That reminds me of that movie "The Serpent and the Rainbow"...the one that is supposedly based on some degree of fact, in regards to the 'zombie dust' concoction that can be made from the powdered derivative of one of those blowfish. It will make someone seem dead for all intents and purposes, but they still retain consciousness and can actually perceive everything around them even though their body is rendered completely useless. Then, when the effects of this supposedly wear off, they "come back from the dead" Pretty scary shit.
I watched that again a little while ago. A friend of mine has the book (written by Wade Davis, whom Bill Pullman's charector is based on). I believe there's been a fair few cases of people who've shown no signs of life whatsoever, then suddenly woke up...in one instance, a mortitian or mortuary worker noticed a tear trickling down the face of one the 'bodies'!