How much of your mind has to be lost before you don't know it?

The stories coming out of Aurora Colorado got me to thinking.
Psychotics, people with Azheimer’s disease, some people on Q and A sites on the Internet – I am sure there is a moment when they know that something is not quite right, but then at some further point they do not realize that there is a problem. Is there a way to express that tipping point in percentages? Does the moment where the corner is turned have a clinical name? What would you call it?