Jesse Mazer wrote:

[quoting Stathis Papaioannou]
Nevertheless, I still think it would be *extremely* difficult to emulate a whole brain. Just about every physical parameter for each neuron would be relevant, down to the atomic level. If any of these parameters are slightly off, or if the mathematical model is slightly off, the behaviour of a single neuron may seem to be unaffected, but the error will be amplified enormously by the cascade as one neuron triggers another.

I don't think that follows. After all, we maintain the same personality despite the fact that these detailed parameters are constantly varying in our own neurons (and the neurons themselves are being completely replaced every few months or so); neural networks are not that "brittle", they tend to be able to function in broadly the same way even when damaged in various ways, and slight imperfections in the simulated behavior of individual neurons could be seen as a type of damage. As long as the behavior of each simulated neuron is "close enough" to how the original neuron would have behaved in the same circumstances, I don't think occasional slight deviations would be fatal to the upload...

Perhaps, perhaps not. For one thing, in the brain's case we are relying on the laws of chemistry and physics, which in the real world are invariable; we don't know what would happen if these laws were slightly off in a simulation. For another, we do know that tiny chemical changes, such as a few molecules of LSD, can make huge behavioural changes, suggesting that the brain is exquisitely sensitive to at least some parameters. It is likely that multiple error correction and negative feedback systems are in place to ensure that small changes are not chaotically amplified to cause gross mental changes after a few seconds, and all these systems would have to be simulated as well. The end result may be that none of the cellular machinery can be safely ignored in an emulation, which is very far from modelling the brain as a neural net. I may be wrong, and it may be simpler than I suggest, but as a general rule, if there were a simpler and more economical way to do things, evolution would have found it.

--Stathis Papaioannou

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