Dyslexia is a developmental language disorder, characterized by deficits in phonological awareness, the explicit awareness of phonemes as separate entities, as operationalized with tasks that require the segmentation of words into their constituent phonemes and the manipulation of these units. These deficits lead to difficulties in reading decoding (i.e., accuracy) that, in severe cases, persist into adulthood. Although reading comprehension is constrained by decoding skill, comprehension is frequently better, in keeping with better listening comprehension, i.e., the ability to understand spoken language, thus distinguishing dyslexia from developmental dysphasia. Prevalence is generally estimated to be 5 to 10% in the school-age population.
The purpose of our research on dyslexia is to determine whether subtle structural brain abnormalities might underlie the variations in brain function observed in persons with dyslexia. Our research study uses anatomical magnetic resonance imaging (MRI) to examine the neuroanatomy in a dyslexic male subject group. As functional imaging findings have pointed to the temporal lobes and the left parietal lobe as the most probable sites of neuropathology underlying dyslexia, our research has focused on these regions as being the most likely to demonstrate anatomical differences between subjects with dyslexia and normal controls.
Structural and morphometric imaging findings
The major finding of our study in dyslexia thus far is a decrease of tissue volume in the temporal lobes of men with dyslexia which was most prominent on the left hemisphere. The subjects with dyslexia also slowed a significant reduction in combined (right + left) temporal gray matter which was also more pronounced in the left temporal lobe. Thus, the decrease in tissue in the temporal lobes is primarily attributable to a decrease in gray matter. These findings in conjunction with the anatomical functional findings of other samples support a view that dyslexia is a disorder whose neurobiology involved bilateral temporal regions.