Understanding Vision Impairment
How the location of the eye problem determines the type of vision problem
Now that we have a good understanding of the structure of the eye, and of how vision works, let’s look at what effect eye problems have on vision. I’ll introduce this by considering which part of the eye is affected, rather than what disease is causing it. We’ll look at specific eye conditions later, but for now it’s enough to consider that most diseases that affect (for instance) the cornea cause similar types of functional impairments — which are quite different from the functional impairments caused by diseases affecting the photoreceptors, which are in turn different from diseases affecting the optic nerve, etc.
Working from front to back, here’s a quick overview. We’ll go through them in more detail in the next pages.
- Front of the eye (cornea & lens): The job of the front of the eye is to let light in, and focus it on the back of the eye. Problems in this area tend to be haziness or distorted shape, so the vision problems tend to centre around blur, reduced contrast and glare. The problems usually affect the entire visual field evenly.
- Back of the eye (retina, optic nerve): The job of the retina is to detect that focused image and convert it to nerve impulses, so the information can be sent through the optic nerve and back to the brain. Most commonly, conditions affecting these structures affect just certain parts of the visual field, and may leave other areas completely unaffected. The part of the field that’s affected is the critical factor in determining what sort of vision problem results.
- Vision centres in the brain: This is where vision problems change from affecting one or the other eye to affecting one or the other side of your vision.
- Other parts of the brain: This all relates to how your brain uses vision, integrating into your overall perception. In many cases you could say that there’s nothing wrong with the vision as such, but the brain isn’t making use of the vision properly, so the effect is still effectively a vision impairment.