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 readi ng aloud concrete and highly imageable words, frequent visual and visu al-semantic errors, morphological errors and very poor reading of nonw ords. The right hemisphere hypothesis proposes that in deep dyslexia t he patient is not reading with an impaired version of the normal left hemisphere reading system, and cannot use that system for reading at a ll. Instead, a different reading system, located in the right hemisphe re is used. The right hemisphere hypothesis was examined in this study by investigating the amount of cortical activation in the left and ri ght cerebral hemispheres of a deep dyslexic patient (L. H.) during vis ual word recognition. Three experimental tasks were devised to isolate a Visual Word Recognition process and a Spoken Word Production proces s and these tasks were administered to the deep dyslexic patient as we ll as another patient with left-hemisphere-damage but a different form of acquired dyslexia (surface dyslexia) and two matched control subje cts. Regional cerebral blood flow (rCBF) was monitored during performa nce 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 activat ion 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 re sults support the right-hemisphere hypothesis of deep dyslexia.