  Developmental dyslexia is characterized by persistent reading and spelling
deficits. Partly due to technical challenges with investigating subcortical
sensory structures, current research on dyslexia in humans by-and-large focuses
on the cerebral cortex. These studies found that dyslexia is typically
associated with functional and structural alterations of a distributed
left-hemispheric cerebral cortex network. However, findings from animal models
and post-mortem studies in humans suggest that developmental dyslexia might
also be associated with structural alterations in subcortical sensory pathways.
Whether these alterations also exist in developmental dyslexia in-vivo and how
they relate to dyslexia symptoms is currently unknown. Here we used ultra-high
resolution structural magnetic resonance imaging (MRI), diffusion MRI and
probabilistic tractography to investigate the structural connections of the
visual sensory pathway in dyslexia in-vivo. We discovered that individuals with
developmental dyslexia have reduced structural connections in the direct
pathway between the left visual thalamus (LGN) and left middle temporal area
V5/MT, but not between the left LGN and left primary visual cortex (V1). In
addition, left V5/MT-LGN connectivity strength correlated with rapid naming
abilities - a key deficit in dyslexia [14]. These findings provide the first
evidence of specific structural alterations in the connections between the
sensory thalamus and cortex in developmental dyslexia. The results challenge
current standard models and provide novel evidence for the importance of
cortico-thalamic interactions in explaining dyslexia.
