I have lost the sides, top and bottom of my vision to retinitis pigmentosa. If I look straight ahead I cannot see the floor or what is immediately above, or to my right or left. It is a bit like looking through a letterbox. Tunnel vision is a misnomer for what I see. There are no big black ugly sides like toilet roll tubes. Instead I have funnel vision, perhaps in time my funnel will change to a tunnel, but right now it’s like watching the world through a video camera lens – things that are far away look normal but close up things get cropped. I can see the whole of a scenic view but only a bit of my computer keyboard.
By scanning a little, my eyes see the rest of the keyboard, and my brain puts it together so I feel that I can see the whole thing. It is a clever trick like stitching together a panoramic photo. I have to really stop to think about how I see. It is not always a conscious thing. My mind just does it automatically.
The problem is if I don’t scan, I won’t see things that are close. There is a big margin for error. I can see the tiniest bit of fluff but will fall over a chair getting to it. When I am moving around I have to consciously tell myself to look – because the moment I don’t, is the moment I will end up making a wally of myself.
RP eyes are constantly putting together a puzzle, and over time the jigsaw pieces will get smaller. In very severe tunnel vision there may not be any overlap between what each eye sees resulting in two separate jigsaw pieces. In the early stages, while the pieces are still big – you don’t really notice the scanning – but as they shrink in size it starts to overwhelm the brain. That is probably when it is time to starting using a cane.
Every Usher person has a different visual field, some will have islands of vision in their far peripheral field and will notice someone waving if they hit the sweet spot. Some will bump their heads on everything but still see the ground. Others may notice overhanging branches but have poor lower vision, making it hard to see signing hands, or to move about. This does not mean that one has person worse damage than another, it is just that we use different parts of the visual field for different things.
Some people with Usher have poor balance due to differences in how their ear canal works, others do not, but since balance relies on a triptych of signals from the eye, ear and muscles (particularly in the ankles and feet), worsening sight can make stability more difficult. Some people report feeling momentarily unsteady even when standing still. They may get dizziness from quick changes in their eye or head movements, and lose track of direction. As vision narrows there may be a sensation of camera shake or juddery vision. I have occasionally experienced this standing at the top of the stairs.
The brain stabilises the images we see, otherwise quick eye movements would make us dizzy. You can try this for yourself. If you fixate on a point and move your head from side to side, the world will appear to stay still even though your head and eyes are moving. The brain is calibrating the image for you.You can override this effect by gently tapping the side of your eye to make your eyeball move slightly (keep your head still). Your vision will jerk and the world will appear to move. By manually moving your eyeball, you are are causing a disconnect between the brain and the eye, and it does not realise that stabilisation is needed.
I wonder with tunnel vision whether there is sometimes not enough visual feedback from our eye movements reaching the brain, which means our sight cannot stabilise quickly, and it takes a little while longer to feel orientated. The less a person can see, the more prone they may become to instability, or dizziness, so for a short time – until the brain catches up – they struggle to see, like trying to steady a zoomed in camera, only it is not zoomed in just narrowed.