Chapter 1
Four Things to Know About Vision
An introduction to some interesting and important concepts about eyes and vision. For instance, did you know that only a small part of your vision sees detail? Do you know what 20/20 means? Read on…
And for vision professionals, a selection of … let’s call them ‘conversation-starters’, some points to get you thinking.
Vision is a fascinating and complex sense. We get about 75% of all our sensory information through our eyes. But few people really understand how we see what we see.
We can’t begin to understand vision impairment if we don’t understand how normal vision works, so we’ll go through it all in more detail later on. But to begin with, I’ll give you four key facts about how vision works. We’ll cover them all in more detail later on, but they are so important I wanted to give you an introduction to them straight away.
Fact #1: Only a small part of our vision sees fine detail
We tend to think of our vision as being all clear. We look straight ahead and we see clearly, with good detail. We look way over to our right and it’s clear. We look to our left, look up, and look down… everywhere we look, it’s clear vision.
But in fact, most of our vision is not very good at seeing detail at all. It wouldn’t really be correct to say it’s ‘blurry’, but the fact remains that there’s just one small area that has really good fine detail vision. It’s called the macula, and it’s the bit that we use when we say we are ‘looking at’ something. It’s critically important though — so important that about half of the vision processing part of our brain is devoted to just that one small spot.
Try this experiment. Look at the X in the line below. Without shifting your gaze, how many of the letters on the sides can you identify? In particular, can you tell what the middle letter in each group is?
BGRPN — X — HKZPF
You’ll notice that as soon as you think “I wonder what the middle letter on the left is?”, your gaze immediately wants to shift towards it. That is, you want to change the position of your eye to the left, so that the image of the R (instead of the X) lands on your macula.
But that’s cheating. You need to keep looking at the X. The way our visual system is wired, if we turn out attention towards something, we automatically point our macula towards it. If you can suppress that reflex and keep your gaze steadily pointed at the X, you’ll notice that it’s really hard to tell what most of those other letters are.
So, why don’t we notice that only one bit of our vision sees fine detail? It’s because that linkage between attention and our eye movements is so tight. As soon as we turn our attention towards something, it’s natural to look towards it. It’s as if we are in a dim area, but we have a torch (flashlight) that we shine around to build up a detailed impression of the world around us.
What’s over there?
It’s as if we are in a dim area, but we have a torch (flashlight) that we shine around to build up a detailed impression of the world around us.
Fact #2: Our non-detail vision is just as important
Detail is important, but it isn’t everything. Context is just as important.
If you lost all your vision except for the macula, you’d struggle to get around safely, you’d find it very hard to find anything you’re looking for, and there’s a lot that you’d just totally miss seeing. In that case it would be more like you were in a pitch-black cave rather than just a dim area, with a very tight-beam torch. With such a tiny area of light, you have to move slowly and scan the light around very actively to figure out your surrounds, and if you’re looking for something small it would be very easy to miss it.
We call the totality of our whole area of vision our visual field. The middle bit is the macular field, and then all the rest is… well, it’s not really accurate to call it our peripheral field, because that refers to just the areas well out to the sides, so let’s just call it our general field.
Macula? Or Macular?
It’s a bit like the difference between circle and circular. The macula is the part of the eye that does the detail vision. Things that relate to the macula are macular. So degeneration of the macula is macular degeneration, and the part of the visual field that is seen by the macula is the macular field.
The main job of the general field is to tell us roughly what’s around us. While I’m sitting here writing this, I’m looking at the screen (so that’s where my macular field is pointed), but I can still see in my general field that my daughter is sitting at her laptop writing an essay, my wife is walking across the room to my left, and there is a large green couch over to my right.
At least, I think that’s what’s there. I can’t be 100% sure that the person who walked across the room isn’t another woman of about the same size and hair colour as my wife. I think my daughter is doing her essay, but actually I can’t tell for sure that she hasn’t switched to looking at her phone. I’m assuming the big green rectangular thing is the couch, but if it had suddenly been replaced by a big shrub of about the same size and colour I probably wouldn’t notice.
