Dyslexia is one of the most widespread learning disabilities, with many different therapeutic and learning strategies used to tackle the reading and writing difficulties associated with the disorder, however to-date there is no cure. The main theory posits dysfunctional processing of auditory speech as a cause of dyslexia, with the reasons for these alterations in speech processing still unknown.

Now, a study from researchers at TU Dresden shows people with dyslexia have a weakly developed structure at the subcortical processing stage. The team states their data shows weak white matter connectivity between the left auditory motion-sensitive planum temporale (mPT) and the left auditory thalamus (MGB). The study is published in the Journal of Neuroscience.

Mapping dyslexia in the brain

Previous studies show dyslexia is characterized by the inability to acquire typical reading and writing skills. Although past neuroimaging research mainly focused on neural mechanisms of dyslexia at the cerebral cortex level, several lines of evidence suggest the malfunctioning of subcortical sensory structures is in play. The current study shows dyslexic male adults have reduced white matter connectivity in a cortico-thalamic auditory pathway between the mPT and MGB.

The current study analyses people with developmental dyslexia in comparison to people without dyslexia and conducts diagnostic tests of the brain. Results show people with dyslexia have less fiber connectivity between the mPT and MGB in the left hemisphere of the brain than people from the control group. Data findings show the control group exhibit very strong fiber connectivity between the mPT and MGB, particularly those who perform well in reading tests.

The lab hypothesized the amount of left mPT–MGB connectivity correlates with dyslexics’ rapid naming of letters and numbers, known as RANln scores. Diffusion tensor imaging indicates dyslexics have reduced structural connectivity between the left MGB and the left mPT, confirming this hypothesis.

Targets in the brain for dyslexia

Results show stronger left mPT–MGB connectivity was not associated with faster RANln scores in dyslexics, only in neurotypical readers. The group states findings provide the first evidence of reduced cortico-thalamic connectivity in the auditory modality as a feature of developmental dyslexia may also affect reading abilities in neurotypical readers.

The team surmises they have identified the neural mechanisms of developmental dyslexia. For the future, the researchers state understanding the neural mechanisms of developmental dyslexia will be decisive for the development of early diagnostics and targeted therapies to treat the disorder.

Source: Technische Universität Dresden

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