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Trained brains become more energy efficient

Fuel efficient A study on monkeys has called into question the fundamental assumption that an active neuron is a hungry one; an assumption that underlies brain imaging techniques such as fMRI.

Researchers compared levels of neuron activity and the amount of glucose uptake in the primary motor cortex of monkeys performing either an unfamiliar, visually-guided sequence of movements, or a familiar, well-learned sequence of movements.

They found similar levels of neuronal activity but much lower levels of glucose uptake during the familiar task compared to the unfamiliar task.

"Our results provide direct evidence for a widespread alteration in the relationship between metabolic activity and neuron activity associated with practice on a skilled sequence of movements," the researchers report today in Nature Neuroscience.

The findings challenge previous research suggesting that neuronal and metabolic activity in the brain are linked - something that also makes physiological sense.

"The motor cortex generates commands to the motor neurons to control muscle activity, so any time you move, you'd expect neurons to be active and to require energy because of that," says lead author Dr Nathalie Picard, research assistant professor in the department of neurobiology at the University of Pittsburgh.

"We know that the neurons are active - that's what the neuron recording studies showed - so the low metabolic activity for [familiar] internally generated tasks, that's really unexpected."

The discovery may have implications for imaging techniques such as functional magnetic resonance imaging (fMRI), which relies on changes in blood flow as a measure of neuronal activity in the brain.

"In imaging studies in humans using something like fMRI, the assumption is that if an area is active, it means that neurons are active," says Picard.

However, if neurons can be firing active but not be metabolically active, it calls that relationship into question.

"We suspect that this is something that's particularly important for long-term practice, long-term skill performance such as in physicians and athletes, but at this point we can't tell when the change occurs and how much practice it takes to get to that stage," she says.

The researchers suggested that the neurons involved in learning these motor tasks may become more efficient over time and repetition, so that they require less energy to maintain the same level of activity.

Possible mechanisms include long-lasting enhancement of the signal transmission between neurons or the formation of more synapses - the structures that enable neurons to transmit signals to other neurons.

The study also provides further evidence that the primary motor cortex is not only involved in generating commands but also learning.

"It's still not in most people's minds that the primary motor cortex really comes to be important for the consolidation and storage of motor skills," says Picard.

"Other areas are probably involved in learning, but the more you do it, the more your motor cortex comes to be represented."