Category differences in brain activation studies: where do they come from?

Differences in the neural processing of six categories of pictorial stimuli (maps, body parts, objects, animals, famous faces and colours) were invest igated using positron emission tomography. Stimuli were presented either wi th or without the written name of the picture, thereby creating a naming co ndition and a reading condition. As predicted, naming increased the demands on lexical processes. This was demonstrated by activation of the left temp oral lobe in a posterior region associated with name retrieval in several p revious studies. This lexical effect was common to all meaningful stimuli a nd no category-specific effects were observed for naming relative to readin g. Nevertheless, category differences were found when naming and reading we re considered together. Stimuli with greater visual complexity (animals, fa ces and maps) enhanced activation in the left extrastriate cortex. Furtherm ore, map recognition, which requires greater spatio-topographical processin g, also activated the right occipito-parietal and parahippocampal cortices. These effects in the visuo-spatial regions emphasize inevitable difference s in the perceptual properties of pictorial stimuli. In the semantic tempor al regions, famous faces and objects enhanced activation in the left antero -lateral and postero-lateral cortices, respectively. In addition, we showed that the same posterior left temporal region is also activated by body par ts. We conclude that category-specific brain activations depend more on dif ferential processing at the perceptual and semantic levels rather than at t he lexical retrieval level.