This PET study concerns changes in regional cerebral blood flow (rCBF) associated with orientation discrimination tasks and with simple detection of the stimulus. The difference in rCBF between discrimination and detection represents the discrimination, or 'task', component of the response. We have examined how such differences are influenced by the position of the visual stimulus and also how they change when a peripheral distractor is added to a relevant central stimulus. We first determined the regions in which the task produced the main effect regardless of stimulus position or distraction with an additional stimulus. In these selected regions, we determined the interactions between task and stimulus position and the interactions between task and stimulus addition. Five regions were more active during orientation discrimination than during stimulus detection: the inferior occipital cortex, the right putamen, the superior parietal lobule, the anterior cingulate cortex and the left lower and the right upper premotor area. Stimulus addition interacted with task only in the lower premotor area. Interactions between the task and stimulus position occurred in the occipital and parietal cortex and in the putamen. When a central stimulus was presented instead of a peripheral one the difference between orientation discrimination and stimulus detection was significantly larger in the inferior occipital lobe and in the right putamen. Conversely, the difference between orientation discrimination and stimulus detection in the superior parietal lobule was significantly larger when a peripheral stimulus was presented instead of a central one.