82 2 Perception, the Eye and Assistive Technology Issues
2.4.5 Binocular Vision and Stereopsis
Despite the fact that the image of the three-dimensional world on the retina is twodimensional, depth vision is important. There are both binocular and monocular cues to depth and distance. Monocular approaches to evaluating depth and distance include parallax or the relative motion of near and far objects when the head is moved from side to side or up and down, perspective and the relative size and position of different objects. However, the most important means of assessing depth, stereopsis, uses the fact that the two eyes receive slightly different images of a scene with depth. Objects whose images are on corresponding points on the two retinas are perceived as being at the same distance, whereas objects which have an outward or inward displacement relative to the retinal image of the point being viewed are perceived as being respectively closer or more distant than this point. About 4–5% of the population without other visual impairments do not have stereopsis, but generally compensate and obtain depth vision by monocular approaches, including parallax, perspective, occlusion and depth perception from movement.
The two retinal images of an object can result in diplopia or double vision in the absence of fusion, namely, the perception of the two images as one. It should be noted that fusion, fixation and focus are all different from each other. Fusion is the perception of the two retinal images as one, whereas fixation involves directing the visual axis of one eye at an object to position its image on the fovea and focus the formation of a crisp retinal image. Fusion of the two images into one is only possible when there is approximate correspondence between the retinal locations of the images in the two eyes. The two images do not need to be located at exactly corresponding points, but the image in one eye needs to be located in Panum’s fusional area, which is a small area round the corresponding point to the image in the other eye. An image is seen as single when located in the fusional area and double outside it.
A pair of Panum’s fusional areas would have similar coordinates on the retina. Corresponding points on the two retinas can be defined either geometrically or functionally in terms of the visual tasks carried out by the two eyes. The term horopter (from the Greek horos for boundary or limit and opter, observer) is used for the lines or surfaces formed by visual points with images on corresponding retinal points. However it is not uniquely defined.
2.5 Visual Impairment and Assistive Technology
Currently the design of assistive technology generally does not take account of the type of visual impairment of the particular end-user community. Instead, most assistive technology is generally designed for people who are implicitly assumed to be functionally blind, or in some cases, such as screen magnifiers, for people who are generically visually impaired without consideration being given to the specific type of visual impairment. However, only an estimated 3% of blind people are functionally blind and most blind people have some degree of useful vision.
2.5 Visual Impairment and Assistive Technology |
83 |
This raises the issue of whether there would be advantages in designing devices for specific groups of blind and visually impaired people. This could lead to the development of devices that are complementary to and take account of residual vision. However, this could have the disadvantage of leading to a very large number of different designs of assistive devices for small numbers of blind and visually impaired people, which may not be technically or financially viable. There may be benefits in developing sets of related devices with different members of the set having features that are appropriate for particular groups of blind and visually impaired people. Thus, this section contains a discussion of the effects of different types of visual impairment, since this may, at some point in the future, have an impact on the design of assistive technology.
2.5.1 Demographics of Visual Impairment
There are an estimated 45 million blind people and 135 million visually impaired people worldwide (WHO 1997a). There are differences both in the percentage of blind people in the population and in the distribution of the different causes of blindness globally. In particular, trachoma, which results from an infection of the conjunctiva by the bacteria Chlamydia trachomatis leading to scarring of the cornea, is responsible for 18–24% of blindness globally (Zhang et al. 2004), but corneal infection is only responsible for about 2% of cases of registered blindness in the industrialised countries. This is probably because it can be easily avoided by washing the hands and face and treated by antibiotics, though a corneal graft from a donor would be required to restore vision after the cornea has become scarred. Some 90% of blind people live in the ‘developing’ countries (WHO 1997b).
Ageing frequently has an effect on the visual system, resulting in some degree of visual impairment. For instance, in the UK 82% of blind and visually impaired people are over the age of 50 (TIRESIAS undated) and nearly 73% are over 80 (Bruce et al. 1991; Walker et al. 1992). In the industrialised countries, the percentage of blind and visually impaired people increases by a factor of three for each decade after the age of 40 (Taylor et al. 2001). Many types of blindness are much more common in older people. For instance, age-related macular degeneration is responsible for 39% of cases of registered blindness, as well as being a common cause of blindness in older people, with an incidence of about 8% for people over 65. Cataract is also increasingly common in older people, with an incidence rate of 40% for people over the age of 70 years, with an appreciably greater prevalence in women. It is currently the major cause of blindness globally and responsible for about 45% of cases (WHO 1998).
Glaucoma is the second major cause of blindness globally and is responsible for about 15% of cases of registered blindness. It again increases progressively with age from about 1% at 65 years of age to 7% at 80 years. Over half the population with glaucoma worldwide are unaware that they have this condition (Quigley 1996). Retinal conditions contribute to about 9% of cases of registered blindness, with the incidence of retinopathy due to diabetes increasing from 2% at 50 years of age to 7% at 75 years.
