From: Anders Sandberg (asa@nada.kth.se)
Date: Tue Jun 22 1999 - 07:04:17 MDT
"Timothy Bates" <tbates@karri.bhs.mq.edu.au> writes:
> > So, for a first approximation, let's say we can custom-design the
> > system so that we can store one synaptic weight per byte. This generously
> > assumes that the connection pattern (i.e. which neuron has the synapse)
> > is hard-wired or hard-coded into the simulation program. The synaptic
> > weights have to change, because that's how the system learns. Since they
> > change, they have to be recorded.
> A synapse in the cortex is an analog element, it passes along cable currents
> in proportion to its inputs as well as discrete action potentials. In
> addition it does spatial and temporal summation of action potential inputs.
> In addition, they can can be mediated by second messengers, voltage gated,
> and they can be auto inhibitory, just for starters. This is MUCH more than 1
> byte. It is several kB plus an on-board processor programmed in OCCAM. Per
> synapse!
That is because we try to simulate it perfectly, not knowing if every
little bump and inclination on the membrane potential time series are
important or not. Most likely this can be simplified a lot (since our
biology has evolved to work in a noisy, messy environment where you
cannot count on perfect fidelity). Still, one byte per synapse is
likely too little, since learning most likely involves several state
variables (just look at the BCM rule, or the "realistic" models people
play with using more than 50 state variables) and you need to store
the kind of receptors that are around.
> > Therefore, the computer needs at least one byte per synapse,
> > 3x10^12 bytes of storage.
> I think that from here on becomes irrelevant given the above limitations.
Not really; his original aim was to show that a micron-sized upload
brain would not work. If his calculations are a million times too
small, they still show that the micron-uploads do not work. The
question seems to be whether we are talking about sugar cubes or small
grains of salt here.
-----------------------------------------------------------------------
Anders Sandberg Towards Ascension!
asa@nada.kth.se http://www.nada.kth.se/~asa/
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