Previous studies on the organization of semantic knowledge indicate that natural (animals) and artifactual (tools) objects activate different cortical areas in object identifications tasks (category-specific effects). This dissociation indicates that specific aspects of semantic knowledge are differentially relevant for representing natural (visual features) and artifactual objects (functional). However the functional and structural organization of the semantic system and the interpretation of category-specific effects are still a matter of controversy. Within a repetition priming paradigm, we tested the prediction whether visual and functional aspects of semantic knowledge are differentially important for artifactual and natural objects by measuring event-related brain potentials (ERPs). A categorization task was used in the learning phase and a visual and functional judgement task, respectively, in the test phase. Stimuli in the test phase were words denoting artifactual and natural objects, which were either new or previously presented (old). Only for the visual judgment, we found greater positivity in natural than artifactual objects at occipito-parietal electrodes (category effect). At the same electrodes repetition priming effects differed between categories. The present results support the relevance of visual features for natural categories (modality-specific subsystems) and show that category-specific brain activation depends on task-demand.