My scenario violates the assumption that a conscious being consists of independent replaceable parts.
Just to be concrete: let's suppose that the fundamental physical reality consists of knotted loops in three-dimensional space. Geometry comes from a ubiquitous background of linked simple loops like chain-mail, other particles and forces are other sorts of loops woven through this background, and physical change is change in the topology of the weave.
Add to this the idea that consciousness is always a state of a single loop, that the property of the loop which matters is its topology, and that the substrate of human consciousness is a single incredibly complex loop. Maybe it's an electromagnetic flux-loop, coiled around the microtubules of a billion cortical neurons.
In such a scenario, to replace one of these "consciousness neurons", you don't just emulate an input-output function, you have to reproduce the coupling between local structures and the extended single object which is the true locus of consciousness. Maybe some nano-solenoids embedded in your solid-state neuromorphic chips can do the trick.
Bear in mind that the "conscious loop" in this story is not meant to be epiphenomenal. Again, I'll just make up some details: information is encoded in the topology of the loop, the loop topology interacts with electron bands in the microtubules, the electrons in the microtubules feel the action potential and modulate the transport of neurotransmitters to the vesicles. The single extended loop interacts with the localized information processing that we know from today's neuroscience.
So what would happen if you progressively replaced the neurons of a brain with elements that simply did not provide an anchor for an extended loop? Let's suppose that, instead of having nano-solenoids anchoring a single conscious flux-loop, you just have an extra type of message-passing between the neurochips, which emulates the spooling of flux-topological information. The answer is that you now have a "zombie", an unconscious entity which has been designed in imitation of a conscious being.
Of course, all these hypotheses and details are just meant to be illustrative. I expect that the actual tie between consciousness and microphysics will be harder to understand than "conscious information maps to knots in a loop of flux".
So what would happen if you progressively replaced the neurons of a brain with elements that simply did not provide an anchor for an extended loop? Let's suppose that, instead of having nano-solenoids anchoring a single conscious flux-loop, you just have an extra type of message-passing between the neurochips, which emulates the spooling of flux-topological information. The answer is that you now have a "zombie", an unconscious entity which has been designed in imitation of a conscious being.
This is done one neuron at a time, though, with the ...
In June 2012, Robin Hanson wrote a post promoting plastination as a superior to cryopreservation as an approach to preserving people for later uploading. His post included a paragraph which said:
This left me with the impression that the chances of the average cryopreserved person today of being later revived aren't great, even when you conditionalize on no existential catastrophe. More recently, I did a systematic read-through of the sequences for the first time (about a month 1/2 ago), and Eliezer's post You Only Live Twice convinced me to finally sign up for cryonics for three reasons:
I don't find that terribly encouraging. So now I'm back to being pessimistic about current cryopreservation techniques (though I'm still signing up for cryonics because the cost is low enough even given my current estimate of my chances). But I'd very much be curious to know if anyone knows what, say, Nick Bostrom or Anders Sandberg think about the issue. Anyone?
Edit: I'm aware of estimates given by LessWrong folks in the census of the chances of revival, but I don't know how much of that is people taking things like existential risk into account. There are lots of different ways you could arrive at a ~10% chance of revival overall:
is one way. But:
is a very similar conclusion from very different premises. Gwern has more on this sort of reasoning in Plastination versus cryonics, but I don't know who most of the people he links to are so I'm not sure whether to trust them. He does link to a breakdown of probabilities by Robin, but I don't fully understand the way Robin is breaking the issue down.