Neurons aren't simple little machines, axons talk to each other.
He and his colleagues first discovered individual nerve cells can fire off signals even in the absence of electrical stimulations in the cell body or dendrites. It's not always stimulus in, immediate action potential out. (Action potentials are the fundamental electrical signaling elements used by neurons; they are very brief changes in the membrane voltage of the neuron.)
"This cellular memory is a novelty," Spruston said. "The neuron is responding to the history of what happened to it in the minute or so before." Spruston and Sheffield found that the cellular memory is stored in the axon and the action potential is generated farther down the axon than they would have expected. Instead of being near the cell body it occurs toward the end of the axon.
Their studies of individual neurons (from the hippocampus and neocortex of mice) led to experiments with multiple neurons, which resulted in perhaps the biggest surprise of all. The researchers found that one axon can talk to another. They stimulated one neuron, and detected the persistent firing in the other unstimulated neuron.
No dendrites or cell bodies were involved in this communication. "The axons are talking to each other, but it's a complete mystery as to how it works," Spruston said. "The next big question is: how widespread is this behavior? Is this an oddity or does in happen in lots of neurons? We don't think it's rare, so it's important for us to understand under what conditions it occurs and how this happens."
The original article (paywall).
Assuming this is all true, how does it affect the feasibility of uploading? Anyone want to bet on whether things are even more complicated than the current discoveries?
ETA: It seems unlikely to me that you have to simulate every atom to upload a person, and more unlikely that it's enough to view neurons as binary switches. Is there any good way to think about how much abstraction you can get away with in uploading?
Yes, I know it's a vague standard. I'm not sure how good an upload needs to be. How good would be good enough for you?
This is precisely what I was fishing for, thank you.
If brains are physical systems, and physics as we know it involves noncomputable processes (c.f. the comment below about CTD), then it follows that brains are doing noncomputable stuff. The question is then whether that noncomputable stuff is necessary to aspects of experience that we should care about, presuming that this conversation is ultimately about uploads. And the simple answer is that we don't know.
I've got enough theoretical and practical experience with physics, computers and nervous systems to have noticed the muddles that conspicuously creep in when I try to make these three topics interface, and am a little mystified whenever it seems like other smart people don't notice them. I suspect a big part of it is just getting a little too happy with metaphors like "brain = computer" without paying close attention to the ways in which these things are dis-analogous.
Well, I would say "that we care about"; it's not clear to me what it means to say about an aspect of experience that I don't care about it but I should. But I think that's a digression.
Leaving that aside, I agree: the only way to be sure that X is sufficient to build a brain is to do X and get a brain out of it. Until we do that, we don't know.
But if supporting a project to build a brain via X prior to having that certainty is ... (read more)