Deep dyslexia and right hemisphere reading - A regional cerebral blood flow study

Deep dyslexia is an acquired reading disorder that is characterized by the production of semantic reading errors, greater success when reading aloud concrete and highly imageable words, frequent visual and visual-semantic errors, morphological errors and very poor reading of nonwords. The right hemisphere hypothesis proposes that in deep dyslexia the patient is not reading with an impaired version of the normal left hemisphere reading system, and cannot use that system for reading at all. Instead, a different reading system, located in the right hemisphere is used. The right hemisphere hypothesis was examined in this study by investigating the amount of cortical activation in the left and right cerebral hemispheres of a deep dyslexic patient (L.H.) during visual word recognition. Three experimental tasks were devised to isolate a Visual Word Recognition process and a Spoken Word Production process and these tasks were administered to the deep dyslexic patient as well as another patient with left-hemisphere-damage but a different form of acquired dyslexia (surface dyslexia) and two matched control subjects. Regional cerebral blood flow (rCBF) was monitored during performance on each of the tasks. For L.H., but not the other three subjects, rCBF in the right hemisphere was greater than in the left hemisphere during Visual Word Recognition. By contrast, there was greater activation of the left hemisphere than the right hemisphere for L.H. during Spoken Word Production; this was also true of the other three subjects, but the effect was statistically significant only for L.H. These results support the right-hemisphere hypothesis of deep dyslexia.