[ExI] cortical architecture/"connectome"

[Jeff Davis]

What do you all make of this?

Blue Brain Project accurately predicts connections between neurons

http://www.eurekalert.org/pub_releases/2012-09/epfd-bbp091212.php

Blue Brain Project accurately predicts connections between neurons
Proof of concept: Researchers identify principles to support brain
simulation models

One of the greatest challenges in neuroscience is to identify the map
of synaptic connections between neurons. Called the "connectome," it
is the holy grail that will explain how information flows in the
brain. In a landmark paper, published the week of 17th of September in
PNAS, the EPFL's Blue Brain Project (BBP) has identified key
principles that determine synapse-scale connectivity by virtually
reconstructing a cortical microcircuit and comparing it to a mammalian
sample. These principles now make it possible to predict the locations
of synapses in the neocortex.

"This is a major breakthrough, because it would otherwise take
decades, if not centuries, to map the location of each synapse in the
brain and it also makes it so much easier now to build accurate
models," says Henry Markram, head of the BBP.

A longstanding neuroscientific mystery has been whether all the
neurons grow independently and just take what they get as their
branches bump into each other, or are the branches of each neuron
specifically guided by chemical signals to find all its target. To
solve the mystery, researchers looked in a virtual reconstruction of a
cortical microcircuit to see where the branches bumped into each
other. To their great surprise, they found that the locations on the
model matched that of synapses found in the equivalent real-brain
circuit with an accuracy ranging from 75 percent to 95 percent.

This means that neurons grow as independently of each other as
physically possible and mostly form synapses at the locations where
they randomly bump into each other. A few exceptions were also
discovered pointing out special cases where signals are used by
neurons to change the statistical connectivity. By taking these
exceptions into account, the Blue Brain team can now make a near
perfect prediction of the locations of all the synapses formed inside
the circuit.

Virtual Reconstruction

The goal of the BBP is to integrate knowledge from all the specialised
branches of neuroscience, to derive from it the fundamental principles
that govern brain structure and function, and ultimately, to
reconstruct the brains of different species – including the human
brain – in silico. The current paper provides yet another
proof-of-concept for the approach, by demonstrating for the first time
that the distribution of synapses or neuronal connections in the
mammalian cortex can, to a large extent, be predicted.

To achieve these results, a team from the Blue Brain Project set about
virtually reconstructing a cortical microcircuit based on unparalleled
data about the geometrical and electrical properties of neurons—data
from over nearly 20 years of painstaking experimentation on slices of
living brain tissue. Each neuron in the circuit was reconstructed into
a 3D model on a powerful Blue Gene supercomputer. About 10,000 of
virtual neurons were packed into a 3D space in random positions
according to the density and ratio of morphological types found in
corresponding living tissue. The researchers then compared the model
back to an equivalent brain circuit from a real mammalian brain.

A Major Step Towards Accurate Models of the Brain

This discovery also explains why the brain can withstand damage and
indicates that the positions of synapses in all brains of the same
species are more similar than different. "Positioning synapses in this
way is very robust," says computational neuroscientist and first
author Sean Hill, "We could vary density, position, orientation, and
none of that changed the distribution of positions of the synapses."

They went on to discover that the synapses positions are only robust
as long as the morphology of each neuron is slightly different from
each other, explaining another mystery in the brain – why neurons are
not all identical in shape. "It's the diversity in the morphology of
neurons that makes brain circuits of a particular species basically
the same and highly robust," says Hill.

Overall this work represents a major acceleration in the ability to
construct detailed models of the nervous system. The results provide
important insights into the basic principles that govern the wiring of
the nervous system, throwing light on how robust cortical circuits are
constructed from highly diverse populations of neurons – an essential
step towards understanding how the brain functions. They also
underscore the value of the BBP's constructivist approach. "Although
systematically integrating data across a wide range of scales is slow
and painstaking, it allows us to derive fundamental principles of
brain structure and hence function," explains Hill.


Best, Jeff Davis

                "Everything's hard till you know how to do it."
                                                 Ray Charles