None of those things seem very likely (except for my daughter switching to her phone!), but I couldn’t tell for sure that they weren’t true unless I shifted my attention to them so that my macular field pointed at them, at which point I would be able to tell for sure.
But the point is that my general field gives me a really good overview of what’s around me. Then I can move my macular field around in an efficient manner, jumping accurately from object to object.
Importantly, the general field is particularly good at seeing spatial information (how objects related to each other in space) and extremely sensitive to motion. This is how we move around without (mostly without) bumping into things or falling over. It’s also the part of our vision that will detect immediately that there is a tiger jumping out at us from those bushes to our right. Or, to be more precise, it won’t be able to tell us it’s a tiger — that’s a job for the macula — but it will tell us it’s a thing, it’s big, it’s fast, it’s on a collision course with us, and we’d better duck right now.
This need for speed is one of the advantages of the general field not carrying a lot of detailed information. Carrying a lot of information takes time, but for survival we want the message to get back to our brains in the fastest possible time. The general field does a great job of keeping a constant flow of just the essential information to keep us alive.
Fact #3: 20/20 doesn’t mean perfect vision
There’s a lot of misconceptions about this phrase. People generally think it means they have great vision — as it it’s an exam score, 20 out of 20, full marks, gold star.
But it’s actually an example of a visual acuity measurement, which just tells us how well your very best bit of vision can see individual fine details. Nothing else. It doesn’t tell you how wide your field of view is, or how well you can see subtle colours, or how sensitive you are to seeing things moving, or how well you see at night, or even how well you can see to read or recognise faces. It only tells us how small is the very smallest detail you can see. It’s quite possible to have 20/20 vision, but still have very significant vision impairment.
It’s quite possible to have 20/20 vision, but still have very significant vision impairment.
20/20 just means that you can read the ’20’ size of letter on the eye chart at a distance of 20 feet. The 20 size of letter is called that because it’s the size that a normal eye should just be able to read at 20 feet. So to rephrase, 20/20 means that you can see the amount of detail that a normal eye should be able to see.
If your detail vision’s a bit worse than that, you might have 20/30 vision — that is, you can’t read the ’20’ sized letters, but you can read the larger ’30’ sized letters, the ones that a normal eye should be able to read from 30 feet away. 20/200 means you can read at 20 feet what a normal eye should be able to read from 200 feet away.
20what?
Nowadays, most of the world works in metric, so we use a six metre reference distance instead. That means instead of 20/20 we use 6/6 to indicate ‘normal’ vision. By the same logic, 6/60 is the equivalent of 20/200.
That very best bit of your eye that you use for reading the chart is the very centre of the macula, which is called the fovea. Just like the macula is the spot in the middle of our general field that gives us detail, the fovea is in the middle of the macula and gives us the very finest detail. It’s kind of the macula of the macula, I suppose. And just as one of the jobs of the general field is to give a rough overview of the world for the macula to rove around, one of the jobs of the macula is to pick up a rough overview of the finer details for the fovea to rove around.
The even-smaller-spot
Just as there are the words macula and macular, there is fovea and foveal. So the part of the visual field that is seen by the macula is the macular field, and the part of the field that is seen by the fovea is the foveal field.
Fact #4: Fluent reading can be devilishly difficult to restore
If you’ve got a vision impairment and you are having trouble reading text, it’s relatively easy to restore your ability to read slowly. We can use magnifiers that go up to really high magnification, and they are generally quite effective in letting you accurately read even small text.
But when it comes to reading quickly for longer periods of time — that is, the sort of reading that we’re doing when we say we’re ‘reading a good book’ — it’s a whole different story. It can be exceedingly difficult to restore that sort of reading. For many people, reading books is one of their main joys in life, and anything that makes reading harder has a very big impact on their quality of life.
Why is it so hard to fix?
We’ll go into that in great detail later on. But to get us started, let’s talk a bit about how we use our visual system to read quickly.
Slow, accurate reading is not much of a challenge for our visual system. It can be as simple as ‘look at the first letter and identify it, then look at the second letter and identify it, then the third, then the fourth…’
And that’s totally valid, it works, it’s reliable. Having that ability to accurately read text is really important for retaining our independence.
