[ExI] Smallest human-equivalent device

[Anders Sandberg]

On 2013-10-13 17:04, John Clark wrote:
> On Sat, Oct 12, 2013 at 1:36 PM, Anders Sandberg <anders at aleph.se 
> <mailto:anders at aleph.se>> wrote:
>
>     > How many transistors are functionally equivalent to one synapse?
>
>
> I don't know but I can figure out how many modern transistors you 
> could fit inside 2 neurons.

In computational neuroscience the typical fine-grained neuron model is 
divided into electrically isopotential compartments, typically 
corresponding to each segment of the dendritic and axonal branches. A 
reasonable estimate is that there are as many compartments as synapses, 
so the total number is twice the synapse number (synapses also count). A 
typical neuron has around 8000 synapses, so 16,000 compartments is 
likely. Each compartment has at least a membrane potential and some 
channel states (in the Hodgkin--Huxley model you have 3-6 depending on 
how you slice the activations). Izhikevich estimated the cost as around 
1000 FLOPS per compartment. This is likely an underestimate when you add 
extra channels and synaptic properties, but they just multiply the 
guesstimate a bit. So I would be surprised if a synapse takes more than 
10,000 FLOPS, even if you try to model a lot of state. Assuming 2000 
FLOPS per compartment gives an overall cost of 32 MFLOPS per neuron.


> Using technology that Intel will mass produce next year they can  
> build a transistor inside a 3*10^3 cubic nanometer box, or about 10^10 
> transistors in the volume occupying 2 neurons.

That ought to be enough. Even if we assume 1000 transistor per 
operation, we should have more than enough. Not to mention a big speed 
advantage.

The deep mess might be the change in configuration that happens during 
plasticity. Synapses grow and find targets on a hour/day timescale. This 
means the network topology is slightly mutable. Just assuming a fixed 
circuit network will not do. I think this is not too hard to handle with 
interconnects, but they are pretty big circuits.

> Granted you couldn't (yet) pack transistors at that density throughout 
> a volume as large as the human brain due to heat considerations, but 
> imagine what will be practical in just a few years.

This is why I have high hopes for quantum dot cellular automata and 
other near-reversible tech.

-- 
Dr Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University

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