Saturday, 3 December 2011

Why it is important to identify a visual impairment early

It is important for you (if you are a class teacher) to be able to recognise some common signs of poor vision in order to spot the visually impaired child in your class. Sometimes you may be the only vision screening they will have. There can be many reasons for a child not achieving their potential and a lack of adequate vision can be one reason. However the younger the child the more important it is to spot a visual impairment. This is because the visual system is still developing in the early years and there is a window of opportunity to do something about it when the child is still young.

With this in mind I want to say a word for those who care for very young children because in some cases it could make the difference between a child being able to see and not being able to see. There is a critical period for vision and it is vital for the eyes to be stimulated during this time. Eye conditions such as amblyopia can develop if the eye is not stimulated during the critical period, and this can lead to poor vision in one or both eyes. Years ago experiments (aspects of which by today’s standards would be considered unethical) were conducted by David Hubel and Torsten Wiesel on newborn kittens. They showed that if a kitten’s eyes are deprived of normal visual experience during a critical period at the start of its life, the circuitry of the neurons in its visual cortex is irreversibly altered. One eyelid of the newborn kittens was sutured shut. When the kitten reached adulthood at six 6 months, its eyelid was opened again. Recordings were then made of the electrophysiological activity in each of the kitten’s eyes. These recordings showed an abnormally low number of neurons reacting in the eye that had been sutured shut, and an abnormally high number in the other eye. Macroscopic observation of the visual cortex showed that the ocular dominance columns for the eye that had been left open had grown larger, while those for the eye that had been closed had shrunk.

Hubel and Wiesel also found that if the eye of an adult cat was sutured shut for a year, the responses of the cells in its visual cortex remain identical in all respects to those of a normal cat. They found that suturing a cat’s eye shut had no effect on its visual cortex unless this visual deprivation took place during the first three months of the cat’s life.

Wiesel and Hubel showed that it is not until the signals from the retinas reach the primary visual cortex that the brain begins to merge them into one, three-dimensional image.

As in other developmental processes for which there is a critical period, sensory deprivation does not have the same effect on adult animals. Closing one eyelid of an adult animal has no effect on the response of the visual cortex cells for that eye or for the other eye. In contrast, during the most sensitive part of the critical period, visual deprivation for as little as one week can have catastrophic effects on the animal’s vision for the rest of its lifetime.

In humans, certain diseases can cause a cataract (a total or partial opacity of the lens) in one or both eyes. Cataracts can occur not only in adults but also in very young children. Cataracts can now usually be removed surgically. Studies of individuals who had such surgery at various times in their lives showed that in humans as in other animals, there is a critical period for the development of the sense of sight. These studies demonstrated for the first time that early environmental influences, and hence particular neural activity patterns during a critical period, could permanently alter the neural connections in certain areas of the human brain.
Parents always want to know that their newborn baby is perfect and they are right to be concerned if all their instincts indicate a problem. Research confirms that in human beings vision has a critical period. If the eyes are not working properly in the first few months of life vision may never be perfect.

To come back to the school years, whenever I am asked to speak to a group of teachers (particularly in primary schools) there are certain things that I feel I have to communicate regardless of whether or not there is a ‘visually impaired’ child in the school. I always feel it is necessary to show them how to identify a child with visual difficulties whatever the precise nature of those difficulties. In other words what sort of things should you be looking for to be able to flag up a difficulty that is visual rather than say simply cognitive, attentional or indeed related to hearing or one of the other senses.

Let me then briefly list in summary and then comment on some of the most obvious behaviours that should flag up a problem with vision. Here are 17 possible indicators of a visual impairment. One on its own may mean nothing but if there are more than one of these present in a child’s behaviour then this should flag up a concern.  

Certain behaviours when occurring together may suggest a problem with vision.
These can include:

1. Close viewing
2. Head turning
3. Habitual clumsiness
4. Consistent off-task behaviour
5. Exceptionally poor spelling despite able reading
6. Regular careless miscuing of known words
7. Underachievement
8. Screwing up eyes in light
9. Consistent copying from a neighbour
10. Eyes that look crooked
11. Eyes that look red or watery
12. Eyes that move eccentrically
13. Difficulty looking directly at your face or eyes
14. Lack of confidence in new environment
15. Distress in a noisy, busy environment
16. Covering one eye to look
17. Turning head rather than moving eyes to follow a target

No comments:

Post a Comment