We obtained fMRI BOLD images of cortical activity while 6 subjects performed fine color and shape discriminations under conditions of stimulus certainty (monitor one dimension) and uncertainty (monitor both dimensions). Accuracy data divided the subjects into two distinct groups: 3 subjects maintained accuracy levels achieved in prior testing outside the scanner; 3 subjects dropped to lower levels, primarily in the color task. For the former group, the pattern of BOLD activity observed in the uncertainty condition resembled the union of patterns observed in certainty conditions, but for the latter group the uncertainty pattern differed widely from such a union. Possible implications for models of attention are discussed.