[extropy-chat] A couple weird questions...

[Anders Sandberg]

Eric Messick wrote:
> Jeffrey Herrlich:
>> What would be a very lower-bound estimate for the physical distance
>> between the Soma/Main-Body of one neuron and the Soma/Main-Body of an
>> adjacent neuron in the Human brain? (Doesn't matter what units are
>> used)

Some somata are probably adjacent, so they are just two membranes and a
little intercellular space away (perhaps 1-10 nm). So somewhere beween
40-90 nm. In most cases the distances are a bit larger, but you get
clustered somata in cortical minicolumns.

>> The reason I ask, is that I'm trying to refine my estimate of the
>> time delay between neuronal discharges by incorporating a discussion
>> about the illusion of Simultaneity of discharges - involving distances
>> between neurons and the speed of light. Any input is greatly
>> appreciated.

There is also a delay 1-2 ms (or more, for some messenger systems) at the
synapse due to starting the vesicle emmission machinery, diffusion and
receptor reaction.

> I doubt that there is any time during which there are not any neurons
> in the process of firing within a normally functioning human brain.

The background firing activity is about 2% active (within a time bin of
one second), with higher rates for cortical on-states that may reach a few
more procent at least locally.

Activity in the brain is likely to a large degree asynchronous. You find a
lot of synchronization papers out there, both because it is easily
detected on the macroscale and because people like to invent elegant
information processing based on synchronized oscillations and spike
timing. In theory neurons can do a lot of cool simultaneity-based
processing, but we do not have any evidence that it is really used.
Outside a few systems (like phase-based auditory orienting in owls) we do
not know that synchronization is used for anything. We do not even know
how to distinguish a widely dispersed synchronization from asynchronous
noise.

-- 
Anders Sandberg,
Oxford Uehiro Centre for Practical Ethics
Philosophy Faculty of Oxford University