The shift was documented by researchers led by Jerzy Szaflarski, MD, PhD, assistant professor in the Department of Neurology at the University of Cincinnati (UC) Academic Health Center, and Scott Holland, PhD, professor in the UC departments of biomedical engineering, pediatrics and radiology. Dr. Holland also heads the Pediatric Brain Imaging Research Program at Cincinnati Children's Hospital Medical Center.
Their results will be published in the February 2006 edition of the journal Human Brain Mapping.
While the site of language activity in right-handed people is originally the left side of the brain, the researchers report, starting as early as age 5 language gradually becomes a function shared by both sides. Between the ages of about 25 to 67, the site becomes more evenly distributed, until language activity can be measured in both hemispheres simultaneously.
This, the researchers say, may explain why young children who have had a large portion of one side of the brain surgically removed often recover completely.
"This knowledge may give new hope for rehabilitation of brain function in adults after stroke or traumatic brain injuries," said Dr. Szaflarski. "The fact that language adaptability is seen even in the older people supports the notion that these patients can be rehabilitated and returned to productive life, possibly even after a devastating stroke."
Scientists have long thought that the hemisphere or side of the brain that controls language and speech is determined before birth. Most people are right-handed and demonstrate more activity during language or speech in the left hemisphere of the brain. In left-handed people language centers are located more symmetrically.
Drs. Szaflarski and Holland studied brain activity in 177 right-handed children and adults aged 5 to 67 at Cincinnati's University Hospital and Cincinnati Children's using functional magnetic resonance imaging (fMRI). The technique shows brain activity, in this case language tasks such as reading or speaking, in a specific color.
"Our research revealed that language activity in the brain increases in the dominant hemisphere from age 5 until about 25," Dr. Szaflarski said, "which may be related to improving linguistic skills and maturation of the central nervous system.
"We observed that the nondominant side of the brain started helping the dominant side during reading or speaking from the age of 25 to 67," Dr. Szaflarski continued. "It's possible that as cognitive systems began to fail in the dominant side of the brain, the other side or hemisphere needs to compensate. Our study showed that older people have a more balanced capacity for language, with activity on both sides of the brain."
From around age 5 until about 25, said Dr. Szaflarski, language capacity in right-handers grows stronger in the left hemisphere of the brain. Similarly, fMRI shows increasing brain activity in the right hemisphere of left-handed persons until age 25.
"We were most interested in why this occurs, and the age at which the hemispheric language dominance began to decrease," said Dr. Szaflarski.
Drs. Szaflarski and Holland and their colleagues are also investigating how the brain handles language when it is damaged by a stroke or traumatic brain injury.
In children, Dr. Szaflarski said, the brain seems able to reorganize and shift the work load to the uninjured side. In adults, this doesn't happen as easily.
With a view to developing better treatment for brain injury in children and adults, the researchers are now trying to learn at what age this transition occurs.
Dr. Szaflarski and Dr. Holland's research is funded by the National Institutes of Health and the Neuroscience Institute of Cincinnati, a center of excellence in neuroscience specialties at the University of Cincinnati College of Medicine and University Hospital. The Neuroscience Institute, of which Dr. Szaflarski and Dr. Holland are members, is dedicated to patient care, research, education and the development of new medical technologies that may help patients with stroke, epilepsy, multiple sclerosis, trauma, Alzheimer's disease, Parkinson's disease and other movement disorders.
Journal
Human Brain Mapping