Video: Brain avalanches
YOU might expect the brain of someone with a mental disorder to be disorganised. But it’s the nature of the disorganisation that’s important – a finding that one day could help early diagnosis of different types of dementia.
We already know that the different regions of healthy brains are linked in a so-called small-world network, which makes communication very efficient. For people with Alzheimer’s or other types of dementia, however, it’s a different story.
In small-world networks – which also emerge, for example, in social networks – each node is connected to a lot of nearby nodes, but also has a few links to distant ones. Because of this, any node can communicate with almost any other in just a few hops.
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This may explain the brain’s formidable ability to process masses of information rapidly. “A small world, in theoretical terms, is the optimal network,” says Willem de Haan of the VU University Medical Center in Amsterdam, the Netherlands.
De Haan’s team used scalp electrodes to measure the brain activity of resting volunteers, of whom 20 had mild to moderate Alzheimer’s, 15 had a rare form of dementia called , and 23 were healthy. The researchers figured out the underlying network structure of the volunteers’ brains from the electrical activity in different regions over time.
In healthy brains, this structure resembled a small-world network, as expected. In people with Alzheimer’s, the nodes were connected more randomly; in FTLD, the network was more ordered, with fewer long-distance links. The researchers think that in both abnormal cases the brain networks would be less efficient at exchanging information, and that this might explain some of the cognitive problems experienced by people with these disorders (BMC Neuroscience, ).
“In one form of dementia, the network was more ordered, with fewer long-distance links”
Although the brain wastes away in all forms of dementia, it has been hard to link the amount of atrophy to the severity of symptoms, or explain why cognitive problems can come and go. Comparing the severity of symptoms with the degree by which the brain’s organisation differs from a small-world network might help explain the anomalies better, says De Haan.
Also, he says, such differences in brain organisation could one day help distinguish early on between different forms of dementia, which can appear similar at onset but diverge dramatically later.
Andreas Meyer-Lindberg of the Central Institute of Mental Health in Mannheim, Germany, agrees that the disruption to the brain’s optimal organisation is “very likely related to the cognitive impairment seen in dementia”.
Meyer-Lindberg also points out that the latest results resonate with earlier findings that the brain works on the border of order and chaos (see “Avalanches and aftershocks help us think”). In 2006 his team showed that a small-world network was just the right one to support such a state.
Given De Haan’s results, Meyer-Lindberg says that the disruption of the brain’s small-world network seen in dementia and Alzheimer’s suggests that these brains have moved away from this critical state. “On the edge of chaos, that’s where you want to be,” he says.
Avalanches and aftershocks help us think
Disordered cascades of electrical activity known as “neuronal avalanches” have been observed in a waking brain for the first time – further evidence that the brain teeters between chaos and order.
The idea that the brain functions in this state, known as self-organised criticality, helps explain its amazing ability to process information. In other phenomena that exhibit self-organised criticality – like forest fires and earthquakes – even small changes can propagate rapidly over large distances. Neuroscientists have long supposed that cascades of electrical activity in the brain might obey the same principle, making them the ideal way to transmit information. Till now, however, they had only been observed in tissue cultures or anaesthetised rat brains.
To investigate further, at the National Institute of Mental Health in Bethesda, Maryland, analysed neuronal activity detected by arrays of electrodes implanted in the motor and premotor cortices of two macaque monkeys. The brain signals revealed avalanches with all the signs of self-organised criticality.
For example, the avalanches followed a power law: the frequencies of the larger and smaller avalanches followed a specific ratio. Even more compellingly, the team observed that each avalanche was followed by a series of smaller ones, like the aftershocks of an earthquake (Proceedings of the National Academy of Sciences, ). Such neuronal aftershocks had never been observed before. “They might help the brain explore new avenues of a thought,” Plenz says.
David Robson
