Infant Directed Speech Facilitates Vowel Category Discrimination in Pre-Verbal Infants

Compared to adult-directed speech (ADS), infant-directed speech (IDS) is acoustically exaggerated. It has been proposed that such exaggerations facilitate speech sound discrimination and phonetic learning in young infants. This proposal was tested here using an abstract mismatch negativity (MMN) paradigm to assess 4- and 9-month-old infants' and adults' neural responses to a vowel contrast produced in IDS and ADS. In 4-month-olds, IDS stimuli elicited both a negative MMN and a positive mismatch response (MMR), but ADS stimuli elicited only an MMR, which is associated with acoustic change detection, typical for infants of this age who are still acquiring their native language's phonemic inventory. In 9-month-olds and adults, both IDS and ADS stimuli elicited MMN, associated with native phonemic processing. The 9-month-olds also generated an MMR for IDS. These results suggest that for 4-month-olds, for whom speech processing is predominantly acoustic/phonetic, the heightened acoustic variability and phonetic saliency in IDS, compared to ADS, augments vowel discrimination, whereas for 9-month-olds, their additional phonemic processing affords vowel discrimination in both augmented (IDS) and non-augmented (ADS) speech contexts. This neural level evidence is consistent with the perceptual attunement argument that early language-general acoustic/phonetic speech processing gives way to a more abstract form of phonemic speech processing as a function of experience in a specific language environment, and also demonstrates that the properties of IDS may facilitate this developmental transition during infants' first year of life.