How the brain's backup system compensates for stroke

Researchers have pinpointed in humans how a "backup" brain region springs into action to compensate for disruption of a primary functional area, as happens during stroke. Their finding offers new insight into how the brains of stroke victims can quickly reorganize to enable the beginning of recovery of movement.

Jacinta O'Shea and colleagues at the University of Oxford reported their findings in the May 3, 2007 issue of the journal Neuron, published by Cell Press.

In their experiments, the researchers focused on a region of the brain called the dorsal premotor cortex (PMd), which is known to govern the selection of an action. The PMd is mirrored in the two brain hemispheres, and the left-hemisphere PMd is dominant in such motor control function. Researchers had known that after a stroke damages one side of the brain and paralyzes a limb, the PMd in the intact hemisphere increases in activity. However, they did not know whether this activity increase represented specific "rescue" of PMd function in the damaged hemisphere.

In their experiments, O'Shea and colleagues used magnetic pulses to temporarily and harmlessly disrupt function in the left PMd in human volunteers. To measure the subjects' ability to select action, the researchers gave the subjects a complex button-pushing task. During this task, they scanned the subjects' brains with functional magnetic resonance imaging (fMRI) to detect changes in brain activity. This brain-scanning technique uses harmless radio waves and magnetic fields to measure blood flow in brain regions, which reflects brain activity.

The researchers found that the magnetic pulses to the left PMd briefly disrupted the subjects' performance on the task, but that performance quickly recovered. The fMRI scans revealed that during this behavioral recovery, the right PMd and other areas involved in premotor function underwent a compensatory increase in activity. The researchers' experiments also established that this activity increase was functionally specific for the behaviors governed by the PMd and not simply a general increase in brain activity.

The researchers also established that the activity increase in the right PMd caused the recovery of performance. When they also used magnetic pulses to disrupt the right PMd after the performance recovery, the subjects showed a renewed deficit in performance on the button-pushing task.

The researchers pointed out that, because the PMd is important for learning new connections between eye and motor function, it "is particularly well placed to mediate learning of new motor strategies after brain damage results in the normal routes to action being compromised."

The researchers concluded that their experiments showed that "In the initial stages of adaptive compensation for neuronal interference, the adult brain exploits pre-established patterns of functional specialization. When a key node in an information-processing circuit is impaired, healthy cortical networks can flexibly reconfigure processing in a way that is rapid, functionally-specific, and preserves behavior."

Source : Cell Press