AbstractCortical memory circuits that represent words in the brain form the foundation of our mental lexicon; yet, theirarchitecture remains poorly understood. A valuable approach to probing these representations is measuring the Mismatch Negativity(MMN), an electrophysiological brain response sensitive to various psycholinguistic variables; however, its specificity to lexicalprocessing remains debated. To scrutinise its properties as a neural index of word-specific memory trace activity, we adapted theclassical passive auditory oddball design to fMRI and recorded the BOLD (blood oxygenation level-dependent) correlate of the MMNelicited by words of different lexical frequencies and by phonologically matched control pseudowords. The results showedsignificant BOLD-MMN responses in bilateral superior-temporal and middle-temporal cortices. Crucially, these activations were moreexpressed for meaningful words than meaningless pseudowords, indicating BOLD-MMN sensitivity to the stimulus’ lexicality. We alsofound the left temporal activity to be more pronounced for high- than low-frequency words, the effect not found for theirpseudoword analogues, further confirming the lexical nature of these responses. This pattern of results is best explained by theautomatic activation of long-term memory traces for real words formed in the process of previous linguistic experience whoseintensity determines the strength of connectivity within these circuits and thus the magnitude of their activation, which therebyreflects the respective stimuli’s status in the brain's mental lexicon.