Binocular Vision

Binocular vision is at the core of everything an optometrist does.

Although sight accounts for just 20% of the five major senses (the others being hearing, touch, taste, and smell) about 80% of the way most of us experience the world is through vision.

In order to navigate this world we need to be able to identify objects, track them and perceive their depth (3D vision). In order to navigate school we need to be able to learn to read so that we can assimilate information easily and accurately.

Most people assume that all vision problems can be corrected with spectacles or contact lenses. But a surprisingly high number of children (with and without spectacles) have functional eye disorders. A functional eye disorder is where a little piece (or more than one piece) of the visual pathway doesn’t work or works inefficiently. To a child this may appear as blurred vision, difficulty tracking along a line of text, problems remembering what they’ve read, being a slow reader, difficulty catching or hitting a ball, headaches, or double vision. To an adult such as a teacher, the child might appear to be a slow worker; they might mix up words when they read; they might try to avoid close-up work like reading or writing altogether; they might become anxious or angry, stare out of the window, get up and wander round the class or distract other children.

These children are frustrating to many optometrists. In our consulting room they may appear to have great vision, reading the bottom line of the chart with ease, or conversely they may struggle to read more than half way yet when we examine them we can find nothing that requires them to wear spectacles. These children are also frustrating to their teachers, who are only too aware that they are underperforming without a seemingly good reason. And they are frustrating to their parents who are worried that their child has been labelled a ‘problem’. At the centre of this is a frustrated child, rapidly losing confidence and feeling a failure compared to their peers.

This is where a specialist binocular vision examination can be very useful. Binocular vision disorders are extremely common, but often intermittant, causing symptoms some of the time in some situations but not all of the time in all situations (at least in the early stages).  Because of this these issues can be missed if they aren’t looked for specifically.

Binocular vision disorders may be helped with spectacles, contact lenses or vision therapy (kind of physio exercises for the eyes), or any combination of these. Please note that in spite of claims made in certain corners of the internet none of these solutions are a cure for dyslexia or ADHD and they are definitely not a magic bullet (though the results can seem like magic sometimes). The aim of any vision appliance or therapy is purely to enable child using their visual system in such a way that clear, comfortable, accurate vision is possible. Many children with dyslexia or ADHD may also display or develop an underlying binocular vision problem, the symptoms of which exacerbate the problems they are having with the dyslexia or ADHD! When the binocular vision problem is fixed the dyslexia or ADHD appear to improve, but in reality we’ve simply removed one problem, so they now have more energy and resources to devote the other problem.

Seeing Monsters

Mrs. C has been a patient of mine for some years. She has advanced Age-Related Macular Degeneration (AMD). She came to see me before Christmas and mentioned that she felt uneasy in crowded places. “It’s hard to explain…I was in the supermarket and suddenly felt quite threatened. I panicked and had to leave.” It’s a complaint I’ve heard numerous times in various forms over the years. Patients with AMD frequently complain of what can only be described as visual unease, especially around lots of people or activity.

Before I go on, it’s important to know that AMD affects only the central area of the retina. In this area mostly one photoreceptor cell (the little light receptors known as cone cells because of their shape) is attached to one nerve fibre, which then transmits the information from that one cell to the brain. Because of this one-cell-to-one-nerve-fibre relationship the ability of a healthy central area of retina to resolve detail is phenomenal. In the other 90% of the retina each single nerve fibre transmits information from many photoreceptor cells (the little light receptors known as rods, again because of their shape). Mostly this peripheral area of the retina is about light collection and motion sensitivity so it makes sense for the receptor cells to pool their resources, but it does means that the peripheral retina is much less good at resolving detail, such as that required for facial recognition.

The human  brain is finely evolved to seek out human faces. If there is just a single face in a large painting then this is what we home in on. We can pick out the faces of those we know well even in a large crowd of strangers. There is even a specialist area of the brain called the fusiform face area which is predominantly tasked with seeking out and recognising human faces. AMD destroys the central retina and deprives the patient of this ability so I always reasoned that may be why they feel so uncomfortable.

Recently I came across an elegant demonstration which may shed even more light on why patients with AMD feel this way (and not just AMD but anyone who has lost their central vision).

You’ll need to click on the image as I could find no way to embed the demo into the post. Follow the instructions on the screen. It’s a rather disturbing but extremely compelling bit of visual psychology. I think it illustrates beautifully why someone with central visual loss, presented with rapidly changing faces, as in a supermarket at Christmas, or at a large social gathering may indeed feel disorientated, uneasy and even panicky.

Seeing Monsters



Just recently a child asked me whether carrots really were good for your eyesight! From the meaningful look her mum gave me I deduced that vegetables were something of a battle ground. I sympathise…one of my children is a vegetable refusnik too.

And the answer isn’t competely straightforward. Carrots won’t improve your visual acuity if you need spectacles. A diet rich in carrots won’t correct blindness or allow you to ditch your glasses. But carrots are immensely rich in the vitamins which promote overall eye health. They contain beta-carotene, a substance that the body converts to vitamin A and which is important in the production of retinol, which in turn is important in the production of the biochemicals which allow us to see in dim lighting conditions.

So yes kids, carrots are good for your eyesight, but so are liver, milk, eggs and fish oil.

And to finish here is a poem by the late great Shel Silverstein (I am a huge fan!)

Shel Silverstein Carrots

Hallucination, Illusion or Misinterpretation?


Most people viewing this pattern feel visually uncomfortable and disorientated because it’s designed to make you feel that way. Now imagine feeling like that all the time, as though every surface has that pattern on it. To a person with dementia changes in colour, or shadows on a path,  or the pattern on wallpaper can produce the same effect. The world can be a visually terrifying and uncomfortable place for them. Continue Reading →

A Question of Ethics

Digital illustration of a dna


Ok this isn’t anything to do with visual psychology, but medical and scientific ethics is also something I think about quite a bit both in relation to medical funding issues and also with regard to the question of how much it’s ‘right’ for us to interfere with nature.

Obviously because of my work the area I’m most interested in relates to vision, and recently I’ve been reading a lot about inherited eye disease. Continue Reading →

Oliver Sacks

Oliver Sacks

I was saddened to read of the death of the wonderful Oliver Sacks today. He announced in February that he had terminal cancer. He was a brilliant neurologist and speaker and I enjoyed his books and lectures. Visual hallucinations are very common among the mainly elderly patients I examine in practice. Often these patients are too frightened to admit to them unless questioned directly. The perception among the general public is that hallucinations = going mad. In fact some 10% of visually impaired people experience hallucinations and these have a name; Charles Bonnet Syndrome. Oliver Sacks himself was visually impaired and experienced these hallucinations first hand. Continue Reading →