Summary: A new study reveals men with dyslexia have altered structural connections between the thalamus and auditory cortex on the left side of the brain.
Men with dyslexia have altered structural connections between the thalamus and auditory cortex on the left side of the brain, new research published in Journal of Neuroscience reveals. The study extends similar observations of the dyslexic visual system and highlights the importance of early sensory processing for reading proficiency.
Neural fibers connect a subcortical structure in the auditory pathway — the left medial geniculate body (MGB) — to part of the cerebral cortex called the motion-sensitive planum temporale (mPT).
Nadja Tschentscher and colleagues present evidence that the strength of this pathway is reduced in adults with dyslexia compared to typical readers. The researchers found left MGB-mPT connectivity was associated with reading fluency only in typical readers, while previous studies reported associations between an analogous visual pathway and reading ability in both dyslexics and typical readers.
Taken together, the results broaden our understanding of dyslexia — one of the most common learning disabilities — to include alterations in lower as well as higher brain structures.
Funding: The research was funded by the Max Planck Society, European Research Council.
Source: David Barnstone – SfN
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to Tschentscher et al., JNeurosci (2019).
Original Research: Abstract for “Reduced structural connectivity between left auditory thalamus and the motion-sensitive planum temporale in developmental dyslexia” by Nadja Tschentscher, Anja Ruisinger, Helen Blank, Begoña Díaz and Katharina von Kriegstein in Journal of Neuroscience. Published January 14 2019.
Reduced structural connectivity between left auditory thalamus and the motion-sensitive planum temporale in developmental dyslexia
Developmental dyslexia is characterized by the inability to acquire typical reading and writing skills. Dyslexia has been frequently linked to cerebral cortex alterations; however recent evidence also points towards sensory thalamus dysfunctions: dyslexics showed reduced responses in the left auditory thalamus (medial geniculate body, MGB) during speech processing in contrast to neurotypical readers. In addition, in the visual modality, dyslexics have reduced structural connectivity between the left visual thalamus (lateral geniculate nucleus, LGN) and V5/MT – a cerebral cortex region involved in visual movement processing. Higher LGN-V5/MT connectivity in dyslexics was associated with the faster rapid naming of letters and numbers (RANln), a measure that is highly correlated with reading proficiency. We here tested two hypotheses that were directly derived from these previous findings. First, we tested the hypothesis that dyslexics have reduced structural connectivity between the left MGB and the auditory motion-sensitive part of the left planum temporale (mPT). Second, we hypothesized that the amount of left mPT-MGB connectivity correlates with dyslexics RANln scores. Using diffusion tensor imaging based probabilistic tracking we show that male adults with developmental dyslexia have reduced structural connectivity between the left MGB and the left mPT — confirming the first hypothesis. Stronger left mPT-MGB connectivity was not associated with faster RANnl scores in dyslexics, but in neurotypical readers. Our findings provide first evidence that reduced cortico-thalamic connectivity in the auditory modality is a feature of developmental dyslexia, and that it may also impact on reading related cognitive abilities in neurotypical readers.
Developmental dyslexia is one of the most widespread learning disabilities. While previous neuroimaging research mainly focused on pathomechanisms of dyslexia at the cerebral cortex level, several lines of evidence suggest an atypical functioning of subcortical sensory structures. By means of diffusion tensor imaging, we here show that dyslexic male adults have reduced white matter connectivity in a cortico-thalamic auditory pathway between the left auditory motion-sensitive planum temporale (mPT) and the left medial geniculate body (MGB). Connectivity strength of this pathway was associated with measures of reading fluency in neurotypical readers. This is novel evidence on the neurocognitive correlates of reading proficiency, highlighting the importance of cortico-subcortical interactions between regions involved in the processing of spectrotemporally complex sound.