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Published by the MIT News Office at the Massachusetts Institute of Technology, Cambridge, Mass. March 10th, 2004


Brain circuitry findings could shape computer design

Guosong Liu, a neuroscientist at the Picower Center for Learning and Memory at MIT, reports new information on neuron design and function in the March 7 issue of Nature Neuroscience that he says could lead to new directions in how computers are made.

While computers get faster all the time, they continue to lack any form of human intelligence. While a computer may beat us at balancing a checkbook or dominating a chessboard, it still cannot easily drive a car or carry on a conversation.

Computers lag in raw processing power--even the most powerful components are dwarfed by 100 billion brain cells--but their biggest deficit may be that they are designed without knowledge of how the brain itself computes.

While computers process information using a binary system of zeros and ones, the neuron, Liu discovered, communicates its electrical signals in trinary --- utilizing not only zeros and ones, but also minus ones.

This allows additional interactions to occur during processing. For instance, two signals can add together or cancel each other out, or different pieces of information can link up or try to override one another.

One reason the brain might need the extra complexity of another computation component is that it has the ability to ignore information when necessary; for instance, if you are concentrating on something, you can ignore your surroundings. "Computers don't ignore information," Liu said. "This is an evolutionary advantage that's unique to the brain."

Liu, associate professor of brain and cognitive sciences, said an important element of how brain circuits work involves wiring the correct positive, or "excitatory" wires, with the correct negative, or "inhibitory" wires. His work demonstrates that brain cells contain many individual processing modules that each collects a set number of excitatory and inhibitory inputs. When the two types of inputs are correctly connected together, powerful processing can occur at each module.

This work provides the first experimental evidence supporting a theory proposed more than 20 years ago by MIT neuroscientist Tomaso Poggio, the Eugene McDermott Professor in the Brain Sciences, in which he proposed that neurons use an excitatory/inhibitory form to process information.

By demonstrating the existence of tiny excitation/inhibition modules within brain cells, the work also addresses a huge question in neuroscience: What is the brain's transistor, or fundamental processing unit? For many years, neuroscientists believed that this basic unit of computing was the cell itself, which collects and processing signals from other cells. By showing that each cell is built from hundreds of tiny modules, each of which computes independently, Liu's work adds to a growing view that there might be something even smaller than the cell at the heart of computation.

Once all the modules have completed their processing, they funnel signals to the cell body, where all of the signals are integrated and passed on. "With cells composed of so many smaller computational parts, the complexity attributed to the nervous system begins to make more sense," Liu said.

Liu found that these microprocessors automatically form all along the surface of the cell as the brain develops. The modules also have their own built-in intelligence that seems to allow them to accommodate defects in the wiring or electrical storms in the circuitry: if any of the connections break, new ones automatically form to replace the old ones. If the positive, "excitatory" connections are overloading, new negative, "inhibitory" connections quickly form to balance out the signaling, immediately restoring the capacity to transmit information.

The discovery of this balancing act, which occurs repeatedly all over the cell, provides new insight into the mechanisms by which our neural circuits adapt to changing conditions.

This work is funded by the National Institutes of Health and the RIKEN-MIT Neuroscience Research Center.

--- Electrons like Gaul come in three parts ---

arslogo (3K) Propagation of orbiton -
quasiparticle state from the orbital properties
of electrons within a material.

By Matthew Francis
Published April 18, 2012 4:30 PM

orbitals (10K)

Free electrons moving through space are fundamental and indivisible: they are not built up of smaller particles, in contrast with protons and neutrons. However, within materials, interactions among electrons and atoms can give rise to quasiparticles, quantum states in which groups of electrons behave as new, particle-like excitations.

Physicists have now successfully created quasiparticles that split the electron's orbital characteristics from its spin. To accomplish this, Justine Schlappa et al. studied a special material in which electrons are confined to one-dimensional interactions at low temperatures, so that electron-electron interactions are dominant. Using resonant inelastic X-ray scattering (RIXS) at the Swiss Light Source facility, they determined that the electron orbital states propagated through the material independently of the spin.

Spin and momentum

Angular momentum is a property of rotating bodies, which is a conserved quantity in fundamental physics. Elementary particles like the electron have intrinsic angular momentum, which we label "spin." The spin is what gives materials their magnetic character. When electrons are part of atoms, they also have orbital angular momentum in some energy states, which influences the detailed spectrum when an atom absorbs or emits light.

The new finding builds on similar results from the 1990s, in which the spin degree of freedom moved separately from the electric charge. The two quasiparticles in those experiments were the "spinon," a neutral quasiparticle that behaves magnetically like an electron, and the "holon," a spinless excitation carrying electric charge.

In the new work, Schlappa et al. produced the spinons seen in the earlier work, but also separated out are dubbed "orbitons," spinless quasiparticles carrying the orbital angular momentum. While orbitons were previously predicted theoretically, this is the first experimental confirmation of their existence, as well as evidence they can move around within a material.

The particular material the researchers used was a strontium copper oxide (Sr2CuO3), which is metallic in character, but doesn't conduct electricity due to electron-electron interactions. (Metals with this odd property are known as Mott insulators.) The RIXS technique excites electrons between different orbital angular momentum states in the copper atoms; at the same time, it allows mapping of the spin configuration.

By measuring the separate contributions from the spin and orbital angular momentum states, Schlappa et al. were able to show that both spinon and orbiton quasiparticle states existed, and propagated through the Sr2CuO3 lattice at different rates. Spinon excitations showed up at lower energy RIXS measurements, with the distinct signature of orbiton excitations appearing as the X-ray energy increased.

Together with the earlier results, this indicates that three properties of electrons—spin, charge, and orbital angular momentum—can in principle be separated into three quasiparticles. What appears indivisible in free electrons splits under interactions to create something strikingly different and wonderful.

Nature, 2012. DOI: 10.1038/nature10974   (About DOIs).

Date of Origination (approx.): Wednesday, 15th May 2013... 9:18 AM
Updated Posting: Sunday, 20th May 2018... 6:02 AM
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Herb O. Buckland