of ep response for each subject the resulting brain state frames and ep response frames for the dyslexic group and the control group were then averaged to form mean frames of each group for each state and stimulus the two groups of mean images were then compared using the t statistic function a further transformation produced a matrix of percentile index values pi whose value is related inversely to t values the pi values permit a graphic localization of regions of maximum difference between the dyslexic group and the control group by topographically displaying the pi matrices for alpha and theta for each brain state and for each ep stimulus it was possible to identify the brain regions which differed between the dyslexics and the controls as a final step a new display matrix was formed which summarized the differences reflected in all of the pi matrices as indicated by the occurrence of a certain pi level on at least one of the underlying pi matrices the map of pi differences having a value of at least 2 identified four brain areas related to dyslexia 1 bilateral medical frontal 2 left anteriolateral frontal 3 left mid temporal and 4 left posterolateral quadrant classic concepts of dyslexia had not suggested the involvement of all of these brain areas in dyslexics the study also indicated that alpha brain activity was involved in dyslexia as well as the theta activity which has previously been viewed as of.