Probably originally an extrapolation from cases of severe brain
injury, where the person apparently still functions normally
afterwards. Always an unreliable way of doing things unless you
beforehand know what that bit of brain did so you can test it or you
have a sufficently broad battery of tests to elucidate every cognitive
mechanism known, which still wouldn't be very reliable. You don't know
what it did before, injuring it apparently has no effect consequently
it does nothing. I think you can see that the premises are both
flawed which doesn't leave much hope for the conclusion! lol.
A newsgroup cliche come to mind:- absence of proof is not proof of
absence.