The oddball effect on P3 disappears when feature relevance or feature-response mappings are unknown.

The P3b component of human event-related EEG potentials is larger with rare than frequent task-relevant stimuli. In a previous study, this oddball effect was much reduced when stimulus-response (S-R) mappings were still undefined at stimulus presentation (being later provided by response prompts). This reduction may reflect P3b's dependence on transmitted information which might be any relevant information (informational value hypothesis) or, more specifically, information about how to respond (S-R link hypothesis).