15 November 2011

MIT's New Silicon Brain Mimic -- One Synapse at a Time

With about 400 transistors, the silicon chip can simulate the activity of a single brain synapse — a connection between two neurons that allows information to flow from one to the other. The researchers anticipate this chip will help neuroscientists learn much more about how the brain works, and could also be used in neural prosthetic devices such as artificial retinas, says Chi-Sang Poon, a principal research scientist in the Harvard-MIT Division of Health Sciences and Technology.

Poon is the senior author of a paper describing the chip in the Proceedings of the National Academy of Sciences the week of Nov. 14. Guy Rachmuth, a former postdoc in Poon’s lab, is lead author of the paper. Other authors are Mark Bear, the Picower Professor of Neuroscience at MIT, and Harel Shouval of the University of Texas Medical School. _MIT

This is an important -- but very small -- step toward the understanding of how individual neurons and synapses work. And since there are 100 billion neurons in the brain, each with roughly 10,000 synapses, there is still a long way to go before one can claim to "mimic" the brain. In fact, with its emphasis on the study of ion channels, the researchers are taking this chip deeper, toward the molecular level, rather than upward to the global level of brain function.
The MIT researchers designed their computer chip so that the transistors could mimic the activity of different ion channels. While most chips operate in a binary, on/off mode, current flows through the transistors on the new brain chip in analog, not digital, fashion. A gradient of electrical potential drives current to flow through the transistors just as ions flow through ion channels in a cell.

“We can tweak the parameters of the circuit to match specific ion channels,” Poon says. “We now have a way to capture each and every ionic process that’s going on in a neuron.”

Previously, researchers had built circuits that could simulate the firing of an action potential, but not all of the circumstances that produce the potentials. “If you really want to mimic brain function realistically, you have to do more than just spiking. You have to capture the intracellular processes that are ion channel-based,” Poon says.

The new chip represents a “significant advance in the efforts to incorporate what we know about the biology of neurons and synaptic plasticity onto CMOS [complementary metal-oxide-semiconductor] chips,” says Dean Buonomano, a professor of neurobiology at the University of California at Los Angeles, adding that “the level of biological realism is impressive.

The MIT researchers plan to use their chip to build systems to model specific neural functions, such as the visual processing system. Such systems could be much faster than digital computers. Even on high-capacity computer systems, it takes hours or days to simulate a simple brain circuit. With the analog chip system, the simulation is even faster than the biological system itself.

Another potential application is building chips that can interface with biological systems. This could be useful in enabling communication between neural prosthetic devices such as artificial retinas and the brain. Further down the road, these chips could also become building blocks for artificial intelligence devices, Poon says.

...When the researchers included on their chip transistors that model endo-cannabinoid receptors, they were able to accurately simulate both LTD and LTP. Although previous experiments supported this theory, until now, “nobody had put all this together and demonstrated computationally that indeed this works, and this is how it works,” Poon says. _MIT
Barely born, and researchers are already getting it high on endocannabinoids! But seriously, the model is not the synapse, and certainly not the brain. Being able to "accurately simulate" a phenomenon is not the same thing as revealing exactly what is going on. Models have the potential to give you better ideas, but if you are not careful they can badly mislead you.

This line of research is good news for neuro-molecular biologists and for neuroscientists who study low level activities of neurons and synapses. It is certainly important research.

Most Al Fin cognitivists, on the other hand, are eager to learn more about how the human brain works, and how we could make it more functional -- better able to achieve its goals. Achieving such a feat would obviously require an entirely different approach to the research than Chi-Sang Poon et al have taken.


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Blogger tisanjosh said...

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Saturday, 25 August, 2012  

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