While the consequences of brain damage can be dire, one kind of language loss may not necessarily affect another. Specifically, as a recent Johns Hopkins study shows, brain damage could dramatically affect one's ability to speak but not his ability to write, or the opposite could be the case.
Even though writing evolved from speaking, the two systems of the brain have diverged greatly. While the concept defies logic, a person unable to speak a grammatically correct sentence might still be able to write it without errors. As an article in Futurity (http://www.futurity.org/brains-speech-writing-communication-919852/) says, "For instance, a spoken ‘The man is catching a fish' could end up as ‘The men is catches a fish' when the same person puts pen to paper." The article, which quotes the publication of the study in Psychological Science, adds that "writing and speaking are supported by different parts of the brain - and not just in terms of motor control in the hand and mouth, but in the high-level aspects of word construction."
According to Brenda Rapp, professor of cognitive science at Johns Hopkins University, "Actually seeing people say one thing and - at the same time - write another is startling and surprising. We don't expect that we would produce different words in speech and writing. It's as though there were two quasi-independent language systems in the brain."
The study was conducted to enable understanding of the way in which the brain organizes knowledge of written language when stroke victims suffer from aphasia, defined by the National Institute of Neurological Disorders and Stroke (NINDS) at the National Institutes of Health (NIH) as "a neurological disorder caused by damage to the portions of the brain that are responsible for language" (http://www.ninds.nih.gov/disorders/aphasia/aphasia.htm).
According to the NINDS website, "Primary signs of the disorder include difficulty in expressing oneself when speaking, trouble understanding speech, and difficulty with reading and writing. Aphasia is not a disease, but a symptom of brain damage. Most commonly seen in adults who have suffered a stroke, aphasia can also result from a brain tumor, infection, head injury or dementia that damages the brain. It is estimated that about 1 million people in the United States today suffer from aphasia. The type and severity of language dysfunction depends on the precise location and extent of the damaged brain tissue."
The idea of the Johns Hopkins study was to determine how reading and spelling were organized, because there appeared to be a "genetic blueprint for spoken language but not the more recently evolved written system." The researchers wanted to figure out whether written language depended on spoken language in literate adults. If that were the case, they would expect that there would be similar errors in speech and writing. If not, researchers would conclude that people do not necessarily write exactly what they say.
In studying five stroke victims with aphasia, they found that "four had difficulty writing sentences with the proper suffixes, but had few problems speaking the same sentences. The last individual had the opposite problem-trouble speaking but unaffected writing."
When the researchers showed the stroke victims pictures and asked them to describe the action, one individual would say, "The boy is walking," but write, "the boy is walked." Another would say, "Dave is eating an apple" and then write, "Dave is eats an apple."
Rapp concluded, "We found that the brain is not just a ‘dumb' machine that knows about letters and their order, but that it is ‘smart' and sophisticated and knows about word parts and how they fit together."