Researchers are reporting direct neurological evidence that the region of the brain that processes brief, rapidly successive sounds is functionally abnormal in adults with the reading disability known as dyslexia. The findings, documented through simultaneous brain imaging and behavioral tests, strongly indicate, the researchers said, that adult dyslexics have an enduring neurological deficit in their ability to process these brief, rapidly successive sounds.
They suspect that the deficiency contributes to difficulties in early speech and language learning, and leads to a weakness in the subsequent mental leap in abstraction to words on a page that enables people to learn to read. The study was published in the May 24 issue of Proceedings of the National Academy of Science.
Perhaps the most provocative aspect of the finding, the researchers said, is the clear and direct neurological evidence that reading deficits are generated, at least in part, by a deficit at a very fundamental level of cortical processing of sound inputs.
"Our findings indicate that there is a basic problem in signal reception, as complex sound information streams into the cerebral cortical system underlying aural speech representation," said the senior author of the study, Michael Merzenich, Ph.D., the Francis A. Sooy Professor of Otolaryngology and a member of the Keck Center for Integrative Neuroscience at UC San Francisco. "The way that the brain processes sound in poor readers is very different from its processing and representation of rapidly changing sound inputs in competent readers."
"Our research indicates that adult dyslexics are representing the sound parts of words by the activation of cortical neuron populations in a weaker and less salient form within their cortical aural speech processing system. We believe that they, therefore, are not delivering the normal forms of representation of the separable sound parts of words to the regions of the brain involved in speech perception and reading," he said.
The authors emphasize that their findings do not discount the additional involvement of higher levels of brain processing in dyslexia, where more complex combinations of information lead to the recognition and interpretation of speech.
At the same time, they argue that the very elementary defect in the brain's processing of sound must be playing an important role in the generation of relatively weak neuronal representations of the sound parts of aural speech. And this elementary neurological deficit, they said, could provide a target for remedial therapies aimed at training the brain to increase the speed and accuracy with which it processes rapidly successive and rapidly changing sounds.
The sound-processing function occurs at a base, or entry, level of sound processing in the brain, and is believed to be a primary step in the brain's representation of normal speech sounds and its creation of speech and language-reception abilities. The process ultimately culminates with a listener learning to recognize the sound parts of words, and to translate these word sounds as written letters.
Previous behavioral studies have suggested that the inability to parse the rapidly successive, changing sounds that make up words, the phonemes of language, may be the primary basis of language-learning impairments in children. Scientists have long argued that children who have difficulty parsing word sounds are destined to have difficulty successfully initiating reading. Other behavioral studies have indicated that most people with dyslexia, characterized by a difficulty with reading, also have impairments in the fidelity of their auditory reception. However, because most dyslexics ultimately develop facile speech reception and production capabilities, the significance of this problem for the origin of reading impairments has been unclear.
The researchers conducted their current study in seven dyslexic adults who were of normal intelligence but severely challenged by reading, spelling and writing. Results were compared with those recorded in seven adults of normal intelligence who were competent readers.
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, the brain separately generated only very weak representations of sound events past the first sound.
"In the normal reader, successive intra-syllabic sound events are separately represented in high fidelity within the processing channels of the 'primary' auditory cortex. In the impaired reader, they are not," he said.
"These findings are consistent with the increasing evidence," said Merzenich, "that language-impaired and reading-impaired children are a very broadly synonymous population. Scientists have historically argued that only a small percentage of dyslexics have a clear history of early language impairment and fundamental auditory processing deficits. To the contrary, we have seen that most poor readers and most language-impaired children share these same fundamental listening and brain processing abnormalities."
Moreover, he said, "The studies show that these fundamental listening problems clearly persist across a lifetime, even while the basic speech reception abilities of these individuals can ultimately achieve a relatively normal competency."
Co-authors of the study were Henry Mahncke, Ph.D., a research fellow in the Keck Center at UCSF; Talya Salz, of Scientific Learning Corporation in Berkeley, CA; Paula Tallal, Ph.D., co-director of the Center for Molecular and Behavioral Neuroscience at Rutgers University, Newark, NJ; and Timothy Roberts, Ph.D., assistant professor of radiology in the Biomagnetic Imaging Laboratory, Department of Radiology, at UCSF.
The study was funded by the National Institutes of Health, the Office of Naval Research, Hearing Research Inc., and the Coleman fund.