But that sort of reading isn’t comfortable and fluent — quite the reverse, it’s slow and laborious. Many younger readers, who have grown up in the time of computers and spell checkers, will never know the bone-crushing tedium of proof-reading a long essay — having to look at every letter in every word, checking for spelling mistakes. It’s really hard work. (I don’t miss it).
In contrast, book reading — fluent, fast, accurate, comfortable reading, pleasurable reading in which you can get immersed in a story — that’s a miracle of the visual system. The key feature of this skill is speed, the need to keep up a certain pace. If we can’t read reasonably fluently, it doesn’t feel like ‘reading’, it feels like a chore. Think of the way we listen to music — sure, you can listen to your favourite song on half speed, and technically you are ‘listening to music’, but it really isn’t a satisfying experience.
Many good readers can (silently) read more than three hundred words per minute. It’s impossible to look individually at each one of those letters so quickly. How do we achieve such speed?
Many avid readers will (silently) read more than three hundred words per minute.
We’ll talk about this wonderful skill more later on, but for now, just understand that it’s a high-performance skill, one that needs all the components of the visual system working together in harmony, just like all the instruments in a symphony orchestra.
Let’s carry that symphony orchestra analogy a little further. If all the players are skilled and working with their fellow players, the music is a thing of beauty. But if there’s even one player that faked their CV to get the job, and really doesn’t know how to play well, the music’s going to start sounding… less than beautiful. And if there are several players (or even a whole section) that aren’t playing well, it’s not going to be an enjoyable experience.
That need for all the bits to be working together is why trouble achieving fluent, comfortable reading is often one of the earliest functional problems noticed by many people with low vision. It’s also why it can be one of the trickiest problems to fix. Still, understanding what’s going on is half the battle. We’ll cover this in great detail in Part 3 of this site.
Key point: high-fluency reading is a high performance task that uses all the components of the visual system to their maximum. This is why many eye conditions impair reading fluency quite early on.
What we’ve gone over here are some important highlights to help you understand how we use our eyes to construct this marvellous sense we call sight. Next we’ll turn to a deeper understanding of the eyes, their structure, what can go wrong, and what we can do about it.
But for those of you who are vision professionals, the page has only just begun! The following are some topics that I think serve well as an introduction to the Professional content in this site. Consider them tasters, or conversation starters. I hope they’ll get you thinking a bit more deeply about vision impairment and how we professionals think about rehabilitation.
Content for Vision Professionals
Don’t believe the VA — click here for more detail.
Don’t — Don’t — Don’t Believe the VA
(Title with apologies to Public Enemy)
Keep in mind what VA is measuring. It’s not a measure of your patient’s overall vision, it’s a measure of their very best bit of vision. That is, in normal circumstances it’s a measure of foveal function.
Lots of eye pathologies can do a lot of damage to vision but spare foveal function until quite late in the disease. The obvious ones are those that take away vision from the outside inwards, like retinitis pigmentosa. But there are other examples we see all the time, like macular degeneration and glaucoma. For instance, it’s really common in geographic atrophy to be the very last part of the macula to go — a patient can be in the very late stages of the disease, with terrible loss of macular field, and still have a VA of better than 6/12.
Here’s the thing to keep in mind: if your patient has poor VA, that tells you their vision is definitely bad. But if they have ocular pathology but good VA, don’t fall into the trap of assuming that means they have good vision. Instead, listen to what they are telling you about how they are seeing, and don’t tell them they’re seeing well when they’re not.
There will be a lot more on this later.
“If they have ocular pathology but good VA, don’t fall into the trap of assuming that means they have good vision.”
Content for Vision Professionals
There might be more to brightness and contrast than you think — click here for more detail.
Task Lighting Doesn’t Improve Text Contrast
In low vision work, we’re know that most of our patients have impaired low contrast vision, which is particularly relevant with tasks like reading newspapers (which tend to have relatively low contrast print). And we’re also know that giving them brighter light on their newspaper tends to help them read a whole lot easier.
