Video for VI Learners
Effects of visual impairment
Narrow visual field
What is meant by a visual field?
The visual field can be described simply as the area which can be seen
by an individual when looking straight ahead without moving the eyes
or gaze. It can be measured in degrees from the fixation point; the
normal field of vision is approximately 160-180 degrees horizontally
and 120 degrees vertically. The nasal fields of vision (towards the
nose) from each eye overlap when the eyes look straight ahead but not
the temporal fields i.e. those towards the temples.
Field defects usually refer to the deterioration of the peripheral
field, that is the retinal area outside the macula area. In these instances
central vision may be unaffected, as for instance with glaucoma and
What are the effects of a visual loss resulting in a narrowing
of the visual field?
Damage to the rod cells of peripheral retinae affects the person in
several ways. The narrowing of the field reduces the amount the person
can see when looking straight ahead and in extreme circumstances this
is known as tunnel vision. You can simulate this for yourself by covering
your eyes with a simple mask made out of cardboard with only a pinhole
for you to see through. Straight way you are aware that you have problems
with moving around without stumbling and bumping into furniture; seeing
details of 'whole objects' - only small parts are visible - or even
recognising objects or people.
Moving objects such as traffic are very difficult to follow or even
to be aware of. Conversing with people you do not know is difficult
as you are unable to watch their body language. Since the rods are concerned
with vision in in dimly lit or dark places, people with this kind of
visual loss suffer from 'night blindness'. The effect is misleadingly
named: it is not restricted to the night but also manifests itself when
there is a change in lighting conditions. For instance, when coming
into a room from the bright sunshine the eyes take far longer to respond
to changes in lighting - in fact many people with field narrowing are
also photophobic and may wear tinted glasses. Similarly, going from
a bright lecture room into a dimly lit corridor or staircase can be
a frightening experience for the person as they will be functionally
'blind' due to slower pupillary response (reacting to bright and low
levels of lighting).
What makes the situation worse, and what helps?
The student with these problems is the best person to advise you -
please ask them!
Bright sunlight and associated glare and low levels of illumination
creates visual havoc. Controlled levels of lighting and same layouts
in rooms can help the student. Good contrast is essential both on the
screen and for any handouts which are given out as supplementary materials.
Good 'black' print on white or yellow paper is best, and remember to
check with the student about their preferred size of print - in these
circumstances 'big' is not beautiful.
Some conditions may be progressive and so effects get worse as time
passes: what is acceptable when the student first joins you may need
How does this affect viewing video screens?
The student needs to be central to the screen in order to maximise
the remaining central vision. Only a small amount of the information
on the screen can be seen at one time, causing problems in tracking.
This will affect the ability to follow fast moving images on the screen
or to read any on screen text, which might be vital to the understanding
of the video.
Taking notes at the same time as viewing can cause problems. It can
be hard to shift focus to the notes (possibly in dim
light) and then to find the place on the screen again.
For some students, there will be difficulties in interpreting colour
on the screen and you may need to decide how colour-related content
can be explained in another way.
Following the lecturer and all her associated mannerisms and body language
will be a challenge. Teachers should use language carefully so full
explanations are given verbally, and should try to avoid relying on
gesture and facial expressions.
What would improve screen access?
The contrast and brightness controls should be adjusted so that they
are 'comfortable' for the students. Try to balance the light levels
on the screen with those on the teacher, her teaching aids, and the
illumination of the student's notes.
Students should be asked about the best seating position for viewing
- a good rule is centrally and at the front.
What names of eye conditions should
I watch out for?
peripheral field loss -
- retinitis pigmentosa
- hemianopia, chorioretinitis
- glaucoma, aniridia
- Marfan's syndrome
- retinal detachment
- Leber's amaurosis