Vision for Doing: Assessing Functional Vision of Learners who are Multiply Disabled
- the ability of the lens of the eye to vary in thickness,
so that a sharper image falls on the retina. To experience its effect try this
exercise. Close one eye and, while looking at your finger at arm's length, slowly
bring your finger towards your nose. You can feel the tension in your eye as
the lens thickens. Some writers have suggested that for learners who are multiply
disabled, they have poorer accommodation. However, no hard evidence exists for
this. If it was true, it would have implications for prescribing of spectacles.
It would mean that it would be more important for glasses to be prescribed for
learners who are multiply disabled - even when the refractive error is quite
- where central visual acuity of an apparently healthy eye
is reduced because of lack of use for certain visual functions. Often it occurs
because the "good eye" is preferred. It is usually associated with squint.
- absence of the iris which controls the amount of light entering
the eye as in a camera's shutter. The learner will be photo-phobic, therefore
it is helpful to use dimmed lights and she may benefit from filtered lenses.
- use of both eyes together so that the separate
images from each eye (which are slightly different) are interpreted by the brain
as a single image. At its highest form - stereopsis - an impression of depth
can be obtained by the brain superimposing two slightly dissimilar pictures of
the same objects.
binocular parallax - for any point you fixate, the images on the two
eyes must be slightly different. But the two different images still allow us
to perceive a stable visual world. Binocular parallax refers to the ability of
the eyes to see a solid object and a continuous surface behind that object even
though the eyes see two different views.
cataract - lens of the eye is cloudy or completely opaque, the result
is a loss of vision for detail. If the cataract is located in the centre of the
lens then dim lighting should be used. If on the peripheral part of the lens
then it is usually helpful to have brighter lighting.
coloboma - present from birth, a cleft or wedge is missing from the
Iris. It is likely that visual field problems will exist. This may affect mobility
and scanning such as symbols, pictures or text.
contrast - the way that a foreground (say an object) stands out from
its background. Contrast is not a property of visual edges on the retina but
of visual edges in space. This is important in allowing us to vary the learner's
visual world in order to enhance contrast.
contrast sensitivity - the term is used in a technical sense to refer
to the minimal amount of contrast needed to perceive a test pattern. Some writers
argue that contrast sensitivity gives a better indication of ability to see the
form of objects than does a measure of visual acuity. Section 14, while not being
as accurate as a true test of contrast sensitivity, does give useful practical
information on the basis of your results. `
cortical blindness - implies that the brain does not interpret the
signals received from the eye. Often the learner appears to have better peripheral
vision. Use good illumination and high contrasted materials.
cortical visual impairment see Cortical Blindness
dark adaptation - refers to the gain in sensitivity as the eye remains
in the dark. It is a relatively slow process, taking around 40 minutes to complete.
Where the ability of the eyes to adapt to the dark is slower, this is likely
to be associated with poorer contrast sensitivity. Should the learner suddenly
exhibit poorer dark adaptation than is usual for that learner, you may want to
request further examination.
diabetes - this is a major cause of blindness in industrialised countries.
Blindness is associated with older people who have diabetes (the reason poorer
countries do not have diabetes as a cause of blindness is that those who contract
diabetes are unlikely to be treated and will die prematurely).
emmetropia - the normal eye focuses light at a point on the retina
(with the lens at rest). (See Figure 5.2.1)
glaucoma - raised pressure within one or both eyes. The cornea or outside
surface of the eye clouds and peripheral vision decreases. The learner may be
uncomfortable in bright light (photophobia) and if mobile have difficulty with
hypermetropia - long sightedness. Parallel rays entering the eye come
to a focus behind the retina. Note that it does not mean that vision is very
good (eyes like an eagle!). It means that the muscle controlling the thickening
of the eye (see accommodation) has to continually work when reading and for other
close work - the eyes literally do then get tired. (See Figure 5.2.1)
light adaptation - is the loss of sensitivity when an eye (or both
eyes) has become adapted to the dark. Complete light adaptation is much faster
than dark adaptation. Much of the adaptation to light takes place within a second,
the remainder requiring only a few minutes.
linear perspective - a painter's cue to represent distance on a two
dimensional photograph. (See Figure 7.11.3 for example of another painter's cue).
