案例分析题
Some people have a more complicated sensory life than others. Most taste tuna only when they eat it. A few, though, taste it when they hear a particular word, such as "castanet". Others link the color red with the letter "S" or make some other inappropriate connection between stimulus and response. Such people are known as synaesthetes, and the phenomenon of synaesthesia has puzzled brain scientists since it was recognized over a century ago.
Most researchers in the field suspect synaesthesia is caused by crossed wires in synaesthetes’ brains, but until recently they have had no way to check this hypothesis. However, the development of a technique called diffusion-tensor imaging has changed that. And researchers at the University of Amsterdam have just applied it to the brains of 18 women (the sex more likely to experience synaesthesia) who have the most common form of the condition. This is called grapheme-color synaesthesia. It is a tendency to see letters and numbers in color.
Diffusion-tensor imaging measures the direction of movement of water molecules. Since the filaments that connect distant nerve cells are surrounded by fatty sheaths which restrict the movement of water, such molecules tend to move along a filament rather than out of it. The upshot is that the technique can detect bundles of such filaments running from one part of the brain to another.
The grapheme-colour synaesthesia was chosen to study for two reasons. One was that it is common. The other was because there is a specific hypothesis as to its cause. Earlier brainscanning studies have shown that the part of the brain which identifies word shape is in an area called the fusiform gyrus. This is next to an area known as V4, which identifies color. Both light up simultaneously in traditional scanners when someone is experiencing grapheme-colour synaesthesia, so an inappropriate link between them is an obvious thing to look for.
Diffusion-tensor imaging did, indeed, show strong connections between these two areas in the brains of the synaesthetes when they slid into the scanner and viewed color-evoking letters and numbers. Other unusual connections showed up too, suggesting the phenomenon is more complex than had been appreciated. By contrast, the brains of 18 non-synaesthetes matched with the volunteers by age and sex showed no such strong connections.
And the revelations went further. Certain types of grapheme-color synaesthetes have more connectivity than others. As part of the study, researchers asked volunteers to fill out questionnaires about how they experience their color sensations. Some reported seeing color projected on to whatever word or number they were shown. Members of this group are known as "projectors". Others, known as "associators", reported color only in their mind’s eye. Although both groups of synaesthetes had much more connectivity than non-synaesthetes, projectors had noticeably more again than associators.
Researchers are not yet sure what form the stronger connections take. They could be the result of more filaments than normal connecting the areas in question. Or the filaments might be broader than normal. Or the fatty coatings of the filaments might be thicker, which would amplify the signal passing along them as well as keeping water molecules on the straight-andnarrow.
How synaesthesia starts is also unclear. There is evidence of a genetic component but learning must be involved as well. People are not born with the concept of the letter "A". And they certainly are not born with a bright red, cherry-colored "A"-however much it feels, as synaesthetes insist, as though they were.