Comparing two phonemic category learning mechanisms in infancy and the factors that modulate their efficacy

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During the first year of life, infants become specialists at perceiving the speech sound differences used to distinguish one word from another (the phonemes) in their native language. This process, termed `attunement', involves an increase in perceptual sensitivity to native speech sound (phonetic) distinctions and a concomitant decrease in sensitivity to non-native ones. In this grant, we compare two phonetic category learning mechanisms and ask whether they equally well yield a phoneme repertoire to distinguish meaningful words. We examine the influence of multisensory information, and the extent to which each mechanism is gated by critical periods. We propose six experimental sets, using EEG and ERP indices to measure learning, neural plasticity, discrimination, and semantic use. Set I compares two learning mechanisms: the first using statistical learning of relative frequency distribution of sounds in input speech, and the second rooted in word form-to-meaning mappings. Here, as well as in Sets 2-4, we test three groups of infants: those beginning attunement (5 months), those toward the end (9 months), and those after (12 months). During the learning phase, we measure an oscillatory biomarker of critical period plasticity to determine whether either (or both) learning mechanisms are gated by critical periods. In Set 2, we add visual phonetic cues (talking faces) and in Set 3, oral-motor manipulations, to probe how multisensory information interacts with learning and/or the timing of plasticity. Set 4 asks whether the two learning mechanisms interface equally well with higher levels of language acquisition, by testing whether the reorganized phonetic categories yield commutable phonemes that can be used to distinguish meaningful words. To probe whether plasticity has closed off entirely or can be reopened, in Set 5, we test older children's and adults' performance in these two learning procedures, and in Set 6, we ask whether externally entraining gamma oscillations can reopen plasticity in adults. Together, these studies query the very foundation of what spoken human language entails, the establishment of `duality of patterning' - how patterning at the level of form (e.g., speech) supports patterning at the level of meaning. To the extent that this foundational achievement is constrained by critical periods in early life and is influenced by the multisensory information inherent in parent-infant enface interactions, we gain deeper insight into the confluence of forces fundamental to the evolution of language. These results will also have important applied implications, both for infants deprived of engaging enface interactions with caregivers (e.g., those who experience extreme early adversity) and for infants born with sensory or motor impairments that limit their experience of multisensory information (e.g., those with visual or oral-motor impairments).