Whether essential processing of spatial information is lateralized asymmetrically in the human cortex is still a matter of debate. In this study, items of an Item Response Theory calibrated test for spatial ability were used to ensure stimulus homogeneity and validity. Subjects were preselected as extreme groups of good and poor spatializers. Mapping of true DC-recorded slow potential shifts (SPSs) resulted in distinctly discriminable topographies with spatial and verbal-analytic material as well as with spatial performance groups within the spatial block. Left fronto-central negativity maxima in the verbal condition clearly contrasted with occipito-parietal peak activity in the spatial condition. Poor spatializers showed higher amplitudes as well as a tendency to asymmetric activity in right parietal (parieto-temporal) areas, whereas in good spatializers the activity was localized symmetrically in occipital and occipito-parietal regions. The findings emphasize the importance of the right posterior cortex for spatial processing (negativity maxima at occipital and right parietal sites) and suggest a task-specific lower cortical efficiency or, seen from a processing perspective, a higher Investment of Cortical Effort (ICE) on the part of poor spatializers.