Does anyone have an up-to-date fix on how much computation occurs (if any) within-cells (as opposed to the traditional neural net level) that
are part of biolgical brain systems? ........especially in the case of animals that have a premium placed on the number of neurones they can support (eg. limited by size, weight or energy supply compared to the need for computational capacity).
Let's put it this way: The idea that neurons are simple little integrating units is an urban legend propagated by computer scientists.
Flipping through my bookmarks, I found "The role of single neurons in
information processing" by Christof Koch and Idan Segev, which I recall is a decent quick intro:
http://icnc.huji.ac.il/Files/Koch_Segev_NN2000.pdf
Abstract:
Neurons carry out the many operations that extract meaningful information from sensory receptor arrays at the organism’s periphery and translate these into action, imagery and memory. Within today’s dominant computational paradigm, these operations, involving synapses, membrane ionic channels and changes in membrane potential, are thought of as steps in an algorithm or as computations. The role of neurons in these computations has evolved conceptually from that of a simple inte-grator of synaptic inputs until a threshold is reached and an output pulse is initiated, to a much more sophisticated processor with mixed analog-digital logic and highly adaptive synaptic elements.
I think Koch has a book about this, but I don't recall the title offhand. In any case, people are still trying to break, e.g., the giant squid axon (the largest nerve known, and hence an oft-studied one) down into enough compartments that it can be computationally simulated with some degree of accuracy. Long-term potentiation is another aspect that is only beginning to be understood. Lots of mysteries. All very expensive to simulate until we figure out what the higher-level functions are, since you have to get every detail, not knowing which details are important. Uploading without understanding is *not* cheap, if that's what you're thinking of. You could easily end up having to go down to the molecular level.
-- Eliezer S. Yudkowsky http://singinst.org/ Research Fellow, Singularity Institute for Artificial Intelligence
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