Understanding Vision Impairment
2.3: When things go wrong with the vision part of the brain
In the end, all seeing is done by the brain. All hearing, all tasting, all touching, all smelling — it’s all just nerve signals, that get processed within the brain and lead to the conscious and unconscious experience of those senses.
Key point: If there is a problem with the vision parts of the brain, it tends to mean a problem with just one side of our visual field, not one eye or the other.
You might already know that if one side of our brain is damaged, it will cause functional problems on the other side of our bodies. For instance, if someone has a stroke that affects the left side of their brain, it might cause weakness or paralysis of their right arm and right leg.
With vision, it’s different. Damage to the vision centres in the left side of the brain will cause loss of vision in the right eye, but only to the vision that is on the right side of the visual field (that is, all the vision that is to the right of the fovea). Whatever vision is to the left is left untouched.
But it will also affect the right side of vision in the left eye as well. So the overall effect is that a person doesn’t lose vision in one eye or the other — instead, they lose all the vision on one side or the other, in both eyes. We call that pattern of vision loss a hemianopia or hemianopsia.
This quirk is because the two optic nerves meet behind the eyes, but from there all the nerve fibres from both eyes that are carrying information from the right side of vision connect back to the left side of the brain, and vice versa.
Depending on the degree of damage to the brain, the amount of visual field on the one side that’s lost might not be 100% of that side, but the split is right down the centre (we call that ‘respecting the vertical midline’). Note that it is also possible to get some loss of visual field on the other side, if the brain injury affects both sides of the brain. In the very worst cases, both sides of the vision part of the brain are badly damaged, and so both sides of vision are blind — this is called cortical blindness. But mostly people have loss of vision on just one side.
The effect can be very different from person to person. Many have a strong sense that there is something wrong with one eye, because of their inability to see to that side, and it can be hard to convince them that both eyes have half their vision remaining.
The general field is there to tell us where everything is around us, and keep us safe. So a substantial loss of vision on one side leaves a person vulnerable to any hazards on that side. People might keep bumping into things they don’t see on that side, even big things like people and doorframes. It’s a big falls risk too, since it’s very easy to trip over things that are on their blind side, or accidentally step off gutters or into holes. Loss of vision to the right side can have a devastating effect on reading fluency (more on that in later sections).
This sort of vision loss can’t be fixed. The main thing is to be aware of it. If you have that understanding (we call it insight), then you can compensate for it by frequently looking towards your blind side (we call it scanning), just the same way all of us need to turn and check behind us if are walking backwards.
Blindsight: There’s a curious quirk with the visual pathways. The vision part of the brain is right at the back, but some nerve fibres from the eye never reach there. Instead, they go off to other parts of the brain. That means that if someone suffers damage to the visual cortex they might lose total vision in that side (or both sides, in the case of cortical blindness), but still have some functions that relate to vision. For instance, someone with cortical blindness might still have their pupils respond to light, and still have their circadian rhythm (sleep/wake cycle) regulated by the different amounts of light during day and night, even though they can’t consciously tell whether it’s night or day.
Others might have a total hemianopia, with complete loss of vision to one side, but not have the typical problems of bumping into things on that side — in fact, they might be able to navigate very well around furniture and through doors just as if they could see obstacles on their blind side, even though they couldn’t say whether there is anything there or not, simply because some of those vision nerve fibres go straight to the parts of our brain dealing with spatial awareness and so weren’t affected by the damage to the visual cortex.
Macular hemianopia, macular sparing
Remember that the macula is only a very small part of our visual field. But it’s so important, with so much detail that actually about half of our visual cortex is devoted just to the macula. It’s quite possible to get damage to only one side of the macula (called a macular hemianopia) or damage to the entire side of vision except the macula (called macular sparing).
A macular hemianopia is a very small, very dense blind spot right next to where you are looking. If the blind spot is to the right, it can have the same devastating effect on reading fluency as a full hemianopia, but without all the terrible effects on your ability to walk around safely. Macular hemianopias can be very difficult to detect by normal visual field tests, because they are so small.
Macular sparing is the reverse. The person is likely to have all the problems getting around (bumping into things, tripping, etc) that you’d expect with a hemianopia, but have no trouble with reading, because their macular field is full on both sides.
Neglect is when a person’s awareness of one side of their body is affected. In effect, they ‘forget’ such a thing as a left or a right side even exists. The most common cause is a stroke. For our purposes here, thinking about vision impairment, the effect is that the person doesn’t react to things on one side of where they are looking. It can be very difficult to tell whether a person is actually blind on one side (a hemianopia) or is just not responding to one side (unilateral neglect) — the effect is pretty much the same. And in fact, many people with strokes affecting their vision have both vision loss and neglect.
In terms of dealing with it, neglect makes things a lot harder. Scanning behaviour to compensate for the visual field loss is difficult or impossible if the person has no awareness that there is a blind side they should be checking.
Damage to higher level brain functions
This category is huge. Entire books have been written about it, people have dedicated their entire careers to understanding just small parts of it.
The eye is extraordinarily complex, but the brain is vastly more so. You might have a totally intact and healthy pair of eyes, along with a totally intact and healthy visual cortex — but if the rest of your brain doesn’t make proper use of that good visual information, you’ll effectively have a vision impairment.
There are many conditions that can cause problems with our ability to use vision, but common ones include traumatic brain injury and dementia.
The effects tend to involve having trouble making sense of some aspect of the world around you. It might be that you can see letters but not be able to have them make sense as words (alexia), or see eyes and mouths but not be able to make sense of them as faces (prosopagnosia). Or you might have distortions of your sense of space and proportion, or interpret a change in carpet colour as a yawning chasm. You might experience discomfort with high contrast vision, such as finding black and white print dazzling.
The possibilities are endless, but they are hard to test for, so they are often a diagnosis of exclusion — that is, we need to have tested all the other parts of your vision and found them to be okay before we conclude that the problem must be with higher level brain functions.
To a person with degradation of the parts of their brain that deal with spatial awareness, their whole world might look like this.