Effects of hemisphere-specific stimulation on academic performance and event-related potentials in dyslexic children

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. (Abstract shortened by UMI.)...