But there’s a common misconception that the reason extra illumination on the newspaper helps is that the extra light has boosted the contrast of the print. It’s not true — putting more light on the page does make the paper brighter, but it makes the print brighter too, by exactly the same proportion, so the contrast remains the same.
It’s easy to see why we’d think the contrast has improved. I mean, brightening up a dim page of text certainly makes it look more… well, ‘contrasty.’ Check out this example.
The progression along both lines looks similar. But A, B, C and D all have the same contrast. The background gets brighter, but so does the text — in each step the brightness of both text and background doubles. All the text on the top line is 80% contrast.
In contrast (geddit?) W, X, Y and Z show increasing contrast. The background gets brighter, but the text stays the same. That means the ratio of the background to the text changes, which means the contrast really does increase.
Do you know a way to illuminate the paper but not the text like this? No, neither do I.
In an example like this, with high contrast print, the difference between increasing illumination and increasing contrast doesn’t seem very meaningful. The difference is more obvious when we consider documents with low contrast print. Look at this next example, which has a constant 20% contrast across the top line. Again, A and W are the same, but on the lower line the text stays dark while the paper brightens up.
This time it’s quite obvious that now matter how much we brighten up the low contrast text, nothing is ever going to make it look high contrast.
Still, even though the contrast isn’t changing, it definitely becomes easier to see. Why?
Well, because it’s brighter — that’s a thing in itself. Retinas function better with better light.
There will be a lot more on this later on. A whole lot more.
Content for Vision Professionals
Reduced reading fluency is a common early complaint — click here for more detail.
Reduced reading fluency is an early complaint
This is going to be covered in very great depth later, but just to get you thinking on it:
I find it useful to think of my patients as being on different positions on two basic spectra:
- The first is whether their condition, pathology, disease is mild, medium or severe.
- The second is whether their demand/desire for fluent reading is low, medium or high.
People who have a low need for fluent and comfortable reading tend to present for help later in the disease process. They only tend to run into significant trouble when their vision is starting to interfere with core tasks of daily living and independence, and basic reading of even larger print. Earlier on, if their reading of print is slower, that might be annoying, but it doesn’t tend to block them from doing what they want to do. And a bit later, if they have to buy a basic magnifying glass from the shop to read print, well okay. It’s only when they are struggling to read at all — even with the magnifier — that they tend to come in needing help with the essential tasks.
On the other hand, those people who are bookworms — habitual readers for whom doing large amounts of reading forms a large part of their life enjoyment — they struggle early, and they’ll be in your chair early, because reading fluently and comfortably is a high performance task, and often gets affected by relatively early changes in the macula (way before the fovea gets affected, so don’t get fooled by the fact that their VA is still pretty normal).
If you’re an optometrist in primary care, you’ll be familiar with the patient who turns up with about eight pairs of reading glasses prescribed by eight different optometrists, and “None of them work!” They want you to prescribe the glasses that work. Their corrected VA is fine — 6/6, maybe even 6/5. “No problem,” you think, “This should be easy.”
But then you check the lens power of the other glasses, and they’re all pretty much what you were just about to prescribe. Given a reading chart, your patient can read even very small print, slowly. But crucially, they’ll still be slow and uncomfortable even on the larger print. The common rule about reserve — that if they can read N5 slowly, they’ll read N12 fluently and comfortably — doesn’t seem to hold.
What are you going to do? Are you going to be the ninth optometrist to prescribe them a ‘correct’ pair of glasses? What’s really going on?
Sure, there’s a range of possible explanations, but the really common one is that your patient has something (early AMD? Supposedly preperimetric glaucoma? Trace diabetic macular oedema? Undetected macular hemianopia?) degrading their macular field integrity, most especially if it’s the area of the macular field just to the right of fixation.
We’ll get seriously into all this in Part 3 of this site.
Content for Vision Professionals
Maybe not all our magnifiers are as good as we think they are — click here for more detail.
Some Low Vision Aids Just Aren’t Suitable for Reading Books
Have you ever given a magnifier to a patient to try, and they use it to slowly read some newspaper text, and you feel proud of yourself, but then they turn around and say something like “But I couldn’t read with this.” And you think “What do you mean? You just did — I heard you.”
