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Adapting Video for VI Learners

Effects of visual impairment

Normal sight

Understanding visual impairment starts with being aware of what it is to see fully. Our visual faculties are more subtle than at first appears. The problem is that sighted people hardly notice how restricted their vision is, and so don't appreciate the ways that visual impairments multiply these restrictions.

What do we see?

Consider your ability to see fine detail. Imagine that you are looking at a television picture of the scene in front of you. Is your vision of the whole scene is as clear as the video image? Surprisingly, the answer is no. The central part of of your vision is great at subtle shades, contrasts and colours, and this is where the TV is. It takes little if any eye movement to scan the picture, so all of it appears sharp, well coloured and bright.

Now look at the scene. The part you are focused on is just as sharp, but if you concentrate on (but don't shift your eye toward) the peripheral areas, you will find that objects here are much less distinct. You might be surprised at how rapidly your perception of detail drops away: can you, for example, see words at the extremes of this page whilst you focus on this one? Central vision is therefore what we most use for fine work, reading, and learning: if we need corrective lenses, they are designed to help that central area focus on the world.

The eye and brain put most resources into the central visual field - more sensory cells and more attention - keeping just a 'watching brief ' on the periphery mainly for movement, and for our position in the world. Contrast and colour shade fades out quite rapidly - you may not be aware of it since your brain keeps a running memory of what colours things are so the illusion of colour completeness across the scene isn't suddenly lost. But have someone bring an object in from one edge of your perception whose colour you don't know, and you will find that you first get light/dark impressions alone.

  • Graphic simulating normal vision(D)
  • View video (315K)
  • >Near, middle and far accommodation

    A person with good vision is able to change their focus between near, middle and distant objects. This change of focus or accommodation is done rapidly and without effort, as the clip below simulates.

    >Accomodation to changing light levels

    As well as being able to focus on near and far objects with ease people with good vision can see in light levels ranging from bright daylight down to candlelight (although as light levels fall the eye's ability to register colour is gradually lost). What is more, this accomodation can be made very quickly.

    >Following movement

    Finally, those with good vision expect to be able to deal with moving objects, no matter how unpredictable. The task is to keep the object of interest in the highly accurate, colour sensitive central part of the eye.

    >Partial sight

    So our view of the world is not as precise to the edges of our vision as to the edges of a photograph or television screen, and our visual system is constantly and without our effort compensating for changes in focus, light levels, and the movement of objects surrounding us. Why does any of this matter?

    For one thing, we need to be aware of how difficult is is to compensate for some visual impairments. Suppose you had lost central vision (say through a cataract): it should now be obvious that using your peripheral area instead is going to be difficult. Detail will be less; colour will be poorer; contrast will be low; and you are predisposed to focus the central part on what you are attending to (so you will be fighting a now unhelpful biological habit). At the same time, light which should be focused on the central area can get diffused and scattered onto the peripheral detectors, compounding the problem by introducing glare right into the remaining working areas. Less obviously perhaps, those people with loss of peripheral vision are going to have trouble in tracking objects.

    For another, it helps explain how the eye and brain deal with blind spots: bits of the retina which can't sense light falling on them. Sighted people have them - one in each eye, where the optic nerve and blood vessels connect to it. But you don't see a black spot. Instead, the eye and brain 'fills in'. And since the blind spot is off-centre, we are more tolerant of lack of detail, so we don't notice. Some eye conditions also generate much larger or multiple defects in field vision, but the visually impaired person may not always be aware of them, because sometimes the same 'fill-in mechanisms operate. If the loss is due to brain damage (after a stroke, for instance), then whole sections of the visual field can be lost leaving the person unaware of anything happening on, say, one side of their body. So the way we design and use learning materials (including video) has to take into account strategies designed to search for detail which might, to the rest of us, be in plain view.

    The main message is that full vision is complex and differentiated, and we have to be careful in what we expect visually impaired learners to be able to do with what remains of theirs. The pages on the effects of visual impairment highlight some of the issues.

     
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