Neurological research into dyslexia
Brain scanning or neuroimaging as we know it today began to be developed in the 1980s and 1990s. The present day Brain Imaging Techniques are:
- Computed tomography (CT) or computed axial tomography (CAT)
- Diffuse optical imaging (DOI) or Diffuse Optical Tomography (DOT)
- Event-related optical signal (EROS)
- Magnetic resonance imaging (MRI),
- Functional magnetic resonance imaging (fMRI)
- Magnetoencephalography (MEG) which also uses superconducting quantum interference devices (SQUIDs)
- Positron emission tomography (PET)
- Single photon emission computed tomography (SPECT).
The neuroimaging techniques mainly used in dyslexia research have been functional Magnetic Resonance Imaging (fMRI) and positron emission tomography (PET), which have produced clear evidence of structural differences in the brains of children with reading difficulties. It has been found that people with dyslexia have a deficit in parts of the left hemisphere of the brain involved in reading, which includes the inferior frontal gyrus, inferior parietal lobule, and middle and ventral temporal cortex.
Brain activation studies using PET to study language have produced a breakthrough in our understanding of the neural basis of language over the past decade. A neural basis for the visual lexicon and for auditory verbal short term memory components have been proposed. Wernicke's and Broca's areas are being recast in terms of localized components of phonological input and output. Some classical regions, such as the arcuate fasciculus, are having their "classical" roles questioned, while other regions, such as the basal temporal language zone, are growing progressively in terms of their recognized importance. with some implication that the observed neural manifestation of developmental dyslexia is task-specific (i.e., functional rather than structural)
A University of Hong Kong study argues that dyslexia affects different structural parts of children's brains depending on the language which the children read. The study focused on comparing children that were raised reading English and children raised reading Chinese. Using fMRI technology researchers found that the children reading English used a different part of the brain than those reading Chinese. Researchers were surprised by this discovery and hope that the findings will help lead them to any neurobiological cause for dyslexia.
A University of Maastricht (Netherlands) study revealed that adult dyslexic readers underactivate the superior temporal cortex for the integration of letters and speech sounds. This reduced audiovisual integration is directly associated with a more fundamental deficit in auditory processing of speech sounds, which in turn predicts performance on phonological tasks. The data also provides a neurofunctional account of developmental dyslexia, in which phonological processing deficits are linked to reading failure through a deficit in neural integration of letters and speech sounds.
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