Individual Differences in Infant Speech Segmentation: Achieving the Lexical Shift

We report a large-scale electrophysiological study of infant speech segmentation, in which over 100 English-acquiring 9-month-olds were exposed to unfamiliar bisyllabic words embedded in sentences (e.g., He saw a wild eagle up there), after which their brain responses to either the just-familiarized word (eagle) or a control word (coral) were recorded. When initial exposure occurs in continuous speech, as here, past studies have reported that even somewhat older infants do not reliably recognize target words, but that successful segmentation varies across children. Here, we both confirm and further uncover the nature of this variation. The segmentation response systematically varied across individuals and was related to their vocabulary development. About one-third of the group showed a left-frontally located relative negativity in response to familiar versus control targets, which has previously been described as a mature response. Another third showed a similarly located positive-going reaction (a previously described immature response), and the remaining third formed an intermediate grouping that was primarily characterized by an initial response delay. A fine-grained group-level analysis suggested that a developmental shift to a lexical mode of processing occurs toward the end of the first year, with variation across individual infants in the exact timing of this shift.