The representation of phonological features in the mental lexicon has been examined using event-related brain responses, such as mismatch negativity (MMN; an automatic auditory change detection response in the brain) or the P350 component (a correlate of lexical activation). This presentation will summarize some MMN studies that demonstrate support for (i) models proposing abstract underspecified representations in the mental lexicon, i.e. not all phonological features are stored; and (ii) top-down influence of the language-specific phonological system on the fine structure of the phonological representations. Constraints in using the MMN for investigations concerning phonological representations will also be discussed.