Instability: Normal State Of Brain's Cortex
Even when we're not doing much of anything, our brain's cortex, or outer mantle, is bustling with activity. In fact, scientists for the first time have detected "avalanches" of cortex activity in awake monkeys at rest.
They've also discovered that these bursts of synchronous neuronal activity aren't just random, but rather precisely ordered. Large avalanches are followed by smaller and smaller avalanches, much like the aftershocks of an earthquake. This type of ordering reveals that the normal state of cortex circuitry is at a tipping point: at the edge of instability — like rocks along an earthquake fault.
"Mental illness may involve disturbances in this delicate balance, and abnormal avalanche patterns are potentially detectable," explained NIMH's Dietmar Plenz, Ph.D. "Being in such a state of instability allows neurons to telegraph information optimally across varying distances and to quickly adapt to new challenges. This makes it possible for the cortex to grow through development and expand through evolution without changes in its architecture."
Plenz and colleagues report on their study of neuronal avalanches online during the week of August 24, 2009 in the Proceedings of the National Academy of Sciences.
Understanding the cortex's complex functional architecture has posed challenges for researchers. Plenz's earlier studies in cell cultures and brain slices suggested that cortex tissue is organized like grains of sand in a sand pile - with the potential for even a few grains to trigger large avalanches. Periods of relative calm are punctuated by the spontaneous, synchronous avalanches. To confirm these findings in intact, awake animals, Plenz and colleagues recorded electrical signals in different parts of the cortex of two monkeys that were resting in a chair.
Results of This Study
The researchers observed avalanches, synchronous bursts of neural activity across varying expanses of cortex, in a pattern that implies a specific structure. The neuronal avalanches seem to obey laws similar to those that characterize their geological counterparts. Again like earthquakes, smaller avalanches are more common than bigger ones. Their size can range from involving clusters of cells to widespread networks.
"Avalanches function at any scale, bridging a 1 mm distance in the same way as they bridge a 10 mm distance as the brain develops or evolves," explained Plenz.
The state of instability appears to be a general property of cortex tissue. There are hints that disorders of thinking, such as schizophrenia, could involve a breakdown in this critical state, leading to aberrant neuronal avalanche activity, say the researchers.
Plenz and NIMH colleagues are studying this possibility using a non-invasive technique called magnetoencephalography (MEG), which images electrical activity deep in the brain. They are comparing cortex activity in schizophrenia patients and healthy controls, looking for quantifiable neural signatures of abnormal avalanche activity.
"If a brain doesn't show the normal, synchronous avalanche pattern, this could signal a brain disorder," said Plenz.
Even though the monkey was just resting in a chair, neuronal avalanches were spontanously sparking in its brain's outer mantle, or cortex. Electrodes (black dots) detected these synchronous bursts (colored circles) of neural activity. The diameter and color of the circles reflects the varying size of the avalanches, which occurred as disparate clusters of synchronous activity. Despite their irregular appearances, the avalanche patterns are highly organized in space and time obeying precise rules similar to those found for earthquakes. This suggests that the cortex is normally organized in a state with the potential for such critical activity.