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Adapting Video for VI Learners

Effects of visual impairment

Visual field defects

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 retinitis pigmentosa. Many conditions, for example, optic atrophy and hemianopia may effect both the central and peripheral fields of vision.

What are the effects of a visual field loss?

Peripheral loss

Damage to the rod cells of the peripheral retinae narrows the field. This has several effects:

  • it reduces the amount the person can see when looking straight ahead. In extreme circumstances this is known as tunnel vision. It leads to problems with moving objects, tracking, and even holding conversations.
  • as the rods are concerned with vision in dimly lit or dark places, people with this kind of visual loss suffer from 'night blindness', and have difficulty with changing lighting conditions.

Combined central and peripheral loss

Damage to the cone cells of the central retina affects the person in several ways in addition to those mentioned above

  • reduced visual acuity - in some instances central vision may be lost
  • reduced colour vision
  • slower pupillary response (reacting to bright and low levels of lighting)
  • photophobia i.e. sensitivity to light-brightness

Example 1: Hemianopia

Hemianopia(D)>Example 2: Central visual field defects

>Example 3: Peripheral visual field defects >What makes the situation worse, and what helps?

Bright sunlight and associated glare and low levels of illumination creates visual havoc. Some conditions may be progressive, so that what is tolerable at the beginning of a course will need checking as effects get worse over time. Controlled levels of lighting and unchanging layouts in rooms can help the student.

Good contrast is essential on the screen and for any handouts which are given out as supplementary materials (good 'black' print on white or yellow paper is best - check with the student about the preferred size of print - in these circumstances 'big' is not beautiful!). The student with these problems is the best person to advise you - please ask them.

How does this affect viewing video screens?

Where there is either peripheral loss or combined peripheral and central loss the student needs to be central to the screen in order to maximise the remaining central vision. Since only a small amount of the information on the screen will be observed there will be problems in tracking and following fast moving images or reading any relevant on-screen text.

Taking notes at the same time as viewing can cause problems, since focusing on the notes in possibly dim light and then finding place on the screen again will be hard.

For some students, there will also be difficulties in interpreting colour on the screen and you may need to decide how colour-specific material can be explained in another way.

Following the lecturer and all his or her mannerisms and body language will be a challenge - use language carefully so full verbal explanations are given.

What would improve screen access?

Contrast and brightness controls should be adjusted so that they are 'comfortable' for the students: when you have achieved this, check that the details you want them to see are not lost through the control adjustments by showing a short, critical video section.

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?

central field loss -

  • macula degeneration
  • Best's disease
  • Stargadt's disease
  • achromatopsia
  • cone dystrophies

peripheral field loss -

  • retinitis pigmentosa
  • hemianopia
  • chorioretinitis
  • glaucoma
  • aniridia
  • Marfan's syndrome
  • retinal detachment
  • Leber's amaurosis
  • combined loss - coloboma
  • optic nerve disorders e.g. optic atrophy
  • optic dysplasia and hypoplasia
  • strokes