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zettabits per brain

Our heads hold only 10^11 neurons but we connect them with 10^19 micro tubes, each storing > 4k in binary or ternary codes. This system would need about 2% of the energy consumed by the brain.

Cytoskeletal Signaling: Is Memory Encoded in Microtubule Lattices by CaMKII Phosphorylation?

Travis J. A. Craddock, Jack A. Tuszynski, Stuart Hameroff

Memory is attributed to strengthened synaptic connections among particular brain neurons, yet synaptic membrane components are transient, whereas memories can endure. This suggests synaptic information is encoded and ‘hard-wired’ elsewhere, e.g. at molecular levels within the post-synaptic neuron. In long-term potentiation (LTP), a cellular and molecular model for memory, post-synaptic calcium ion (Ca2+) flux activates the hexagonal Ca2+-calmodulin dependent kinase II (CaMKII), a dodacameric holoenzyme containing 2 hexagonal sets of 6 kinase domains. Each kinase domain can either phosphorylate substrate proteins, or not (i.e. encoding one bit). Thus each set of extended CaMKII kinases can potentially encode synaptic Ca2+ information via phosphorylation as ordered arrays of binary ‘bits’. Candidate sites for CaMKII phosphorylation-encoded molecular memory include microtubules (MTs), cylindrical organelles whose surfaces represent a regular lattice with a pattern of hexagonal polymers of the protein tubulin. Using molecular mechanics modeling and electrostatic profiling, we find that spatial dimensions and geometry of the extended CaMKII kinase domains precisely match those of MT hexagonal lattices. This suggests sets of six CaMKII kinase domains phosphorylate hexagonal MT lattice neighborhoods collectively, e.g. conveying synaptic information as ordered arrays of six “bits”, and thus “bytes”, with 64 to 5,281 possible bit states per CaMKII-MT byte. Signaling and encoding in MTs and other cytoskeletal structures offer rapid, robust solid-state information processing which may reflect a general code for MT-based memory and information processing within neurons and other eukaryotic cells.
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So if the brian "encodes" at this density, why does a brain have to be a big as it is?

There's no end of theories of neuronal function that predict a greater information density than has ever been observed in a functioning organism. That brains need to be large to be smart is a constraint on such theories.
+Peter Meilstrup Actual observation by reading and writing nano memory seems possible given the details of the paper. Perhaps it will be useful to distinguish between reasons of quantity and reasons of quality. Advantages of increased resolution offer trivial reasons for big memory. Emergent phenomena - like self awareness or empathy - anticipate by simulation of memories about additional often inconsistent perspectives.
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