motion parallax - information at the retina caused by relative movement
of objects as the observer moves to the side (or his head moves sideways). Motion
parallax varies depending on the distance of the observer from objects. The observer's
movement also causes occlusion (covering of one object by another), and as movement
changes so too does occlusion. This can give a powerful cue to the distance of
objects from the observer.
myopia - short sightedness. Parallel rays entering the eye come to
a point in front of the retina. (See Figure 5.2.1)
nystagmus - Rapid uncontrollable movement of the eyes, impaired vision
for detail, although peripheral vision may be better. Problems in depth perception
may result. It is unusual for nystagmus to occur in isolation, usually indicating
other difficulties. Experiment to find if there is a stable point ; ie position
of objects in relation to the learner's eyes in which the movements become less
occlusion - provides information as to the relative depth of objects.
Very helpful when depth cannot be perceived by binocular coordination (or binocular
optic atrophy - nerve fibres transmitting information from the eye
to the brain are affected. Use bright illumination of objects and high contrast
optic expansion pattern - the information at the eyes which specifies
an approaching object (see Section 11 for further details).
painter's cues - These include linear perspective, in which separation
of lines decreases with increasing distance (like railway tracks going into the
horizon); interposition or occlusion (one object covering another) gives information
on object distance as does a differences in height between objects.
photophobia - when light hurts the eyes and the person keeps her eyes
away from bright lights. In extreme forms the person performs best in very low
proprioception - sensations from the joints and other sites of the
body. These help the individual to sense the positions in space of parts of the
prosopagnosia - a rare condition which describes when faces cannot
be recognised even though other visual abilities would suggest that this would
refractive error - light coming from a distant object (6 metres or
more) enter the eye in parallel. If the object is to be seen clearly, the eye
must focus on a point on the retina. This depends on four things:- the amount
the eye curves; the length from the cornea at the front to the retina at the
back of the eye; the position of the lens inside the eye; the state of that lens.
The normal eye is emmetropic. (See Figure 5.2.1 for illustrations of refractive
retina - made up of specialised cells called rods and cones (around
120 million rods and 7 million cones) this acts a bit like the film of a camera.
The most sensitive part of the retina is the fovea, which you use for seeing
close detail &emdash; like reading this book.
retinitis pigmentosa - where peripheral vision is affected first. There
are several different types of RP. With reduced vision in dim light and blurring
of images, the condition is often progressive. May be associated with deafness
(Usher syndrome), in which case the learner is born deaf and later may become
retinopathy of prematurity - where the retina is scarred due to immature
blood vessels in the eye reacting to changes in oxygen pressure soon after birth.
Some learners affected will have residual vision and this will usually be accompanied
by short sight (myopia) and a squint may be present. Good illumination and plus
lens refraction may be helpful.
Retrolental fibroplasia see RETINOPATHY OF PREMATURITY
scotoma (pl. scotomata) a blind spot in one or both eyes, or occurring
between the eye and part of the brain which interprets information detected through
size constancy - whenever the perception of object size remains constant
even with changes in the object's or observer's position. (See Section
squint - imbalance of the eye muscles may result in the eyes turning
either towards the nose or outward, the latter having sensitivity to bright light.
As a result if one eye is straight this takes over, leading to the image from
the affected eye being ignored (amblyopia). Often with severely physically disabled
learners one eye does not suppress the other and double vision (diplopia) occurs.
This results in difficulties with discriminating form and patterns of distinguishing
figure from background and with depth perception, eye/hand coordination and fine
motor skills. Good contrast is important, as well as using good even illumination.
strabismus see squint
stereopsis - highest form of binocular vision (see above). Great fun
used to be had in Victorian times with the use of Stereoscopes, devices which
artificially created slightly different views of the same scene. The same technique
is used in 3-Dimensional cinema
trachoma - along with conjunctivitis this is a very common cause of
blindness in poorer countries, and is transmitted by certain types of flies.
Usher syndrome - see RETINITIS PIGMENTOSA.
visual acuity - the ability of the eyes to see detail. There are a
variety of ways of measuring visual acuity.
visual field - see Section
13.- this is the major cause of blindness in pre-school
age children in many poor countries. It is caused by a severe shortage of Vitamin
A in the diet.