I’ve quite rapidly become short sighted, meaning I do not see things in the distance clearly any more, and need quite strong glasses, which is something new for me. This is because the lenses in my eyes have become harder, which makes them bend light more. This has been quite difficult to adapt to. So I’ve chosen to wear under-corrected lenses for walking about, so they are not quite as strong as my prescription. But what was the cause of my difficulties? The answer rests in the way the periphery of my spectacle lenses bend light. Glasses for short sightedness make everything look smaller, which means that the periphery of what I see through the spectacle lens appears to have been moved inwards.
This means that when I wear my new spectacles while moving around quickly, things aren’t where my mind has mapped them to be. So I can trip, and worse, I get a sensation of the world slewing around as I change direction. It’s more than a visual slew though. It also causes a nauseous sensation of imbalance, where everything is slightly off kilter, which is very difficult to describe in words.
Yet I don’t have the same problem when I’m driving while wearing my full spectacle correction.
So why the difference?
The answer is that I’m not actually moving my body through my surroundings when I’m driving, I’m moving the car, which appears equally shrunk with my glasses, and my mind has no problem matching these two elements of the scene, the car and its surroundings.
”What on earth has this got to do with cerebral visual impairment?” you may be asking.
The answer rests with what the posterior parietal lobes do for us. Without us knowing, they instantly map the moving scene of our surroundings with respect to our bodies as we move, from moment to moment seamlessly linking this with our brain’s timing and balance systems.
In the car, I’m not moving with respect to my surroundings, the car is. So I don’t experience the illusion of the world slewing around, but when out and about and moving around with my glasses on, my mental representation of the moving world around me is not being immediately co-located with reality, and my posterior parietal lobes are not yet coping, they need to be trained for a bit longer to adjust.
What happens then when the posterior parietal lobes are not working so well?
The key to getting to understand this is to recognise that the mental map of the surroundings becomes limited in its capacity. It can only map fewer items in the surrounding scene. We have learned from what others have told us, that if say only five things can be mapped, then on an empty beach with only four things to see, the whole scene can be taken in, but on a busy street, the vision shuts down to encompass only the central five things, because there is not enough mental computing capacity (like RAM) to process anything more. In other words vision shuts down.
When it comes to moving around, less information is available, so the map to guide movement is also limited. It is coarser. This leads to moving less accurately and a risk of bumping into things. These are the key issues that we describe in our new section on posterior cortical atrophy, where we explain ways to adapt and to cope.
What about me and my glasses? I’m practicing wearing them and waiting for my brain to catch up, just as it did the second and third time I walked onto an airport moving pavement, when my brain had to learn to compensate for being launched forward (as I was the first time), by predicting this a moment in advance, and automatically shifting my weight to match the now anticipated change in momentum.
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