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Computer Modeling Yields Clues to How the Brain Works

A computer simulation developed by Salk researchers led by investigator Terrence J. Sejnowski reveals that neurons in the thalamus, the central switchboard that processes and distributes incoming sensory information to all parts of the visual cortex, engage in a coordinated effort to get their message out loud and clear.

Their findings, published in Science, hold important clues to how the brain encodes and processes information, which can be applied to a wide variety of applications, from understanding psychiatric disorders to the development of novel pharmaceuticals and new ways of handling information by computers or communication networks.

Historically, neuroscientists have been limited to recording the activity of single brain cells, but communication between neurons is not limited to one-on-one interactions. Instead, any Computer Modeling Yields Clues to How the Brain Works given cell receives signals from hundreds of other cells, which send their messages through thousands of synapses.

For this reason, nobody could answer a very basic question: How many neurons or synapses does it take to reliably send a signal from point A to point B? This question is particularly pressing for the thalamus. Thalamic input only accounts for five percent of the signals that so-called spiny stellate cells in the cortex receive, even though they drive a good portion of activity throughout the cerebral cortex.

"That is a paradox," says Sejnowski, a Howard Hughes Medical Institute investigator and professor and head of the Computational Neurobiology Laboratory. "How can so few synapses have such a big impact? If the average spiking rate were the determining factor, thalamic input would be drowned out by the other 95 percent of the inputs from other cortical cells."

Based on the assumption that the brain cares about the reliability and precision of spikes, Sejnowki's team developed a realistic computer model of a spiny stellate cell and the signals it receives through its roughly 6,000 synapses. They found that it is not the number of spikes that's relevant but rather how many spikes arrive at the same time.

The team's model predicted that it only takes about 30 synapses out of 6,000 firing simultaneously to create extremely reliable signaling. And the prediction lined up with currently available in vivo measurements. The researchers hope that their findings will give them new insight into the holy grail of neurobiology: decoding the neural code or language of the brain.