The classic explanation for this is that the patient is not yet adjusted to their vision loss — that they are looking for vision restoration, not rehabilitation, and so they are simply not emotionally ready to accept doing things a different way.
I think that sometimes that’s true. But sometimes, it’s a sign that the solution we’ve proposed isn’t as ‘fit for purpose’ as we thought it was, or that we’ve got a communication breakdown.
I’ve noticed that most of the time people saying this are people who miss reading books — that is, they are searching for a solution that will let them read comfortably and fluently. The truth is, some magnifiers are very effective for reading slowly and accurately (spot reading) but are simply impossible to use for fluent and comfortable reading (by anyone, not just this patient).
It’s important to communicate carefully and precisely to make sure that you understand what the patient is meaning when they say ‘reading’ in order to find a successful solution. It’s also important to have a clear understanding of which magnifiers have potential to deliver that success and which ones aren’t even worth trying.
We’ll get seriously into all this in Part 3 of this site.
Content for Vision Professionals
What mindset do you need to be a low vision practitioner? — click here for more detail.
Keep a Rehabilitation Mindset
Many vision professionals reading this will be primary care optometrists or opticians, dealing mostly with healthy people — giving their patients clear vision with lenses of one sort or another, or detecting eye disease and referring them to (hopefully) be ‘fixed’ by ophthalmologists. Other readers might be those secondary care ophthalmologists, dealing mostly with people who have an eye disease, and (hopefully) ‘fixing’ them with medications or surgery, or halting (or at least slowing) their loss of vision.
Low vision work is different. It’s tertiary care, where ‘improving vision’ has been pretty much been taken off the table. I mean, sure — if you can improve their glasses, do it; if they have untreated disease, treat it. But the emphasis in low vision is rehabilitation — finding ways to help the person find ways to achieve what they want to do even though you can’t improve their vision.
Three key things to keep in mind are:
- Your patient needs to understand that your role isn’t to improve their vision.
- You need to understand that you’re not expected to do the impossible.
- Instead, your role is to be their expert guide.
Think of a mountain guide. A client comes to them — they used to live in a city on the far side of the mountain range, and they would like the guide to take them over the range back to where they used to be. Can they do it?
Sometimes the answer is a simple yes. There’s a clear path over the range, and the mountain guide can use their expertise to guide them there. That’s a clear success.
Sometimes the answer is a simple no. There’s no way across that mountain range. Sure, it’s disappointing, but that’s still a success — the guide’s expertise has been valuable to the client, because they can stop spending time and energy on something futile, and put their efforts towards something else, something achievable. A good guide might even be able to suggest an alternative destination that the client might enjoy, somewhere easier to get to.
But often, the answer is a maybe. It might be possible, but with conditions.
- A path that most people would find easy might be more difficult or impossible for that client because they have other cognitive or physical impairments. Is there an alternative path? What difficulties are on that path?
- Perhaps a path might be possible, but travelling it would require specialised equipment. Is the client prepared for the expense involved?
- There might be a path, but it’s going to be hard going. Is the client prepared for that? Or were they really expecting an easy stroll?
The guide’s role here is to use their expertise to do their best for the client, in a range of ways.
- To expertly assess their client’s capabilities.
- To know about the equipment available.
- To be prepared to search for alternative paths.
- To communicate with the client about the difficulties involved.
In essence, the guide’s job is to be an expert pathfinder, using their expertise and creative thinking to give their clients the very best chance of succeeding in getting to their desired destination. The aim is always to find a viable path and deliver the client safely there. But if a path can’t be found for that particular client under those particular circumstances, that doesn’t mean the guide is a failure.
In primary and secondary care, criteria for success are generally a lot more straightforward. Vision professionals moving into rehabilitation sometimes struggle with the shift of goalposts, feeling pressure to ‘fix’ their patients. So keep your mind clear on this.
Be the expert pathfinder. If there is a path, help your client find it. But expert pathfinders know that sometimes there’s no way over the mountain.
