The dyslexic adults performed poorly on standardized reading tests. And, as has been shown to be the case with the great majority of adult dyslexics, these poor readers (ages 18-42) also performed poorly on a variety of tests that measured their ability to discern rapidly successive sound stimuli.
In one of these sound-discerning tests, adults were exposed to two sounds that differed in frequency and that occurred a tenth or a fifth of a second apart. They were then asked to identify the sounds and to replay the sequence in which they were presented. Their brain activity was simultaneously recorded using magnetoencephalographic brain imaging, which measures magnetic field fluctuations generated by spatially localizable human brain activity with millisecond precision.
In these studies, the UCSF team focused on the activity generated by the rapidly successive sounds evoked from the "primary" auditory cortical areas, where information about aural speech flows into the cerebral cortex's processing system for language.
Poor readers did report hearing the two very brief sounds, and often knew that in some way they weren't the same, but they were unable to identify them, or to reliably reconstruct the sequence in which they were represented.
"The reason," said Srikantan Nagarajan, Ph.D., an assistant adjunct professor of otolaryngology and a member of the Keck Center for Integrative Neuroscience at UCSF, and the lead scientist of the study, "was demonstrated by the abnormal way that the brain of the poor-reading subjects responded to these rapidly successive sound events."
"In normal readers, the auditory cortex generated clear, separate
representations of sounds occurring within the time dimensions of a syllable,"
said Nagarajan. "In poor readers,
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Contact: Jennifer O'Brien
jobrien@pubaff.ucsf.edu
415-476-2557
University of California - San Francisco
25-May-1999