Infants use phonetic detail in speech perception and word learning when detail is easy to perceive

Infants successfully discriminate speech sound contrasts that belong to their native language's phonemic inventory in auditory-only paradigms, but they encounter difficulties in distinguishing the same contrasts in the context of word learning. These difficulties are usually attributed to the fact that infants' attention to the phonetic detail in novel words is attenuated when they must allocate additional cognitive resources demanded by word-learning tasks. The current study investigated 15-month-old infants' ability to distinguish novel words that differ by a single vowel in an auditory discrimination paradigm (Experiment 1) and a word-learning paradigm (Experiment 2). These experiments aimed to tease apart whether infants' performance is dependent solely on the specific acoustic properties of the target vowels or on the context of the task. Experiment 1 showed that infants were able to discriminate only a contrast marked by a large difference along a static dimension (the vowels' second formant), whereas they were not able to discriminate a contrast with a small phonetic distance between its vowels, due to the dynamic nature of the vowels. In Experiment 2, infants did not succeed at learning words containing the same contrast they were able to discriminate in Experiment 1. The current findings demonstrate that both the specific acoustic properties of vowels in infants' native language and the task presented continue to play a significant role in early speech perception well into the second year of life.