The researchers used magnetoencephalography (MEG) to look at the brain functioning of patients who recently experienced minor strokes. MEG is a non-invasive neuroimaging technology that employs very sensitive magnetometer sensors to make high-speed recordings of naturally occurring magnetic fields produced by electrical currents inside the brain. The subject typically sits under or lies down inside the MEG scanner, which resembles a whole-head hair drier.

Once inside the scanner, patients had their magnetic fields recorded as they completed word and picture matching tasks. These tasks all involved memory, memory search or identification. In some tasks the patient needed to speak the answer, while others required them to press a "yes" or "no" button. An age-matched control group of people who had not suffered a stroke also completed the tasks and were recorded.


Then the two sets of MEG recordings were compared. The stroke patients' recordings exhibited distinct characteristics that were different from the control group. For example, the signals within their brains were noticeably more subdued, appearing more like rolling hills rather than mountain peaks. This is an indicator that the brain is processing less efficiently. The stroke patients also took about twice as long as the control group to complete the tasks. In addition, they were not able to modulate their brain activity at different stages of performing the tasks, a further indicator of neural sluggishness.

These patterns of inefficient processing suggest a dysfunction in the brain's distributed network--a disruption of the network's dynamics.

"If the problem is not because of the lesion itself, affecting visual processing regardless of where the lesion is located, and you can't see what is wrong in an MRI or CAT scan, then we conclude that the issue is more global: how the brain talks to itself," says Professor Simon. "That is where the MEG technology shines--in showing us the dynamics of neural processing."

"This shows that even a minor stroke, whose visible signs of damage are small, has a profound impact on the brain as a whole," says Simon. "Damage to a local neural community affects the global neural community."

"A single little lesion can disrupt the entire network and result in global dysfunction," Marsh says.

After six months, the stroke patients returned for a second time to complete the same tasks. They not only performed better on the tests, but also anecdotally reported their symptoms of impairment had largely resolved. However, surprisingly the scans themselves looked relatively the same.

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