How does acoustic variability in speech affect infant word recognition and word learning

Understanding how infants learn about acoustic variability is a neglected issue in accounts of language acquisition, yet variability is ubiquitous in infants' language experience. The present experiments investigate how infants deal with processing both the fine-grained acoustic-phonetic details of speech and the broader analysis that supports word recognition across contexts. Experiment 1 examined infants' ability to recognize familiar words in a visual fixation task. Fifteen- and 18-month-old infants were able to recognize words under conditions of indexical variability (i.e., listening to several different talkers) and to detect mispronunciations in the midst of indexical variability. The word recognition performance of older infants (and those with larger vocabularies) was more resilient to changes in the words' indexical features. Experiment 2 investigated the role of indexical variability in the development of flexible representations of new words. Infants were presented with two novel object labels. When the labels were introduced in a variable teaching context (by multiple talkers), infants successfully generalized one of the labels, recognizing it when a new voice was used to probe recognition. When the labels were introduced in a consistent teaching context (by a single talker), infants failed to recognize either word. This study demonstrates both the facilitative effects of variability in learning and the limits of infants' capacity to generalize. Infants' ability to perform a broad and yet detailed analysis of the speech stream during word recognition, as demonstrated by our tasks, suggests that infants have access to the multiple levels of acoustic-phonetic information that are key to building experience with sounds and words.