When the dominoes are stacked closely together, toppling one will flatten them all. When the dominoes are set far apart, toppling one will have no effect.
Neither of those scenarios works well in the brain when a neuron in one corner wants to communicate with a neuron in another corner, Plenz says.
For the message to get from point A to point B, he says, the brain needs a series of short, domino-like cascades that gets the message across without activating every neuron.
Such a condition, he says, is found at the critical point, when the neurons are set up just right to allow cascades of all sizes to occur.
In a domino board set up to operate at the critical point, domino A can trigger domino C without toppling domino B.
"So the critical point describes the state of a system where you can selectively transmit information to other sites—where local inputs can lead to big effects very far away—without activating the whole system," Plenz said.
At the same time, the system is highly responsive to additional inputs that can easily reroute the message, he adds.
While working in Plenz's lab, Beggs helped perform experiments that suggested the brain operates at the critical point.
Researchers placed tiny slices of rat brain on a microelectrode array and discovered that local groups of brain cells activated each other in cascades of activity.
Most often, one local group did not fire any other groups of neurons. Less often, one group would trigger one other group of neurons, leading to a cascade size of two.
Even less often, the second group of neurons would trigger a third group—a cascade size of three—and so on.
Statistical analyses of the number of neuron groups activated in each cascade suggest that they follow a pattern similar to mathematical equations that describe events such as avalanches, which are also believed to operate at the critical point.
The researchers named the cascades "neuronal avalanches."
Dante Chialvo is a physiologist at Northwestern University in Evanston, Illinois, and a leading expert in a branch of this research known as "self organized criticality."
For Chialvo, the concept of neuronal avalanches is built on a mathematical framework developed in statistical physics and the fundamentals of neuroscience," a convergence of 50 years of research in both fields.
He lauds the finding of neuronal avalanches as "one of the most beautiful and fundamental experimental results I have seen in the last few years."
This August Plenz published research in the Journal of Neuroscience showing that chemicals produced in the brain that are known to affect cognitive function also control the generation of neuronal avalanches.
"This opens the exciting possibility that by optimizing avalanches in the brain, we might be able to enhance cognition," he said.
According to Chialvo, the brain being tuned to operate near the critical point makes perfect sense. For example, he says, consider how a child learns.
"In order to engage a kid to learn, you have to give it tasks to challenge it, but not too tough nor too boring. You have to choose it to be just at the edge of failure or success to make the kid feel challenged," he said.
That edge, he says, is the critical point. And because the process of learning is ongoing, when the child meets the challenge, teachers raise the bar to maintain the edge.
Although scientists aren't exactly sure how, he said, "the brain is able to stay always at the edge."
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