A visual three-stimulus (target, nontarget, standard) paradigm was employed in which subjects responded only to the target. Nontarget stimulus properties were varied systematically to evaluate how stimulus typicality (non-novel vs. novel) across task discrimination (easy vs. difficult) conditions affects P3a scalp topography. Nontarget stimuli consisted of letters, small squares, large squares, and novel patterns; discrimination difficulty between the target and standard was varied across conditions. When the discrimination was easy, P300 amplitude was larger for the target than the nontarget with parietal maximums for both. In contrast, when the discrimination was difficult, nontarget amplitude (P3a) was larger and earlier than the target P300 over the frontal/central electrode sites, whereas target amplitude (P3b) was larger parietally and occurred later. P3a was largest when elicited by either the large square or novel pattern stimuli. The findings suggest that stimulus context as defined by the target/standard discrimination difficulty rather than stimulus novelty determines P3a generation.