Our human minds are a fantastically complex system of inputs and processing, and while it works well a lot of the time, it can be tricked into misinterpreting data - for example with an optical illusion.
Another more troublesome result of the way our brains work is the problem of amputees experiencing "phantom pain" in a limb that is no longer physically present. Can you imagine having an itch that you literally cannot scratch, or a non-existent limb or joint feeling as if it is stuck in an awkward or painful position?
While the mechanism is not fully understood, one approach seems to work for some patients is the use of augmented reality to trick the mind (again) into seeing a representation of the missing limb, and allowing software to arrange the limb in a less painful position (for example). This seems to let the brain think all is well, and relieves the phantom pain condition.
TechCrunchIf the idea that moving a virtual limb around in AR could relieve pain strikes you as strange, don’t worry — it is. But phantom pain is a poorly understood phenomenon and sometimes the effectiveness of treatments is matched only by their strangeness.