New Insights Into Old Puzzles From Infants' Categorical Discrimination of Soundless Phonetic Units

Abstract

For 4 decades, serious scientific debate has persisted as to whether infants' remarkable capacity to detect and categorize phonetic units is derived from language-specific mechanisms or whether this capacity develops out of general perceptual mechanisms. The heart of this controversy has revolved around whether the young human brain is specialized to detect the underlying contrasting patterns in language or whether it simply processes general auditory perceptual features of sound that, over time, become utilized for language learning. This article takes a novel look at this question by using soundless phonetic units from a natural signed language as a new research tool. Research finds that 4-month-old hearing infants categorize soundless phonetic units on the basis of linguistic category membership, whereas 14-month-old infants fail to do so-thereby exhibiting the identical initial capacity and classic developmental shift in infant categorical discrimination of native and nonnative (foreign language) phonetic units in speech. These results suggest a novel testable hypothesis: Infants may begin life with the capacity to detect specific patterned units with alternating contrasts unique to natural language organization and to categorize them on the basis of linguistic category membership.

FIGURE 1

(a) Habituation and test stimuli. Shown on the left are still image examples of the real-time moving sign-phonetic units that were produced by a native deaf signer (always shown full face) and that all participants viewed in habituation; the top image is an American Sign Language (ASL) phonetic variant from the phonemic category /5/, and the bottom image is an ASL phonetic variant from the phonemic category /flat-0/. During the habituation phase, infants were habituated to movies of an ASL signer repeating one of these two phonetic variants. On the right are the test phonetic units (the Familiar stimulus is the exact same phonetic unit that the infant saw during the habituation phase; the In-Category stimulus is a phonetic variant from the same phonemic category as the phonetic unit that was used in the habituation phase; the Out-of-Category stimulus is a phonetic variant from a different category than the phonetic unit that was used in the habituation phase). The four stimulus items in the top row constituted one habituation condition, and the four stimulus items in the bottom row constituted a second habituation condition; infants were randomly assigned to one of these two habituation conditions, (b) Hearing infants’ looking times and standard errors for each of the test trials for the test stimuli. Analyses revealed that only the hearing 4-month-olds looked significantly longer at the Out-of-Category trial compared to the In-Category trial, suggesting that they were discriminating the handshapes on the basis of linguistic–phonetic category membership just as infants do in speech. *p < .032, for In-Category versus Out-of-Category significance.