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The Adaptive Eye

by Peter Duthie(more info)

listed in vision and eye sight, originally published in issue 34 - November 1998


The human eye is believed to have evolved out on the plains of Africa, with distant horizons, bright natural light giving a small pupil size (and without paperwork), it could keep itself perfectly adapted to its environment. Today we spend our time indoors with relatively poor light giving a larger pupil; a major part of our day is spent focusing closely at paperwork or a computer screen and any focusing difficulty is immediately 'corrected' with prescription lenses. These factors conspire together to deny our eyes the opportunity to recover on their own. A little understanding of the processes involved can help us to guide our eyes back towards normal unaided vision

The Anatomy of Focusing

The purpose of the eye is to form a clear image of the object being viewed on the retina, and then to pass this to the brain for processing.

The eye, like a camera, does this with lenses. The cornea, the clear window at the front of the eye, does most of the focusing work. The light passes through the pupil which controls the amount of light entering the eye and is then focused further by the lens inside the eye. This lens is flexible and can be made fatter or thinner to give more or less focusing power to view near and far objects. To bring a distant object into focus the circular ciliary muscle relaxes against the wall of the eye, pulling the suspensory ligament to stretch the lens into a flatter, thinner form. For close focusing the ciliary muscle contracts, slackening the suspensory ligament and allowing the lens to return to a fatter, more rounded shape.

Human eye and its parts

Finding the Focus

To find the focus the eye uses two cues. The brain can quickly bring both eyes to bear on the object of interest so that the images from both eyes join up, or fuse together. The brain can tell the eye roughly where to focus, from how much it had to pull the eyes together to fuse the images. Secondly, the eye moves the focus back and forth to find the maximum contrast, thereby bringing the image into focus. Try this experiment: hold a finger in front of your eyes as close as you can comfortably focus, now look at a far object and then look back at the finger. Notice how long it takes to come into focus. Now repeat the process using one eye only. If your vision is balanced between both eyes you will notice how much slower you focus using one eye on its own.

Presbyopia: Small Print Problems

The lens in the eye is built up in layers, like an onion. New layers are constantly being added to the outside, compressing and hardening those in the centre. This results in a gradual stiffening of the lens over time. The ciliary muscle grows stronger to compensate for this, doubling in size between the ages of 20 and 40. Unfortunately, as we do our close work in relatively poor light with enlarged pupils, the eye is unable to focus as well as it might and the ciliary muscle does not get as much exercise as it needs. The growth of the ciliary muscle falls behind what is required for the closest focusing. This 'near point' gradually moves further away until by the mid-40s it starts to interfere with reading small print. This is called presbyopia. The conventional treatment for presbyopia is to place a lens in front of the eye to add the extra focusing power that the eye is unable to provide. This immediately brings print into focus but also leaves the ciliary muscle less work to do. With habitual use of reading glasses the ciliary muscle gets less exercise and starts to get weaker, further reducing the ability to focus close up.

What to do about Presbyopia

The ciliary muscle needs more exercise rather than less. It is not under conscious control so we have to find a way of encouraging the eye to focus itself more fully. The answer is to provide an artificially small pupil (a pinhole) in front of the eye, to help the focusing system work more fully. This is the principle behind Trayner Glasses. These have an array of small holes through black screens that replace the lenses in a pair of glasses. By reading with the aid of Trayner Glasses for fifteen minutes or so every day, the ciliary muscle is exercised and grows stronger. Vision is clearer while using Trayner Glasses because the ciliary muscle and lens are focusing beyond their habitual range. The unaided vision improves gradually over a few weeks as the muscle gets stronger with the extra use.

The Anatomy of Focusing

Myopia, or short sight, is the condition where the range of focus of the eye is shortened, making it unable to bring distant objects into focus.

Some people are genetically more predisposed to myopia than others. Having two short-sighted parents will give a strong risk of developing myopia. The way an individual uses their eyes, as well as their genes, will determine how, or even if, they will become myopic. There are two processes involved in the development of myopia: emmetropization, which controls the growth of the rear of the eyeball and adult onset myopia, which controls the stretching of the central part of the eyeball.

Emmentropization: Moving Towards Normal Vision

While the eye grows, hormones control the growth of the rear part of the eye. These growth hormones are released in response to the degree of blur experienced when viewing a distant object. If the distance is too sharp then the eye deems itself too long sighted and growth is increased, lengthening the eye and shortening the sight. If the distance is too blurred then short sight is assumed and growth reduced at the back of the eye while the rest of the eye continues to grow, thereby lengthening the sight. This is called emmetropization, that is, seeking emmetropia, or normal vision.

