From: Martin Striz (mstriz@gmail.com)
Date: Mon Oct 24 2005 - 14:14:04 MDT
On 10/24/05, Richard Loosemore <rpwl@lightlink.com> wrote:
> Remember what uploading would involve. You probably have to know the
> size, shape and possibly the chemical state of every neural cell and
> every one of its connections to all others, and maybe even the state of
> all the synaptic buttons, in three dimensions, in circumstances where
> the darn things are not broadcasting much EM but hiding most of their
> signals in chemical waves.
You may not need single-neuron resolution, since most neural pathways
are columns of dozens or hundreds of redundant neurons firing in
synchrony. Since postsynaptic responses operate probabilistically,
this ensures signal propagation and separates signal from noise.
Tononi et al. suggest this (see below). Understanding how
computation occurs during signal integration at the dendrites may be
interesting and useful, but probably not crucial. Brain damage from
mechanical or chemical insult demonstrates that the specific
synapse-level organization of the brain can change dramatically while
retaining the original function.
> Does anyone out there on the SL4 list have to hand the spatial and
> temporal resolution of the best of the fMRI systems? Anyone care to do
> a quick scale comparison with a synaptic button and an action potential?
You can get less than 1 mm^3 voxels for a single scan (it's actually
considered to be a 2D scan with a set thickness), or a short series of
scans, although much lower resolutions are used for large sequences
over time. 1 mm^3 is about 1000 neurons. But fMRI only measures one
variable, usually "overall activity" taken from oxygen or glucose
metabolism. What you really need is the ability to measure all the
neuromodulators that a neuron(s) can produce and how it uses them.
Current technology is woefully insufficient at constructing the kind
of brain sim that we want.
> The only good argument against this is that we might not need the
> resolution. But at the moment we are clueless about the exact
> functional significance of the detailed structure. For example, maybe
> spike synchrony in the dendritic tree is crucial at the functional level
> ... and if it is crucial, then getting a brain approximately right but
> with the fine details a bit fuzzy would be about as valuable as
> uploading two fistfuls of porridge.
Tononi has proposed creating a connection matrix at "mesoscale"
resolution (minicolumns of neurons being the basic functional units of
the brain).
http://striz.org/docs/tononi-connectome.pdf
Even with adequate funding, we're at least 10-15 years away from a
useful connectome database.
Martin Striz
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