Significance probability mapping (SPM) of brain electrical activity first described by Duffy et al. is useful tool for studying functional aspects of brain disease. Z-statistic SPM is able to identify the area of brain electrical activity deviated with statistic significance from the control group. The problem of this method is, however, that the nature of deviation, i.e., whether it is increase or decrease of electrical activity, can not be displayed. From this point of view, we attempted to use modified z-statistic method. Statistically deviated region and its nature can be clearly displayed on the same picture by analyzing EEG with this method. This method can be applied to SPM of evoked potentials. SPM is not yet complete method for the assessment of brain electrical activity, but there is much room for adopting other statistic method that is more suitable for the aim of the study. Functional aspects of the brain will be more readily clarified by the use of modified SPM and by combination with findings of CT scan, NMR and PET that can give morphological and metabolic information.