Infant Brain Responses in Speech Perception: A Review of the Factors Influencing the Polarity of the Mismatch Response

Auditory discrimination can be electrophysiologically measured by the mismatch response (MMR). Whereas in adults, this response occurs with a negative polarity (mismatch negativity; MMN), in infants it may either occur with a positive (pMMR) or a negative polarity (nMMR). Infant age has been identified as a key factor influencing MMR polarity, as the pMMR seems to change towards an adult-like nMMR with increasing age. It has also been suggested that pMMR and nMMR reflect different underlying processes, and growing evidence suggests that age triggers these processes in concert with other individual and methodological factors. The current review assessed the factors influencing the discrimination of speech sound contrasts in 0- to 24-month-old typically developing monolingual infants, with focus on vowels, consonants as well as lexical tones. We found that the MMR polarity is affected not only by age but also by interindividual, experimental, and data-analysis factors. Based on our findings, we offer methodological considerations for developmental EEG researchers for each study phase. In sum, our systematic approach disentangling the factors affecting the MMR polarity elucidates the processes underlying pMMR and nMMR and thus informs future study design and data interpretation in the widespread application of the MMR in developmental studies.