Neuronal asymmetries in primary visual cortex of dyslexic and nondyslexic brains

Dyslexic brains exhibit histologic changes in the magnocellular (magno) cel ls of the lateral geniculate nucleus, and consistent with these changes, dy slexics demonstrate abnormal visually evoked potentials and brain activatio n to magno-specific stimuli. The current study was aimed at determining whe ther these findings were associated with changes in the primary visual cort ex with the prediction that magno components of this cortex would be affect ed. We measured cross-sectional neuronal areas in primary visual cortex (ar ea 17) in dyslexic and nondyslexic autopsy specimens. There was a significa nt interaction between hemispheres and diagnostic category; ie, nondyslexic brains had larger neurons in the left hemisphere, whereas dyslexic brains had no asymmetry. On the other hand, cell layers associated with magno inpu t from the lateral geniculate nucleus did not show consistent changes in dy slexic brains. Thus, there is a neuronal size asymmetry in favor of the lef t primary visual cortex in nondyslexics that is absent in dyslexic brains. This is yet another example of anomalous expression of cerebral asymmetry i n dyslexia similar to that of the planum temporale, which in our view refle cts abnormality in circuits involved in reading.