The amplitude of the P3 event-related potential (ERP) elicited by task-relevant target ("oddball") stimuli has been shown to vary in proportion to the length of time between targets. Here we use functional magnetic resonance imaging (fMRI) to identify neural systems modulated by target interval in a large sample of healthy adults (n=100) during performance of an auditory oddball task that included both target and novel stimuli. A positive relationship was found between target interval and hemodynamic activity in the anterior cingulate and in bilateral lateral prefrontal cortex, temporal-parietal junction, postcentral gryi, thalamus, and cerebellum. This modulation likely represents updating of the working memory template for the target stimuli. There was no such effect of novel interval, suggesting that neuronal modulation may only occur for task-relevant stimuli, possibly in the service of strategic resource allocation processes.