Left Out by a Stroke: Right-brain injury may upset attention balance

People who suddenly ignore everything to their left after suffering a right-brain stroke display disturbed activity in uninjured parts of a widespread neural network associated with attention, a new brain-scan study indicates.

The finding suggests that structures on both sides of the brain typically maintain a delicate balance in regulating visual attention, proposes a team led by neurologist Maurizio Corbetta of Washington University School of Medicine in St. Louis. In some stroke patients, underactivity of damaged right-brain attention areas leads to hyperactivity in intact, left-brain attention structures, the scientists assert.

As a result, patients focus their visual attention primarily to the right and display various forms of left-side neglect, such as failing to notice or eat food on the left half of a plate and behaving as if they didn’t have a left arm.

Corbetta and his coworkers present their brain-scan findings in the November Nature Neuroscience.

The new study probes the neural basis of spatial neglect, a condition that annually affects as many as 5 million people worldwide. Symptoms are usually most severe in the weeks following a stroke but can last for a year or more.

About 90 percent of cases involve right-brain damage with left-side attention loss; in the rest, left-brain damage undermines right-side attention.

Corbetta and his coworkers studied 11 people who displayed spatial neglect following right-brain strokes. A functional magnetic resonance imaging scanner measured the amount of blood flow throughout their brains, an indirect marker of neural activity, as they performed attention tasks. For each volunteer, scanning and testing occurred about 1 month after the stroke and again around 6 months later, when spatial-neglect symptoms had substantially improved.

The 1-month scans showed minimal activity in attention-controlling parts of the injured right brain as well as intense activity in intact, left-brain attention regions. Right-brain areas involved in vision also exhibited unusually low activity, despite having escaped damage.

Scans obtained 6 months later revealed higher activity levels in attention- and vision-related parts of the right brain and a moderation of neural bustle in the left brain. Participants who recovered most completely from spatial neglect had the most-balanced neural activity, the investigators note.

“We [now] are scanning more stroke patients to see how different lesions affect the attention network and the visual brain,” Corbetta says.

The new findings “provide an important foundation on which to build an approach to remediation” of spatial neglect, remarks neurologist Argye B. Hillis of Johns Hopkins University School of Medicine in Baltimore. For instance, investigators could examine whether devices that deliver brief magnetic pulses to the brain can reduce hyperactivity of left-brain structures and speed recovery after a stroke.

Much remains unknown about the neural roots of attention and spatial neglect, says neurologist M. Marsel Mesulam of the Northwestern University Medical School in Chicago. Parts of the left-brain attention system studied by Corbetta also play crucial roles in language use, he notes, adding that no one knows how the brain orchestrates both attention and language using the same patches of tissue.