Hrm... I wanted to say 10^-7, but then I realized I actually do have evidence that is different from the type of false justifications associated with anosonosia.

The latter, as I understand it, tends to be associated with making excuses for why one isn't moving the arm, etc... I notice though that I actually can move my arms. That is, I observe myself waving both arms freely, typing using both hands, etc.

I'd have to spend a bit more type to work out reasonable numbers associated with it, but these observations are different from the "making excuses for why one can't move the arm in question"