While the eye is growing, up to around 14 years old, it will tend to reduce refractive error (focusing problems) provided it is seeing the world directly, without lenses. Unfortunately this is rarely allowed to happen. The tendency has been for any refractive error measured in a child to be 'corrected' using lenses. The eye is then left to adapt to the world it sees through the lens, dutifully changing its shape to cancel out the prescription within a year or so. This results in stronger and stronger lenses being prescribed and is called progressive myopia. This tendency can be kept in check by allowing the eye to see the true distance (albeit blurred), without glasses and out of doors, even for just one or two hours a day.

Adult Onset Myopia

To bring near objects into focus the circular ciliary muscle contracts allowing the lens to fatten providing the extra focusing power required.

The ciliary muscle has two parts: the circular fibres controlling the lens and fibres that run along the eyeball itself spanning the part of the eye between the ciliary body and the beginning of the retina. These longitudinal fibres work in opposition to the circular fibres so that when the circular fibres are relaxed for distant focusing, the longitudinal fibres are tensed and when the circular fibres are tense for close focusing, the longitudinal fibres are relaxed. These longitudinal fibres have two functions: they help to tense the suspensory ligament for distant focusing and they control the elongation of the eyeball.

The net result is that, in response to prolonged close focusing, the eyeball lengthens, shortening the sight. The eye gets better at the work it is used for. Unfortunately, as it gets better at doing close work it may lose the ability to focus the distance, creating myopia. This process is known as adult onset myopia although it can happen at any age.

Once glasses are worn to bring the distance back into focus, the eye must work even harder overcoming the prescription in order to focus close up. This results in the myopia continuing to increase. This will happen to anyone who continues to do a lot of close work through a distance prescription.

What to do about Myopia

There are various vision improvement systems for reducing myopia which have a common theme; relaxation of the eyes as well as the use of weaker than normal prescriptions. This makes sense, as both will help the eyes to be more distance focused. Many vision improvement practitioners now use Trayner Glasses alongside their traditional exercises. The artificially small pupil provided by the Trayner Glasses encourages the eyes to focus beyond their habitual range, giving a more active encouragement to distance focus than is possible with the passive relaxation techniques.

Use Trayner Glasses for at least half an hour at the end of the day to focus further away than you normally can (take out your lenses first).

For some this will be reading, for others watching TV. This will leave your eyes more distance focused overnight.

Avoid using your distance prescription for close work. For students who alternate between looking at the blackboard (distant) and their notebook (near), some opticians will now prescribe bi-focals. For office workers, a reading prescription will stop the deterioration. This will be slightly weaker than your distance prescription allowing your eyes to stay more relaxed while you do close work. Wear this prescription as much as you can do safely. Use your distance prescription for driving. If you wear contacts you can wear the weakest reading glasses from the chemist over your standard lenses to make a reading prescription.

Trayner Glasses

Trayner Glasses

Hyperopia: Long Sight

Long sight is the opposite of short sight. The eyes focus further away than normal and are unable to focus close up. In milder cases it can be compensated for by the ciliary muscle working harder. A consequence of this compensation is that the effects of presbyopia become apparent earlier than normal when the ciliary muscle struggles to keep up with the stiffening of the lens. As with presbyopia, the conventional glasses worn to give extra focusing power for close work prevent the eyes getting the exercise they need to get better.

What to do about Long Sight

Encouraging the eyes to focus closer on their own will help them adapt, the sight gradually changing towards normal. Read with Trayner Glasses for a few minutes a day, starting with perhaps five minutes a day and building up to half an hour or more. Initially this will just be strengthening the ciliary muscle to cope with the extra work, but soon the eyeball will start to adapt and get longer until it loses its long sight.

Unbalanced Eyes and Eyestrain

Some people have eyes that focus differently to each other. If the eyes cannot work together the world will be perceived without any depth.

If the imbalance is mild the only symptom may be a tendency to get headaches or eyestrain when reading. This is basically a sign that the brain is having difficulty merging the images from the two eyes together, one is sharp the other fuzzy. The brain tenses the muscles around the outside of the eyes as it tries to steady them to get better images. The muscles start to ache after a while. To resolve this problem use Trayner Glasses to read for five minutes with just the weaker eye and then for another 10 minutes with both eyes. Repeat this daily to strengthen and encourage the weaker eye and to encourage the brain to use both eyes together.

In some cases the imbalance is sufficient for one eye to be long-sighted and the other short-sighted. The eyes have become specialised. The brain uses one eye for the distance and the other for close work. In these cases it is necessary to exercise the eyes individually. Use Trayner Glasses, covering the short-sighted eye while reading with the long-sighted one and covering the long-sighted eye while watching TV with the short-sighted eye. In addition, it is important to spend time using both eyes together through the Trayner Glasses. Over time the eyes will move back towards balance and start working together giving depth to the world.


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About Peter Duthie

Peter Duthie has been researching the mechanisms behind vision changes since joining Trayner Pinhole Glasses in 1990.

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