Are you short or long-sighted? Go and lower your glasses so that your visual field is split in half horizontally (in other words, perch your glasses further down on your nose). Now move your head from left to right, and look through your glasses. Then do the opposite, and look above your glasses.

You should have noticed that in the half of your visual field which you weren’t looking at, it seemed like things were out of sync – they weren’t moving exactly together with the half that you were looking at. Why is this so?

It’s all to do with the vestibulocular reflex, or the VOR. This reflex, which is hooked up to your eyes and your balance centres in the brain, compensates for the movements you make with your head. This allows to you distinguish between movements that you’ve initiated (e.g. moving your head) and those that you have not (e.g. falling off a tree). Clearly having a VOR is important, or else every time you moved your head it’d seem like the world was spinning around you. You don’t tend to notice you have a VOR, but the simply fact that the world doesn’t appear to move when you move your head shows it.

When you’re born, you don’t come with a VOR built in – you have to learn it, by comparing information coming from your eyes with information from your balance centres and other brain areas. Unfortunately you can’t remember what this was like when it happened because you were too young, but you can replicate the effect somewhat by becoming short or long-sighted and putting glasses on for the first time.

When you do this, the world seems sharper (obviously) but everything seems a bit out of kilter, like the world isn’t moving properly. You feel a little off-balance, and if you’re unlucky, you might fall down some stairs. This happens because your VOR isn’t calibrated for the new visual input it’s receiving – after all, a pair of glasses with significantly alter the way light reaches your retinas. After a while, though, you get used to it and flights of stairs don’t seem to be that much of a danger any more.

This is all very well and good. But consider the experiment that we did earlier – since the world seemed stable whether or not you looked through your glasses, you were able to effectively switch between two differently calibrated VORs at will. You do this every time you take off or put on your glasses, of course, but the effect is more salient when you are half-wearing them.

The fact that you can do this is really astounding, when you think about it. It’s not as if we had glasses when we evolved, so why should our brain be able to handle two differently calibrated VORs? And how is it that you can switch between them using solely an internal input (your decision to look through your glasses or not)? This is one of the more interesting questions in the visual sciences at the moment, if you ask me.

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