Grace, Gloria Mary2018-06-212018-06-2119902018-06-21http://hdl.handle.net/1828/9488There exists a paucity of research addressing the treatment of dyslexia despite its high prevalence and its potential debilitating effects on an individual's cognitive and emotional status. One of the few research programs addressing this issue is headed by Dirk Bakker. Bakker proposes a balance model of dyslexia, specifying L- and P-type dyslexics and an associated treatment technique called hemisphere-specific stimulation (HSS). In general, Bakker's findings suggest that HSS may improve the accuracy and efficiency of reading performance, particularly in L-type dyslexics. In addition, HSS appears to modify the electrophysiological activity of the brain, reflected by a relative change in the left- and right sided amplitude of the event-related potential (ERP), elicited by flashed words. Although Bakker's findings suggest that HSS may be a potentially successful treatment approach to dyslexia, no studies attempting replication of his results by independent investigators have been published. The present study was designed to replicate the major findings reported by Bakker (Bakker & Vinke, 1985). In the present study, 26 reading-disabled children were classified into 17 L-type and 9 P-type readers (mean age = 11.0 years) and then randomly assigned to experimental and control groups. Disabled readers in the experimental groups (L1, N=8; P1, N=5) received 18 sessions (45 minutes twice weekly) of HSS according to Bakker's specifications. HSS involved the stimulation of the right or left cerebral hemisphere through the computerized presentation of words and letters flashed in the left and right visual fields for L- and P-types respectively. Control subjects (L2, N=9; P2, N=4) received no treatment, and remained in their classrooms during training sessions. All subjects were tested with reading and spelling measures prior to training (pre-test), halfway through training (mid-test), and after training (post-test). In addition, ERPs, in response to centrally flashed words, from left and right parietal and temporal locations were collected from all subjects before and after training. In general, neither the academic nor the electrophysiological results of this study replicated those of Bakker and Vinke (1985). Although all groups tended to show improvements over time on most academic variables, experimental groups did not improve significantly more than controls on most measures of reading and spelling. In terms of ERP changes, the P300 component of the waveform corresponded most closely to the portion of the ERP on which Bakker and Vinke based their findings. In contrast to Bakker's results, L1 in the present study showed a "leftening" effect (a relative increase in left hemisphere activity) relative to L2. However, consistent with Bakker's findings, P1 showed a leftening effect relative to P2. Although replication of Bakker's results did not occur, the study was of considerable heuristic value in the critical appraisal of Bakker's model of dyslexia and associated treatment. Critical analysis of the L/P classification system revealed that the most serious difficulty is poor internal validity. It was argued that Bakker's classification system is in need of revision and further testing before external validity, i.e., the efficacy of HSS, can be accurately assessed.enAvailable to the World Wide WebDyslexiaDyslexic childrenTreatmentThesis