The mental centers for speech and writing, long thought to be in the same side of the brain, can reside in different hemispheres. This surprising finding, reported in tomorrow's Science , was discovered by examining an epileptic patient whose brain was surgically divided to control her seizures. It suggests that spoken and written language can develop separately--and could lead to a new understanding of learning disorders.
The patient, V.J., had suffered severe seizures. By cutting her corpus callosum, the fibrous portion of the brain that carries messages between the hemispheres, surgeons hoped to prevent the seizures from spreading. They succeeded, but V.J. developed an unexpected side effect: She lost the ability to write at will, although she could read and spell words aloud.
To explore what had happened, Kathleen Baynes, a cognitive neuroscientist at the University of California, Davis, and colleagues tested which skills each side of her brain could perform. For example, when they showed words and pictures to V.J.'s left hemisphere (by flashing them in her right visual field), she could read and name them aloud, but she couldn't write the corresponding words. The researchers concluded that her left hemisphere controls speech and reading, but not writing. Similar tests suggested that her right hemisphere controls writing, but not reading, speech, or the neural functions that allow people to find the right word for an object.
It's difficult to know how far to extrapolate from one person, particularly someone with a history of seizures, cautions Baynes. But researchers say the study has implications for learning disorders and language development. It suggests, for example, that spoken and written language may have evolved independently, says Baynes. And to understand learning problems such as dyslexia, "people want to figure out the connection between oral and written language skills," says Richard Ivry, a cognitive neuroscientist at the University of California, Berkeley. This work suggests that they are not necessarily neighbors in the brain.