|MadSci Network: Neuroscience|
Dear Gaby, The majority of us learn language with our auditory systems, associating meaning with various sounds. It is only later that we associate the sounds with words on a page, and thereby link meaning to visual representations of language. It seems as if the auditory association never weakens; there is a distinct feeling of sound even when we read silently. Indeed, functional brain imaging even shows that those regions of our brain responsible for hearing become active when we read. How then does a congenitally deaf person perceive written language? Do those regions of their brain responsible for hand movements become active, because they associate written language with sign language? It is important to preface our discussion by saying that the brain is highly plastic. That is, the brain is very good at reorganizing itself (for further discussion, see the question, “Why do humans use only a part of their brain? at www.hhmi.org/askascientist/neuroscience.html). Therefore, the brain regions that we commonly associate with various tasks in the general population may be doing very different things in the congenitally deaf. You can imagine that in the absence of auditory stimulation, the region of the brain dedicated to processing sound would be used for something else. Thus, in a brain scan you can see that that part of the brain is being activated by some task, but you cannot then say that that task is giving rise to auditory perceptions. Activity in that region may underlie visual or somatosensory perceptions. Thus, it is very difficult to interpret functional imaging experiments in terms of how a deaf-born person perceives written language. With that caveat in mind, what do you see in the brain of a deaf person who is reading? A recent study by Dean Shibata and colleagues (as described in a poster presentation at a scientific conference; see www.apnet.com/www/journal/hbm2000/6825.html) compared brain activity in deaf and hearing people who were reading English. They found that the deaf subjects seemed to have more even activation of visual and auditory cortex, which may indicate that the subjects utilize the actual image of words more. The researchers also found that areas of the brain used for hand motor activity were activated in the deaf, which may suggest that the deaf are mentally converting written words with signed words as they read. If this is true, it is analogous to us “hearing” the words we read. This study is not conclusive, but it does give you a taste of how people are investigating your question. To get at the issue in a different fashion, you propose arranging the letters of one language in such a way that, when read aloud, they sound like a word in the second language; meanwhile, the arrangement of letters means nothing and perhaps sounds like nonsense in the first language. The only link between the two languages is the sound. Would a deaf person understand it? Because he cannot hear or intuit the sound, he would be faced with a meaningless assortment of garbled words. If you can think of other experiments, please propose them. You never know if they might prove useful to a neuroscientist investigating these issues. Best regards, Michael
Try the links in the MadSci Library for more information on Neuroscience.