84 2 Perception, the Eye and Assistive Technology Issues
2.5.2 Illustrations of Some Types of Visual Impairment
Many people think that blind people have no vision at all and would be quite surprised to see a person travelling with a long cane reading a book or looking at a map. However, this is quite common and many people who are registered blind do have some useable vision. An indication of what visually impaired and blind people with different types of visions impairments experience is given in the following figures. A typical busy road scene (Figure 2.11) was chosen, as it illustrates one of the important barriers faced by visually impaired and blind people, namely crossing the road or, more generally, finding their way about urban environments safely. Crossing a road can be a difficult and dangerous activity, even for people with no vision or hearing impairments, as cars do not necessarily obey speed limits or other road rules and may seem to zoom up out of nowhere. However, sighted and hearing individuals can use information from both these senses to make decisions as to when it is safe to cross and how fast they need to cross. In the case of visually impaired and blind people considerably less information is available. Therefore unless additional environmental cues are provided by assistive technology, such as ultrasonic or laser canes or audio or tactile ‘safe to cross’ indicators at crossings, it will be very difficult for visually impaired and blind people to cross roads safely.
Currently, only the following two measures of vision and visual impairment are used:
•Visual acuity or the ability to detect fine details and a quantitative measure of this ability to see an in-focus image at a given distance. Visual acuity is expressed as a fraction or a decimal. For instance, 6:30 means that a person requires to stand at 6 m to see objects that a person without a visual impairment can see
Figure 2.11. Traffic scene
2.5 Visual Impairment and Assistive Technology |
85 |
at 30 m. Whilst 6:6 vision implies an ability to understand static images from a distance, it does not necessarily imply perfect vision, as it could be combined with colour blindness, presbyopia or an inability to track fast moving objects.
•Field of view; which is the part of the visual world that a person can see at any given time. People have a 180◦ forward facing field of view out of each eye, but only a 140◦ field of view from both eyes together.
To give a better appreciation of the quality of a person’s vision and their type of visual impairment, there is a need for the development of further measures, including of the following:
•Visual interference effects.
•Motion vision.
•Depth and binocular vision.
•Colour vision.
Loss of visual acuity
Loss of clarity of vision can be specifically localised in extent or vary in quality across the complete field of view. Figure 2.12 shows a case where the central vision is very blurred but with the clarity of vision improving in the peripheral regions of the view. Thus, a person with this form of loss of acuity will be aware of cars at the sides, but will not see them clearly even when they are directly in front of them. They may also have difficulties in estimating the width of the road. Elderly people with age-related macular degeneration due to damage to the pigment epithelium experience this type of view. There are two forms of macular degeneration: the dry and wet forms. In the dry form, material is discarded from the cone outer segments and builds up a layer of deposits behind the pigment epithelium, which impedes the diffusion of nutrients and dispersal of water products. In the wet form, fluid invades the pigment epithelium-photoreceptor space, the cone photoreceptors degenerate and, complete loss of retinal function eventually follows. Although the wet form of the condition may be ameliorated by laser coagulation, vision cannot be restored in either the wet or dry case.
Mild global loss of acuity across the whole field of view is shown in Figure 2.13 and a significant loss of global acuity is shown in Figure 2.14.
In Figure 2.13, the cars and the road are visible to a certain extent, but not very clear. Meanwhile in Figure 2.14, it is very difficult to distinguish exactly how many cars there are or even where the far side kerb is. These two views might be typical of those experienced by people with a mild and an advanced cataract condition, respectively. Vision reduction results from the lens becoming opaque.
Causes of cataract include genetic factors, heat, trauma, uncontrolled diabetes mellitus and ageing. Cataract due to ageing is increasing with increased longevity and may occur in the cortex or nucleus of the lens. The mechanisms involved are not fully understood, but thought to involve a decline in the oxidative status of the lens fibres and an aggregation of crystallins which form water-insoluble aggregates. The sequestration of metals such as iron and manganese leads to a brown/black lens
86 2 Perception, the Eye and Assistive Technology Issues
(brunescence). Cataract is treated very successfully by removal of the cataractous lens, leaving the posterior capsule intact (extra-capsular extraction), followed by insertion of a substitute acrylic lens of the appropriate refractive power.
Another cause of reduction in visual acuity is amblyopia, where the visual acuity of one eye is reduced due to inadequate visual stimulation during early
Figure 2.12. Traffic scene as viewed with localised central loss of acuity
Figure 2.13. Traffic scene as viewed with mild global loss of acuity
2.5 Visual Impairment and Assistive Technology |
87 |
childhood, sometimes in response to other visual impairments. Once established this reduction in visual acuity is in general irreversible. Amblyopia is relatively common with an incidence of 2.0–2.5%. The major causes are occlusion (caused by a congenital cataract), squint (strabismus) or unequal refraction between the two eyes. There are two main categories of strabismus: inward (esotropia) and outward
Figure 2.14. Traffic scene as viewed with severe global loss of acuity
Figure 2.15. Traffic scene as viewed with nystagmus
88 2 Perception, the Eye and Assistive Technology Issues
(exotropia) deviation of one eye, with inward deviation more common. A major cause of esotropia is uncorrected hyperopia (long sight), as a result of which a child uses accommodation to increase refraction for distance vision. Unfortunately, this also causes convergence so that one eye serves as the viewing eye while the other becomes amblyopic.
