How do our eyes focus?
There’s more to it than you might think:
- Your eyes need to tell the brain that something interesting is out there.
- Your brain then has to send messages out to control your eyes.
- Six little muscles on the outside of your eye not only have to co-ordinate with each other, but also have to co-ordinate with the six in the other eye.
- Lining up both eyes wherever you look allows binocular vision. If they are not lined up you see double.
- As well as lining up vertically and horizontally, you also need to converge your eyes whenever you look at close objects so you don’t see double of everything as it comes towards you.
- Once the eyes are both lined up the slightly different view each eye sees allows us to see in 3D (stereovision).
- At the same time the lens inside each eye needs to be focused on the same point so that it stays clear. This is called accommodation.
- Eye alignment (convergence) and focusing (accommodation) are usually coordinated in a fairly stable relationship so that each tends to accompany the other.
- As things move backwards and forwards in space, all these processes have to be constantly updated as you, your head, your eyes or the object moves.
Clever, isn’t it? And all this goes on all day, every day, all your life without you needing to think about it.
What happens if it goes wrong?
Most babies learn to control their eyes in the very first weeks of life, but it is also a system that can easily be disrupted, especially if it happens in early childhood. If the binocular vision system develops abnormally, or is disrupted in later life, it can lead to many common eye problems which affect well over 5% of us all. They include:
- Squint (strabismus, crossed eyes)
- Lazy eye (amblyopia)
- Double vision
- Eye strain
Questions we are answering through research (For more detailed information, see our publications)
Normal Vision Development
- How does focusing for close things develop and change throughout life?
- Are premature babies different?
- When a tiny baby goes cross-eyed should you be worried?
- When do genuinely crossed eyes start?
- What information do we use from the outside world to tell us where to point our eyes?
Children’s Vision Problems
- Do children with squints use their focusing differently?
- Do long sighted children focus as well as other children?
- Is focusing always symmetrical in each eye?
- How flexible is eye control?
- How common is eye strain. When should you worry?
- How do common exercises for eye strain work?
- How can stress affect focusing?
Vision Screening in Young Children (the EUscreen Project)
Babies are born with more or less straight eyes, and will often stare intently at their mother’s face or a bright light such a window. However, they are initially not very good at changing focus easily from distance to near, and are even worse at doing the opposite. They focus earliest on interesting, single, slowly moving targets.
Their eyes might turn inwards dramatically or look as if they are turning outwards at times, but it’s usually nothing to worry about as long as it has gone away by four months.
By about eight weeks, they are usually adept at converging their eyes and focusing (accommodating) on close objects, although they are not as accurate as adults for a few more weeks. By three to four months they have adult-like control of their eyes and start to use a very accurate form of 3D vision called stereopsis.
We now have a unique database of babies as they develop. It started with normal babies, but now includes premature babies, toddlers, older children, teenagers, young and older adults. Because they were all tested under the same conditions, so we can see how focusing changes throughout life.
We found that premature babies learn to converge and accommodate at the same time after their due-date as full-term babies do.
This means that they may have longer with poorly controlled eye movements which then overlaps with another “critical period” when abnormal development can start. We think this might be why premature babies are at greater risk of eye problems such as squint and need for glasses.
Some babies go cross-eyed a lot in their first few weeks (although others never do). Research we did a few years ago showed that this can be perfectly normal – as long as it is intermittent and is getting better by two months and gone by four months. We have shown that this is because they are better at converging to look at near things than they are at looking back at distant things. They try to look at something close then either “get stuck” with convergent eyes, or even correct the squint in the wrong direction. They usually sort themselves out after a few seconds when they realize they have made a mistake.
At the time that normal babies stop going cross-eyed, babies who go on to develop genuine squints tend to be getting worse. Babies who do not develop 3D stereovision seem unable to correct their eyes back to being straight. The very early bias they have towards turning their eyes inwards seems to persist and get worse. If your baby’s squint is getting worse at three months, speak to your Health Visitor or GP.
Another type of convergent squint, caused by long sight (hypermetropia) usually starts a bit later, between 18 months and 3 years of age.
When we use our eyes to look at anything we get lots of visual information that helps tell us how far away things are and where we should focus our eyes. The main clues are:
- how blurry it is
- the different views we see coming from each eye
- how it changes size as it moves.
You might not think this is very important, but by knowing how normal people do it, we can find out how it might be different in many common eye problems such as squints, eye strain and long and short sight where some of these clues are abnormal, weak or absent.
