Unaffected Hemisphere Assumes Language Function After Perinatal Stroke
Babies sometimes have stroke around the time of birth. Birth is hard on the brain, as is the change in blood circulation from the mother to the neonate. At least one in 4,000 babies have stroke shortly before, during, or after birth.
But a stroke in a baby, even a big one, does not have the same lasting impact as a stroke in an adult. A study led by investigators at Georgetown University Medical Center in Washington, DC, found that a decade or two after a perinatal stroke damaged the left language side of the brain, affected teenagers and young adults used the right sides of their brains for language.The findings demonstrate how plastic brain function is in infants, said Elissa L. Newport, PhD, Professor of Neurology at Georgetown University School of Medicine, and Director of the Center for Brain Plasticity and Recovery at Georgetown University and MedStar National Rehabilitation Network.Her study found that the 12 individuals studied, aged 12 to 25, who had a left-brain perinatal stroke all used the right side of their brains for language. “Their language is good—normal,” she said.
The only signs of prior damage to their brains are that some participants limp, and many have learned to make their left hands dominant because their right hands had impaired function after stroke. They also have executive function impairments—slightly slower neural processing, for example—that are common in individuals with brain injuries. But basic cognitive functions, like language comprehension and production, are excellent, said Dr. Newport.
Furthermore, imaging studies revealed that language in these participants is based entirely in the right hemisphere, in the region opposite to the normal language areas in the left hemisphere. This result had been recorded in previous research, but earlier findings were inconsistent, perhaps because of the heterogeneity of the types of brain injuries included in those studies, said Dr. Newport. Her research, which was carefully controlled in terms of the types and areas of injury included, suggests that while “these young brains were plastic, meaning they could relocate language to a healthy area, it does not mean that new areas can be located willy-nilly on the right side.
“We believe there are important constraints on where functions can be relocated,” she continued. “There are specific regions that take over when part of the brain is injured, depending on the particular function. Each function, like language or spatial skills, has a particular region that can take over if its primary brain area is injured. This is an important discovery that may have implications in the rehabilitation of adult stroke survivors.”
This finding is consistent with the behavior of young brains, said Dr. Newport. “Imaging shows that children up to about age 4 can process language in both sides of their brains, and then the functions split up: the left side processes sentences, and the right processes emotion in language.”
Dr. Newport and her colleagues are extending their study of brain function after a perinatal stroke to a larger group of participants. They are examining stroke in the left and right hemispheres and also whether brain functions other than language are relocated and where. Her group is also collaborating on studies that may reveal the molecular basis of plasticity in young brains. This information might help promote plasticity in adults with stroke or brain injury.
Why Do We Sleep?
Evidence supports the synaptic homeostasis hypothesis about the function of sleep, said researchers. The debate about sleep’s function has continued for a generation and arose following observations that people and animals sicken and die if they are deprived of sleep.
Chiara Cirelli, MD, PhD, and Giulio Tononi, MD, PhD, psychiatrists at the Center for Sleep and Consciousness in Madison, Wisconsin, proposed the synaptic homeostasis hypothesis in 2003. This hypothesis holds that sleep is the price we pay for brains that are plastic and able to keep learning new things. They subsequently undertook a four-year research effort that could show direct evidence for their theory. The result was published in February 2017 in Science and offered direct visual proof of the hypothesis.
Striking electron-microscope pictures from inside the brains of mice suggest what happens in our own brains every day. Our synapses grow strong and large during the stimulation of daytime, then shrink by nearly 20% while we sleep, thus creating room for more growth and learning the next day.
A large team of researchers sectioned the brains of mice and used a scanning electron microscope to photograph, reconstruct, and analyze two areas of cerebral cortex. They were able to reconstruct 6,920 synapses and measure their size.
The team remained blinded about whether they were analyzing the brain cells of a well-rested mouse or one that had been awake. When they finally correlated the measurements with the amount of sleep the mice had had during the six to eight hours before the image was taken, they found that a few hours of sleep led on average to an 18% decrease in the size of the synapses. These changes occurred in both areas of the cerebral cortex and were proportional to the size of the synapses.
Dr. Cirelli’s laboratory is now looking at new brain areas, and at the brains of young mice, to understand the role that sleep plays in brain development.