I remember reading a paper about the possibility of our universe being a simulation.

What the brain knows about the body is indeed being rebuilt continuously using perceptual information. The process is called calibration, and is critical – every time you pick something up the dynamics of your body change, and if you couldn’t keep up you would be screwed. It is of course then ‘hard to break the illusion’ because it’s not, strictly speaking, an illusion: it’s recalibration, and while it’s being maintained by information it will persist (and even has a level of stability if it’s not being maintained).

The essence of a calibration process is taking a measurement and placing it on some scale. The visual perception of distance is intrinsically unitless and so of no use; calibration uses other perceptual information to provide an action relevant scale. So you perceive the length of your arm via vision and dynamic touch, and then measure distance via vision using that arm length as your ruler. You end up perceiving ‘can I reach it?’, not some abstract number.

So Ed:
Life doesn’t come with scale bars for us to measure size against.
Actually it does: ourselves. The perception-action system has access to information about ourselves as well as everything else, and it perceives the world in terms of our ability to act on it – we perceive affordances. This ability changes (as you pick things up, or become fatigued, etc) hence the calibration process must be an ongoing one – a static ‘brain model’ would never work.

This study is an interesting example – but people have been manipulating perception via recalibration for years. Geoff Bingham, at IU, has done a lot of this with prehension (he’s demonstrated things like the dynamic stability of recalibration); Daniel Wolpert is famous for his distance perception studies too.

@Ken – Indeed, and that the 10 minutes of overwriting is so stable that it’s actually quite hard to break the illusion unless you do the out-of-time stroking.