute about 40% of all input to the brain! Unfortunately the nerves are susceptible to damage from many causes, for example, raised intra-ocular pressure, which can cause permanent visual field loss. This combination of damage is called glaucoma.
The route taken by the visual signal once it leaves each eye on its way to the visual parts of the brain is shown in Figure 1.5a. The optic nerve passes back to the brain through the bony cavity in the skull which encloses the eyeball (the orbit). In the skull it meets the optic nerve from the other eye at what is called the optic chiasm or crossover point. From there the visual message is transmitted further into the brain by connecting pathways called optic tracts. In general the right side of the brain responds to and controls things on the left side of the body whereas the left side of the brain correspondingly responds to and controls things on the right side of the body (thus a stroke affecting the right side of the brain may cause a left sided paralysis). This organisation of the brain applies to vision as well so that although each eye sees objects in both sides of the visual field, at the crossover all the information from the right side of the visual field (from both eyes) is passed to the left side of the brain and all information from the left side of the visual field (from both eyes) is passed to the right side of the brain (see Figure 1.5a). Thus the crossover is very important when considering visual impairment because any damage to the left eye or optic nerve still leaves a full visual field from the right eye (which you can simulate by closing the left eye) but any damage to the left side of the pathway behind the chiasm may interfere with all vision from the right side of the visual field of each eye (a right hemianopia, meaning half blind on the right side) and likewise any damage to the right side of the pathway behind the chiasm may produce a left hemianopia (see Plate 15).
Figure 1.5a The visual pathways from the eyes to the back of the brain
The seeing brain
From the chiasm the visual signal travels in nerve bundles (called optic tracts) to the back of the brain to a region called the visual cortex. The cortex consists of the surface layers of the brain and is the site of what is called higher brain function (eg; conscious visual perceptions, or deliberate decision making). The size of the cortex is much bigger than in most animal brains, corresponding to the advanced intellectual function of humans. It is the cortex where the visual signals are translated through increasingly complex interconnections between brain cells (only a few of which are beginning to be understood) into our amazingly detailed and colourful perceptions of the visual world.
Figure 1.5b Side on view of visual pathways
The first area of the cortex involved in processing the visual image is situated at the back of the brain (see Figures 1.5 and 1.6) and is called the primary visual cortex. The first cells in the visual pathway which respond to input from both eyes are situated here and they give rise to the ability to join pictures from both eyes (binocular vision) and perception of depth. The visual system is exquisitely organised. If you look, for example, at someone's nose, then at the same time your peripheral vision is good enough to register a single left eyebrow hair as a signal in each eye. These two signals are carried to exactly the same cell at the back of the brain in the visual cortex.
Although the majority of visual information does arrive at the cortex of the brain by the route described, there are alternative quicker routes which bypass the cortex and which are important for quick visual reflexes. The way our pupils react instantly to bright light is an obvious example but there may be other pathways which allow reflex responses to visual stimuli. Birds, for instance, flying through trees at high speed require very quick visual responses to avoid crashing. This unconscious awareness (conscious vision only occurs in the cortex) of certain objects, moving or stationary, is present to a more limited degree in humans and is manifested as so-called 'blindsight' (unconscious visual awareness of surroundings) in people with damage to the visual cortex.
Seeing contrast and movement The brain is arranged in such a way that the electrical signals coming from the eyes are broken down into the components of the picture. The mind detects edges and boundaries, and their shape and orientation. The picture is built up in the mind from its parts.
The imagery is then compared with pictures which have been 'stored' from the past. If the picture which is seen matches with the memory store it is recognised. If it is not recognised then the object is explored with as many senses as possible and a cross-referenced memory map across the different senses is created. This 'cognition' takes place ready for future recognition.
The brain connections involved
Edges and borders are what help define shapes and contours in our three dimensional world. The brain is able to 'see' such contrasts by the complex reactions of cells in the retina to light reflected from objects. When light is coming in from a flat surface or uniform colour, all the receptor cells of the retina are stimulated, and send corresponding signals to the visual cortex of the brain, where the signals are translated into the visual images which we 'see'. If the cells are stimulated by an edge, that is, a border between a lighter shade and a darker one, a stronger signal than that resulting from the uniform stimulus is sent to the visual cortex to be processed. Further levels of complexity of processing mean that some cells respond selectively to vertical and some to horizontal edges, and various others to different orientations between the vertical and horizontal. Other cells (on-off cells) do not respond to a constant light stimulus but only respond if the light is switched on or off. Groups of these cells may be wired to another cell (a movement cell) further along the line of processing, in such a way that a light source moving in a particular direction will sequentially stimulate the on-off cells, and this will be registered as movement by that next cell, the movement cell.
