DAVIS, Calif. -- Researchers have shown for the first time that, in some
left-handed people, writing may be controlled completely and independently
from the right hemisphere of the brain.
The new findings will be presented Nov. 19 at the annual meeting of the Society for Neuroscience in Washington, D.C. The research eventually may help researchers explain why left-handed people, who make up an estimated 10 percent of the population, are also more likely to have some language skills guided by the right half of their brains.
Conducted by scientists at the University of California, Davis, and Dartmouth College, the study also provides evidence that human brains did not develop specifically to use written language, but instead have adapted older brain operations for reading and writing.
In most people, all language abilities such as reading, writing and speaking are controlled by one side of the brain -- usually the left hemisphere. But, in a woman known as V.J., researchers have found that one of these language functions -- writing -- is separated from the others. While V.J's left hemisphere continues to control her reading and speaking, it possesses only the most rudimentary writing abilities. It is the right side of V.J.'s brain that controls her writing.
V.J. is left-handed, and the researchers believe her right-hemisphere writing ability may be linked to her left-handedness. V.J., 43, lives in San Jose, Calif.
"This is the first solid evidence that there is a clear difference in how language is processed in some genetically left-handed individuals," says Kathleen Baynes, an assistant professor of neurology at the UC Davis Center for Neuroscience. Baynes is the lead author on a paper discussing these findings, now under peer review. Baynes also is a neuropsychologist with the VA Northern California Health Care System, Martinez, Calif.
V.J.'s case also offers a rare window into the hotly debated issue of how closely related the brain systems are for spoken and written language, says Michael Gazzaniga, head of Dartmouth's cognitive neuroscience program. Gazzaniga, well-known for his pioneering "split-brain" research, is a co-author of the paper and principal investigator for the research project.
"Human language is an instinct. Reading and writing, on the other hand, are human inventions," Gazzaniga says. "In this case, the dramatic separation of functions clearly reveals that the invented process of writing is independent and not tied necessarily to processes associated with spoken words."
In all people, the right side of the brain generally controls the left half of the body. The left side of the brain handles the right side of the body. The visual system is similar, with your eyes sending information on the right half of the space in front of you to the left side of your brain, and information from the left visual field to the right hemisphere. To see the big picture, both halves of the brain normally share their information.
V.J. is known as a "split-brain" patient because she had a "corpus callosotomy," a surgical separation of the two halves of the brain achieved by severing the major band of fibers that relays information between the two hemispheres.
This surgery is done only for treatment of severe epilepsy that cannot be controlled with medication. It is usually accompanied by changes that are so subtle that special techniques are required to observe them. Unlike prior patients who have had this procedure, V.J. has not been able to write fluently with either hand since the surgery was completed.
To try to understand why this was happening, Baynes and her colleagues used a special experimental technique, in which words and numbers are shown briefly to just the right or left visual field but not both. V.J. would then try to write or read aloud the words.
When words or letters were displayed to her left visual field (right hemisphere), the researchers discovered, V.J. could not read them out loud, but surprisingly she could write them with her left hand.
When words or letters were displayed to the right side (left hemisphere), V.J. could almost always read them out loud correctly, but she could not write them with her right hand. Because she could write displayed numbers with both hands, the researchers discarded the explanation that V.J.'s right hand naturally was clumsier than her left.
"It appears that although her left hemisphere has the mental code that translates letters into sounds, it lacks the code for writing," Baynes says. "In contrast, her right hemisphere cannot translate letters into sounds, but it can control writing."
Before her surgery, the left and right hemispheres would have easily worked together to seamlessly coordinate reading, writing and speaking. After surgery, V.J.'s left hemisphere still attempts to assert control over language processes, dominating and interfering with the writing ability in her right hemisphere, Baynes says.
Since the late 1880s, scientists have known that writing and reading were separate mental functions. A French scientist named Dejeurine had noticed at the time that, following a stroke damaging the left side of the brain as well as the fibers that connect the two hemispheres, some patients lost the ability to read, although they were still able to write. The case of V.J. is different because, unlike stroke patients, who suffer damage to large areas of the brain, split-brain patients experience only disconnections between brain areas.
The findings are important at two levels -- one, understanding how language is processed differently in left-handed people and, two, the evolution of written language in all people.
If V.J. represents the prototypical way genetically left-handed people process language, then scientists now may have a marker to chart the genetic basis of some aspects of reading, writing and speaking, Bayne says. (Defining left-handedness can be tricky. Because V.J. is what scientists call "familially left-handed" -- her mother, her only sister and her only daughter are all left-handed -- Baynes believes V.J. has genetically determined left-handedness.)
On another level, the findings bolster one side of the debate about whether spoken and written forms of language are one system or two independent systems in the brain. Both historically and developmentally, spoken language forms appear first, Gazzaniga points out, so many researchers have concluded that the written forms arise out of those processes as part of the same system.
Other linguists believe the written form of language is a newer and independent system. "Data to support the independent view have been hard to find," Gazzaniga says.
V.J. may be evidence that human brains did not evolve to use the written language per se. Rather, written language may exploit an older brain system, such as the one used for facial recognition, and hence be able to arise in either the left or right hemisphere, independent of the system for auditory speech and language, the researchers say.
The research was funded by the National Institutes of Health and the McDonnell-Pew Foundation.