A Silicon Chip that Mimics How the Brain's Synapses Change in Response to New Information

In November 2011, a group of MIT researchers created the first computer chip that mimicked how the brain's neurons adapt in response to new information. This biological phenomenon, known as plasticity, is analog, ion-based communication in a synapse between two neurons. With about 400 transistors, the silicon chip can simulated the activity of a single brain synapse — a connection between two neurons that allows information to flow from one to the other. 

"There are about 100 billion neurons in the brain, each of which forms synapses with many other neurons. A synapse is the gap between two neurons (known as the presynaptic and postsynaptic neurons). The presynaptic neuron releases neurotransmitters, such as glutamate and GABA, which bind to receptors on the postsynaptic cell membrane, activating ion channels. Opening and closing those channels changes the cell’s electrical potential. If the potential changes dramatically enough, the cell fires an electrical impulse called an action potential.

"All of this synaptic activity depends on the ion channels, which control the flow of charged atoms such as sodium, potassium and calcium. Those channels are also key to two processes known as long-term potentiation (LTP) and long-term depression (LTD), which strengthen and weaken synapses, respectively. "

"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" (http://www.mit.edu/newsoffice/2011/brain-chip-1115.html, accessed 01-01-2014).

Rachmuth, G., Shouvai, H., Bear, M., Poon, C. "A biophysically-based neuromorphic model of spike rate- and timing-dependent plasticity," Proceedings of the National Academy of Sciences 108, no. 459, December 6, 2011, E1266-E1274, doi: 10.1073/pnas.1106161108