Maturation of cortical mismatch responses to occasional pitch change in early infancy: effects of presentation rate and magnitude of change

Previous studies have reported two types of event-related potential (ERP) mismatch responses in infants to infrequent auditory changes: a broad discriminative positivity in younger infants and a negativity resembling adult mismatch negativity (MMN) in older infants. In the present study, we investigated whether the positive discriminative slow wave and the adult-like MMN are functionally distinct by examining how they are affected by presentation rate and magnitude of change. We measured ERPs from adults, 2-month-olds, and 4-month-olds to a repeating piano tone (standard) that occasionally changed in pitch (deviant). The pitch changes between standards and deviants were either small (1/12 octave) or large (1/2 octave) in magnitude, and the stimulus presentation rate was either slow (800 ms SOA) or fast (400 ms SOA). As the presentation rateincreased, both adults and 4-month-olds showed an MMN response that decreased inlatency, but was unaffected in amplitude. As the magnitude of the pitch change increased, MMN increased in amplitude. On the other hand, only a broad positive mismatch response was seen in 2-month-olds. As the presentation rate increased, 2-month-olds' responses to standard tones decreased in amplitude while their responses to deviant tones were unaffected. The magnitude of the pitch change did not affect 2-month-olds' responses. These results suggest that pitch is processed differently in auditory cortex by 2-month-olds and 4-month-olds, and that a cortical change-detection mechanism for pitch discrimination similar to that of adults emerges between 2 and 4 months of age.