LOS ANGELES - A faulty connection between where the brain stores the auditory building blocks of language and where it processes them may be to blame for dyslexia, a new study suggests.
The findings represent the first neuroanatomical evidence that the vexing spelling and reading disorder lies in a connectivity problem in the brain's white matter, where nerve fibers relay electrochemical signals.
Various degrees of dyslexia strike about one in 10 people, making it difficult for them to analyze and assemble letter combinations and relate them to the auditory packets of learned language, called phonemes.
A Belgium-based research team mapped the brain activity involved in phoneme sorting. They used functional MRI machines to scan the brains of 45 college-age adults (23 with dyslexia, 22 without) while they distinguished among a variety of sounds, some of which differed in subtle ways, then built a sophisticated correlational map.
Both groups were able to distinguish and sort those sounds accurately, though the dyslexic group did so more slowly, according to the study, published online Thursday in the journal Science. Both groups also showed consistent neuron firing patterns as they sorted these sounds, the study found.
"Quite to our surprise we found that the phonetic representations are perfectly intact in dyslexics," said Bart Boets, a clinical psychologist at Catholic University of Leuven, in Belgium, lead author of the study. "They are just as robust and distinct as in typical readers."
The researchers then examined how well the 13 studied areas of the brain communicated with one another. They measured synchronized low-frequency activity - a kind of background chatter characteristic of circuits that tend to work in concert. For both groups, those signals suggested functional connectivity was normal between hemispheres of the auditory cortex - the area associated with phoneme recognition.
But the white matter tracts connecting the phoneme area to a part of the brain that contains the Broca's area, which regulates the conversion of language to speech, was weaker for the dyslexics as a group, the study found. That connectivity deficit correlated with low scores on such tasks as word or non-word reading, verbal memory, and phoneme discrimination.