Many textbooks say that the main clue to both convergence and accommodation was how blurry things go as they move in depth.
We belong to a smaller, but growing, group who have found that as long as both eyes are open, the difference in the images coming from each eye (binocular disparity) is a much more important clue used for both convergence and accommodation than how blurry things are. Many people seem very happy to leave things a bit blurry sometimes, as long as their eyes are straight.
Children with the type of convergent squint (crossed eyes / esotropia) that is controlled with glasses do care whether things are clear – more so than typical children. This might be why they give up keeping their eyes straight in the first place. Being long sighted (hypermetropic) means their visual system has to choose between clear vision but with a squint, or blurred vision and straight eyes, Children who have this type of squint seem to prefer keeping things clear, even if it means squinting.
We used to think that some children with outward-turning eyes (divergent squint or exotropia) controlled their eyes by deliberately over-focusing. We don’t think that is the case – any over-focusing seems to be a consequence, not a cause, of controlling the squint.
Unless a child has a squint, it is common to under-correct any glasses on the assumption that their own focusing will “make up the difference”. Many babies, who are often long sighted at first, will do this, and often grow out of the long sight before about 2 years of age. We found that children who are still long sighted by school age don’t actually make up the difference and often leave things blurred.
You might think that this means that they should be given stronger glasses, but we are still not sure how much a lot of this blur matters. Our most recent studies (funded by Fight for Sight) are looking at how common and important blurred vision for near is in both longsighted children and those who have otherwise normal vision.
Nearly always. It is usually assumed that it has to be, but we have found that a few children who need different glasses strengths in each eye (anisometropia) don’t always accommodate equally, and can even do it completely in reverse in their worse eye. When we looked at how well these children did with patching treatment, we found that they responded not quite as well
This might help us explain why some children with lazy eyes due to anisometropia do worse with treatment than others and maybe by identifying unequal focusing early on we can predict who might different treatment .
The next challenge is seeing if we can change the focusing or the treatment.
Many textbooks imply that accommodation and convergence are really accurate. However, our “naturalistic” testing setup, where people are free to focus or not as they choose, suggests that many people are very happy to tolerate inaccurate focus, particularly accommodation which must give them some level of blurred vision.
We are not sure that the fixed links between convergence and accommodation often quoted in the textbooks are quite as fixed as people think.
When visiting an optician, they may advise you perform eye exercises because your eyes don’t work well together. Or you may get symptoms when doing close work, such as headaches, eye strain, double vision or losing your place, and wonder if it is to do with your eyes.
We have just finished a large study of university students, none of whom thought they had a problem with their eyes. Many had “symptoms” when doing close work, and many also had what professionals would call “a clinical problem” – but they were rarely the same person! Many people with symptoms didn’t have anything wrong with their eyes, and many people with a clinical diagnosis didn’t have any symptoms!
A common symptom questionnaire was very bad at picking up genuine problems, but over-referred a lot of people with normal eyes.
If you get eye strain symptoms it’s a good idea to get your eyes tested, but there are many reasons for difficulty with close work, or headaches, that have nothing to do with your eyes. These include working when with poor posture, when tired or in poor light – but also may be because you are doing difficult or boring work, or you may have additional difficulties such as dyslexia.
There are many different types of eye exercises for eye strain. The most common are very simple – just practicing looking at very close things little and often for a few weeks, but some are much more complicated. We wanted to see whether the more complex ones did a better job, and what type of exercises actually changed how the eyes worked.
A bit to our surprise, the best exercises were the simplest ones. We also had two “control groups” who didn’t do any exercises before their second testing session. One of these control groups were really encouraged by their tester the second time, and this group did even better than the people who had been practising eye exercises for two weeks. This shows us that a good therapist to show you what to do, trying hard and doing simple exercises are probably the best combination.
Eye professionals often see patients with no organic eye diseases, but whose focusing seems to have “got in a muddle”, so they suffer from blurred or double vision. Focusing is usually an unconscious process and it can easily start to go wrong by trying too hard or thinking about it too much. This is very commonly due to a stress- or anxiety-related cause such as school or social issues, bullying, bereavement, or dyslexia. We have been working with Dr Polly Waite at the Anxiety and Depresssion in Children and Young People (AnDY Clinic) at the University to develop some simple resources to explain what is going on and help these problems get better as quickly as possible
If you or your child has focusing problems that you have been told might be anxiety-related, you might find this information helpful Information about anxiety-related eye problems