Vision is a learnt skill that embraces the ability to identify, interpret and understand what is seen. It requires not only correct focusing but also the efficient mechanical functioning of the eyes and brain and a high degree of coordinated activity of the two eyes.
Some common children’s vision problems affect their ability to see clearly and can be detected in routine school screening checkups.
There are a large number of problems which are not as easily identified. These problems can be a result of the eyes not teaming together well, visual perception difficulties that interfere with learning or the visual system functioning inefficiently.
Clear distance vision (visual acuity)
Sharpness of vision (visual acuity) is measured using the familiar letter chart (at a distance of 6 metres ) which has letters of steadily decreasing size. Children who cannot read are assessed using shapes or pictures rather than letters.
Change in focus (accommodation)
The child must be able to change focus from distant to close objects and vice versa and to see both clearly. The ability to achieve this change in focus (called accommodation) should be quick and effortless. The need for quick and efficient focusing is understood if we think about the copying from a blackboard. The child must look quickly from the blackboard to the book on a desk and back to the blackboard, rapidly changing accommodation each time.
Aiming the eyes (fixation)
Fixation is the ability to point the two eyes together, exactly aiming at an object of interest. If the eyes do not point precisely at the same object the brain will attempt to compensate for this imbalance. This will result in increased effort within the eye muscles, which will lead to early fatigue. In severe cases of poor fixation double vision can result. Common symptoms include:
- Headaches and eyestrain
- Losing the place while reading,
- Poor comprehension
- Double vision
Eye movements (saccades and tracking)
There are two main types of eye movements. Both require the eyes to work together as a team. The first type of eye movements involve quick and accurate movements that are used, for example, when the eyes move from one word to another while reading. These rapid jumping eye movements are called ‘saccades’.
The second type of eye movements are known as ‘tracking’ and these should be smooth and accurate. Tracking movements are used when the eyes follow a moving object such as a ball in flight or vehicles in traffic.
Children who frequently lose their place while reading, or who have difficulty in watching the ball while playing sport, may have poorly developed eye movement skills.
Depth perception (stereopsis)
Depth perception is the ability to judge distances. Accurate depth perception is required to hit or catch a ball while playing sports or to park a car accurately. Stereopsis is one of a number of cues which the brain uses to judge relative distance and requires coordinated functioning of both eyes.
Peripheral or side vision is the ability to see and interpret what is happening to the sides of the field of vision while looking straight ahead. It is clear that being able to see to the sides is important while driving a car, but not so obvious for its importance in writing.
Poor peripheral vision can lead to crooked and messy handwriting. The child learning to write may not see the line on either side of the point at which he or she is writing, and this would result in poor judgment.
Eye-hand coordination is a skill in which the brain guides movement of the hands based on the information it receives from the visual system.
This ability is important when tracing a line with a pencil or throwing a ball against a wall and catching it again. Eye-hand coordination is also important in team sports, driving, bike riding and running – in fact in just generally moving about in the world.
To see clearly and without confusion the two eyes must be aimed precisely at the objects the child is trying to see. Unfortunately not everyone develops this ability as they should in childhood.
Inaccurate alignment of the eyes can result in
- visual fatigue
- blurred vision
- poor judgement of depth
- sore eyes
- mental fatigue.