We can therefore see that there is a progression of increasing complexity along the chains of brain cells which begin with the receptors being stimulated by uniform light, but more strongly stimulated by edges and borders. Further along the chain are cells which are stimulated by changes in light, (eg on-off or movement) and it is hypothesised by scientists that as the connections between the cells become increasingly intricate, so some cells will eventually learn to respond to a particular image eg a familiar face. However, although this represents a considerable recent advance in the understanding of the processing of visual information by the brain, the fundamental process by which electrical signals in such cells are ultimately interpreted as the images of the visual world by the 'mind's eye' remains an enigma.
Light is the part of the electromagnetic spectrum which people happen to have receptors for, and it is our minds which are responsible for imparting the concept of colour for each wavelength of light. In the retina we have three types of cell (or cones) each of which absorbs different components of the light coming in. One type of cone mainly absorbs green, one red and one blue. The colour we see depends on how much each is stimulated by the incoming light. It is no coincidence that the colours of the dots or pixels on a TV screen are matched to the colours seen by our receptors (red, green and blue). Just as any colour can be created on the screen from these pixels, colours can be created in the mind by the signals from the three cone types in our retina. If one of the cone types (usually red or green) is deficient there may be difficulty in distinguishing reds from greens. This is the cause of impaired colour vision in about one in twelve to one in fifteen men.
A separate area at the back of the brain is responsible for processing colours and for 'setting' the colour balance, so that for all types of lighting, from the pink of the dawn to the black of a thunderstorm, the colour of grass for instance remains green to the mind's eye because the brain 'knows' that grass is green. This part of the brain can occasionally be damaged, for example by carbon monoxide poisoning. People with such damage see the world clearly but in black and white, like watching black and white TV.
The nature of colour
Imagine a grey ball. Above it is a white ball and below, a black ball, and between them there are balls of all shades of grey arranged in order. The central grey is often used as a base paint in shops supplying paint. If you add red the grey will gradually become redder until it can't become any more red and the hue of red gradually becomes more red until it is fully saturated as a primary colour. (You can't get redder than red.) If you were to do the same with each of the colours you can imagine a circle around the central grey in which red blends into orange, orange into yellow, yellow into green, green into blue and blue through purple back to red again. This is called the colour circle with inner (imaginary) circles of equivalent, washed-out looking desaturated hues. If you now choose to add white or black to say your red hue, you would create either pink (a tint) or brown (a shade) which would ultimately blend into light pink through to white or through dark brown into black. This is what is known as colour contrast. The greater the degree of 'blackness' or 'whiteness' present within adjacent colours, the greater the amount of contrast between them and the clearer the boundary between the two colours.
The way that colour vision is coded in the brain means although we can mix certain colours to see for instance a reddish blue (magenta) or a reddish yellow (orange), there are certain colours that cannot be mixed in this way but instead tend to contrast starkly with each other across a colour border. For instance, we can never see a reddish green or a bluish yellow, but yellow writing shows up clearly against a blue background and is a good combination to use in visual aids because of this.
Dark shades absorb light (and get warmer because the energy is converted into heat) and light tints reflect light and feel cooler, in bright sunlight in particular, as less light is absorbed. A blind person can therefore feel a dark colour as truly being warmer to the touch than a light colour when these colours are brightly lit.
Learning to use vision
Increasingly complicated levels of processing and coding of various aspects of an image take place throughout the visual pathways (eg; from retina to primary visual cortex and from one area of the visual cortex to another). The primary visual cortex is the first part of the cortex to receive the image and it sends messages to numerous other areas of the cortex (sometimes known as visual association areas) where not only does more complex analysis of the visual information take place but this information can be combined with other sensory information (such as that from touch and hearing), and can be stored in memory so that it becomes the basis for future recognition, imagination and dreams. As visual information is continually arriving, it is compared and matched with the information already stored in the memory bank. A pattern match leads to recognition and reinforcement. If the incoming information has not been met before and has not been stored in the memory bank, no match can occur. The information will then be learned, so that if met again, it will be recognised. Repeated viewing enhances recall but lack of such reinforcement makes subsequent recognition more difficult. Damage to the brain responsible for this highly complex system of information storage, retrieval and matching can therefore cause impairment of vision at an intellectual level. Children with multiple disabilities due to brain damage may show evidence of a wide range of such intellectual or 'cognitive' visual disorders, many of which remain to be classified and understood, but this should not prevent us trom having a very open mind when caring tor children with problems of this nature.
(often inaccurately labelled colour blindness - Plate 3)
Some people are completely lacking in one of the three types of cone systems. Although their world does not appear black and white they do experience a smaller variety of colours. These people are called dichromats (meaning 'two colours'). A more common type of colour confusion is called anomalous colour vision which means that although such people require mixtures of three coloured lights to make their colours. they use different proportions and therefore see colours differently from those with normal colour vision. Two colours matched as identical by someone with normal colour vision may also be seen as identical by a dichromat but may look different to someone with anomalous colour vision.
