New Light Shed On The Relationship Between Thought And Language
NASHVILLE, Tenn. -- Individuals who have been blind from birth use different parts of their brain when they read Braille than do those who lost their sight later in life -- a difference that sheds new light on the relationship between thought and language.
That is the conclusion of a study performed by Vanderbilt researchers and reported in the November issue of the scientific journal Human Brain Mapping. The research is one of the latest efforts to understand the structure of the human brain and how it responds to injury.
Twenty years ago most neuroscientists thought that the adult human brain had a very limited ability to reorganize itself following major injuries to the brain itself or to the peripheral nervous system that provides it with sensory information. In recent years, however, researchers have discovered that mature brains, as well as developing brains, display more flexibility than they had thought.
One way to study the phenomena associated with the brain's plasticity -- its ability to adapt to changes -- is to examine differences in brain organization in people who have lost their sight at an early age. In sighted individuals, nearly one-third of the brain is devoted to processing visual information. Cutting off all sensory input to such a large region of the cortex creates a situation where recruitment of some of the unused areas by the other senses seems likely. So scientists have looked for, and found, evidence that some of the areas of the idle visual cortex can be recruited to process other types of sensory information.
In 1996 a Japanese scientist, N. Sadato, working at the National Institutes of Health, reported that positron emission tomography (PET) scans averaged from the brains of several blind subjects displayed activity in parts of the visual cortex while reading Braille. It was unclear, however, to what extend this activity might represent activation of visual memories that individuals had acquired before they were blind and how much represented an actual repurposing of visual areas of the brain to handle touch information.
To help answer this question, Psychology Professor Ford F. Ebner and Research Assistant Professor of Psychology Peter Melzer -- with technical assistance from members of Vanderbilt's radiology department -- turned to functional Magnetic Resonance Imaging (fMRI), a technique can detect active areas of brain activity by measuring activity-induced changes in blood flow.
In order to distinguish between activation of visual memories and processing of touch information, the investigators recruited a group of five men and five women, half of whom had been blind since birth and half of whom lost their sight early in life. The researchers reasoned that those who had been blind since birth would not have had an opportunity to store up visual memories, so all the brain activity that they exhibited in areas associated with the visual system would be areas that had been recruited to process sensory touch information.
The subjects were asked to read a series of single words in Braille. Ten percent of the words were nouns that referred to abstract concepts while 90 percent were nouns that referred to objects associated with visual images.
FMRI takes snapshots of brain activity much more quickly than PET scans, which must average brain activity over several minutes and pool the data from several individuals. By asking their subjects to alternate reading and resting, the researchers were not only able to measure which areas become active but also when the activation occurred relative to the initial stimuli. This allowed them to differentiate between areas involved in the response to the touch information and those that were not activated by act of reading but by some type of higher-level processing.
Surprisingly, the researchers did not find major differences in the magnitude and expanse of activation in the visual cortex between the two groups. But they did find striking differences in the activation behavior, the relationship between the timing of the activation of specific visual areas and the task. In the blind-from-birth group, activation of a region in the posterior temporal lobe that is involved in phonological word processing -- keeping track of the sound patterns and rules of pronunciation in speech -- was more strongly correlated with reading than it was in the group with some visual experience. Conversely, an adjacent region associated with semantic word processing -- determining the meaning of similarly sounding words like flour and flower -- had the stronger correlation with the task in the group with some visual experience than it did in the group that was blind from birth.
The researchers hypothesize that even a short period of early visual experience may make it harder to recruit certain areas in the visual cortex than is the case for those who are blind for birth. They propose that this may be one reason why those who are blind from birth tend to be much better Braille readers than those who loose their sight later in life.
The study also provides new support for the proposition that a kind of mental imagery exists which is independent of the five senses. The subjects who were blind from birth reported having non-visual associations with some of the words. The areas of the brain that are involved in high-level processing of the words in the study strongly suggest that this non-visual imagery is closely related to language. So there is a good chance that further studies may shed new light on an outstanding issue in philosophy and psychology: the relationship between language and thought.
Also contributing to the study were Assistant Professor Victoria L. Morgan, Associate Professor David R. Pickens and Professor Ronald R. Price from the department of radiology and radiological services, and Robert S. Wall, assistant professor of hearing and speech at Vanderbilt's Bill Wilkerson Center.
Funding was provided by the National Institute of Neurological Disorders and Stroke, the John F. Kennedy Center for Research in Human Development at Vanderbilt University, the Vanderbilt Vision Research Center, the Hobbs Foundation and Dr. and Mrs. Irwin Eskind.