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An idle mind is a workshop

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Nature carries a little review of what is known about inactive minds:

[S]ome researchers have been adding a bit of down time to their study protocols. While subjects are still lying in the functional magnetic resonance imaging (fMRI) scanners, the researchers ask them to try to empty their minds. The aim is to find out what happens when the brain simply idles. And the answer is: quite a lot.

Some circuits must remain active; they control automatic functions such as breathing and heart rate. But much of the rest of the brain continues to chug away as the mind naturally wanders through grocery lists, rehashes conversations and just generally daydreams. This activity has been dubbed the resting state. And neuroscientists have seen evidence that the networks it engages look a lot like those that are active during tasks.

A set of experiments in the mid-1990s first suggested that the brain never really takes a break. Bharat Biswal, then a PhD student at the Medical College of Wisconsin in Milwaukee, was trying to find ways of identifying and removing background signals from fMRI scans, in the hope that it would improve interpretations of the signals from tasks. “The assumption was, it was all noise,” says Biswal, who is now a biomedical engineer at the New Jersey Institute of Technology in Newark. But when he looked at scans taken when people were resting in the scanner, he saw regular, low-frequency fluctuations in the brain. Biswal’s experiments suggested that neuronal activity was causing these fluctuations.

Several years after Biswal’s discovery, studies of the resting state in its own right began to emerge. A team led by Marcus Raichle, a neuroscientist at Washington University in St. Louis, Missouri, characterized activity in one such network as the brain’s default mode — what they considered its baseline setting. During tasks, default-mode activity actually dropped, coming back online when the brain was no longer focusing so intensely.

The default-mode network has been joined by dozens of other flavours of resting-state network — some of which resemble the circuitry that contributes to attention, vision, hearing or movement. They seem very similar across study participants but are also dynamic, changing over time. “The fact that it’s always present but modifiable tells you that it’s got its importance,” says Michael Milham, director of the Center for the Developing Brain at the Child Mind Institute in New York.

[U]sing fMRI and electroencephalography (EEG) recordings, [Andreas] Kleinschmidt and his team confirmed that various resting-state networks are correlated with real neural activity.

Shmuel and David Leopold, a neurophysiologist at the US National Institute of Mental Health in Bethesda, Maryland, did much the same, imaging resting states in monkeys while recording the animals’ electrical brain activity using probes implanted deep in the visual cortex. They found correlations between resting-state networks and electrical activity in a band of frequencies around 40 hertz. Such ‘γ activity’ is associated with communication between distant brain areas, and seeing it convinced Shmuel that resting-state networks represent actual brain activity.

Some researchers now think that resting-state networks may prime the brain to respond to stimuli. “The system is not sitting there doing nothing and waiting,” says Kleinschmidt. Cycling activity in these networks may be helping the brain to use past experiences to inform its decisions.

But idling networks might not just save time. They may also influence perceptions — albeit unconsciously. To study how spontaneous resting activity affects perception, Kleinschmidt and his colleagues scanned10 the brains of people who were looking at a picture that can be perceived as a face or as a vase. Study participants who reported seeing a face had more spontaneous activity in the fusiform face area — a brain region that processes faces — before they were shown the picture. Kleinschmidt suspects that the brain is running several models of the world in the background, ready for one of them to turn into reality. “Ideally, you’re always prepared for what happens next,” he says.

Several teams have reported changes in resting connectivity after language and memory tasks and motor learning. Chris Miall, a behavioural brain scientist at the University of Birmingham, UK, and his colleagues have shown that spontaneous activity at rest can be perturbed by what has just happened. The team scanned volunteers at rest, and then asked them to learn a task involving using a joystick to track a moving target. When the participants were scanned at rest again, the team could see the effects of motor learning in the resting networks. That study, and subsequent work along the same lines, suggests that “the brain is not only thinking about supper coming up, but it’s also processing the recent past and converting some of that into long-term memories”, says Miall. The network changes are specific to the tasks performed.


Written by Arhopala Bazaloides

September 20, 2012 at 2:00 pm

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