Plate 3 Colour confusion
Red-green colour confusion is surprisingly common with around eight per cent of men markedly deficient. though it is extremely rare in women because it is inherited in an X-linked fashion (see Glossary). Anomalous (incomplete) red-green confusion is more common (6%) than dichromatic (complete) red-green confusion (2%). The Ishihara colour plates (to be discussed more fully in Chapter 2) were designed to test for and distinguish the specific types of inherited colour confusions mentioned above. These inherited forms of colour confusion do not constitute visual impairment and studies have shown that children are not adversely affected by it in educational terms.
Colour vision defects can also be caused by a variety of diseases that damage the retina, optic nerve or visual cortex. Although such deficiencies do not fall into the neat colour categories above (because. for example all three types of cone might be affected) there is a tendency for macular disorders to cause more blue-yellow confusion and for optic nerve disorders to cause more red-green confusion. With increasing severity of the disease the defect tends to affect all types of colour discrimination.
Figure 1.6 Specialised areas of the brain
Figure 1.6 shows some examples of specialised areas of visual cortex (sometimes called visual association cortex) including those with particular responsibility for the processing of motion and face and shape recognition. These areas are only beginning to be recognised and understood. Some of these types of processing occur on both sides of the brain (eg colour and motion perception) whereas other types tend to occur predominantly on one side. For example, damage to the left side of the brain may lead to poor appreciation of internal detail of an object. whereas damage to the right side may lead to poor appreciation of the outline and orientation of an object. The left side of the brain seems to be important for reading and recognising shapes, and the right side of the brain seems to be important for recognising faces, and for the ability to work out where we are and to find our way and to know where we have put things.
Another example of higher visual function processing involves speech. In most right-handed people speech is processed in the left side of the brain. There is one (sensory) centre where the brain receives and processes speech and language information and another (motor) centre where our own speech originates (see Figure 1.7). The sensory speech area is situated close to the visual association cortex (as well as to the corresponding auditory association cortex). Different parts of visual association cortex process the visual information about mouth movements and facial expressions of a person who is speaking, or about written words as they are read, and they send this information to the nearby sensory speech and language area.
Figure 1.7 Diagram showing the probably nervous pathways involved in
reading a sentence and repeating it aloud.
A similar process operates for the corresponding auditory association areas, where the sounds of spoken words are processed. The sensory speech area having received all this information, then integrates and interprets it and sends signals on through a bundle of nerve fibres to the motor centre where corresponding speech is initiated. Visually impaired children lack the visual experience of facial expressions and mouth movements which accompany various spoken words and therefore their whole concept of the meaning of words and the way in which they learn such meaning is affected, making it important to use 'larger than life' expressions, gestures and signs and body language. Such stimulation needs of course to be within the measured capacity of the child to see, which highlights the importance of regular assessment of the functional vision and working well within the child's limits.
In the future, increasing knowledge of the specialised functions of different areas of the brain will no doubt aid our understanding of the particular learning difficulties encountered by children with brain damage in these areas.
We have said that for visual perception to become meaningful, images must be compared with those previously experienced. In order for visual (or indeed any sensory) memory to be laid down there must be an element of 'plasticity' of the connections between brain cells throughout life. What this means is that not all the wiring patterns between brain cells are established at birth, but rather that these connections are established and then reinforced by repeated stimulation during early life. Thus the process of learning through repetition can be thought of as giving rise to the laying down of wiring patterns between sets of brain cells. This process is maximal during childhood but continues into old age. A simple example of the importance of this in early life is found by returning to visual cortical cells. Certain of these cells each respond to a line of a particular orientation.
It has been shown that if a kitten is reared in an environment of vertical lines only, then no brain cells will develop which will respond to edges in other orientations. If the kitten is reared in complete darkness then this response to edges may fail to develop completely. Such sensitivity of the brain to visual disturbance is known to be greatest in young children, and if the young child's visual system is deprived of well focused images, then permanent visual impairment may develop at the level of the visual cortex, even if the focusing problem is later resolved (hence the importance of diagnosing and treating all causes of treatable visual impairment at an early age). This condition in which the brain does not develop the capacity to see well owing to the poor quality of the pictures presented to it during early life is called deprivational amblyopia.
The implication of this plasticity of the young visual system is that every effort should be made to stimulate vision fully from an early age. This will mean different strategies for different children. Those children with cataracts, for instance, may need surgery at a very young age and then encouragement to wear their glasses or contact lenses as much of the time as possible; children with one damaged eye and one healthy eye may need the healthy eye to be patched for a number of hours every day to encourage development of the pathways from the damaged eye; and visually impaired children for whom no surgical or optical intervention may be possible should be provided with visually interesting stimuli to maximise the signals which are sent from the eyes to the brain in order to optimise the development of these pathways.
Richard Bowman, Ruth Bowman & Gordon Dutton
First published by RNIB in 2001