A different type of reduced global acuity is seen in Figure 2.15. In this case, the view is seen as uniformly blurred and it is due to involuntary, rhythmical oscillatory movements of one or both eyes (nystagmus). Nystagmus affects 0.01–0.02% of the population. It may be hereditary, due to another neurological condition or temporarily induced by alcohol and other drugs. The resulting visual impairment varies greatly. Other conditions which have nystagmus as a symptom include head trauma, which is the most common cause in young people and stroke, which is the most common cause in older people.
Loss of field of view
Figure 2.16 illustrates the portion of the traffic scene that might be seen by a person with ‘tunnel vision’. A person with tunnel vision will not be aware of what is in the peripheral field of view and the person will only be able to see the cars that are directly in front of them, but not those at the sides. They may be able to see across the road and therefore know how wide it is but they may find it difficult to know if their exit on the other kerb is free of obstacles. People with a tunnel vision condition will also experience mobility problems with obstacle avoidance becoming a key issue. Visual field reduction and tunnel vision are typical stages in the various types of retinitis pigmentosa. This is an inherited condition, with an incidence
Figure 2.16. Portion of the traffic scene that might be seen by a person with ‘tunnel vision’
2.5 Visual Impairment and Assistive Technology |
89 |
of 0.03%, in which there is progressive loss of rod photoreceptors resulting in night-blindness. The visual field gradually reduces until only the most central field remains. This is the stage of tunnel vision. Visual acuity also eventually declines, reflecting the loss of cones so that eventually only 5% of the photoreceptors remain.
A condition that leads to a progressive loss of visual field and eventually, if untreated, total blindness is that of glaucoma. It is the result of damage to retinal ganglion cell axons due to an inability to tolerate the prevailing intraocular (inside the eye) pressure. In most cases of glaucoma, the intraocular pressure is raised above the accepted range of 12–21 mm Hg due to impaired drainage of aqueous humour through the canal of Schlemm. However, there are cases of glaucoma without raised intraocular pressures. Glaucoma may have an acute onset, when it is accompanied by considerable pain. However, the usual progression is generally much less apparent so that by the time the person is aware of visual field loss, considerable damage to the retina has already occurred. This could be avoided by regular measurement of visual field and intraocular pressure. Treatment to increase the drainage of aqueous humour may be in the form of eye drops, inhibitors to reduce the formation of aqueous humour or anti-cholinesterases. The eye drops used include timolol or carbonic anhydrase, an enzyme that assists the conversion of carbon dioxide and water into carbonic acid Anti-cholinesterases suppress the action of the cholinesterase enzymes which allow neurons using the neurotransmitter acetylcholine to return to their resting state after activation. In some cases laser treatment or drainage surgery will be required.
A significant reduction in information occurs when a person has only half the field of vision in each eye (hemianopia), as shown in Figure 2.17. Consequently, they will be aware of the cars at one side of the road, but not at the other and
Figure 2.17. Portion of the traffic scene seen when half the field of vision is lost
90 2 Perception, the Eye and Assistive Technology Issues
may not be aware how wide the road is. Hemianopia is due to damage to the optic pathways in the brain and can result from acquired brain injuries caused by stroke, tumour or trauma. Hemianopia is classified according to which half of the visual field is absent, with the most common type right homonymous hemianopia, that is both eyes only see the right half of the visual field.
Manifest visual interference effects
Sometimes stationary or dynamic effects can cause a reduction in the quality of a person’s vision. These kinds of disturbances can be fixed or transient. However, these visual disturbances obstruct the person’s view to the front and/or distract them from the activities they are trying to carry out.
One example in Figure 2.18 shows mild blurriness and blots giving localised loss of vision, typically caused by diabetic retinopathy. This impairment is static. Diabetic retinopathy can result from widespread disturbance of the circulation with consequent hypoxia (reduced oxygen supply) due to uncontrolled diabetes. To try and rectify this new blood vessels are produced in the retina, but may leak and cause haemorrhages. The resulting visual impairment is particularly evident in the blue/yellow colour balance due to a disproportionately great effect on the blue/yellow ganglion cells. Treatment is by laser coagulation to reduce the formation of new blood vessels.
Another form of visual disturbance is the transitory experience of a long slow flash of light often appearing as a slow moving and evolving ring of light as shown in Figure 2.19. This tends to be very distracting, so while this long slow lightshow lasts, the person tends to focus on it rather than the scene. Therefore, although in
Figure 2.18. Traffic scene as viewed by a person with mild blurriness and blots