The Typical Developmental Trajectory of Perceptual Narrowing
Infants’ first year of life is a critical period (Bosseler et al. 2013), in which they experience significant developmental changes in speech perception. Infants begin life with the capacity to discriminate phonetic contrasts across all languages, but between the ages of six to twelve months infants begin to show perceptual narrowing, where their discrimination abilities improve for their native language and decline for non-native languages (Khul et al., 2006). This progressive specialisation in perceptual capacities for their native language is viewed to facilitate later acquisition of more complex language skills, such that the process of language acquisition happens even before children are capable of producing their first words (Bosch, 2011). Indeed, retrospective and prospective studies have provided evidence that there is continuity between infants’ early phonetic discrimination abilities and later language development (Jansson-Verkasalo et al., 2010; Newman, Ratner, Jusczyk, Jusczyk & Dow, 2006; Tsao, Liu & Kuhl, 2004). This essay, in evaluating the fundamental role of perceptual narrowing in later language learning, seeks to demonstrate that deviations from the normal perceptual narrowing process may have clinical implications on the early identification of children at risk of language impairments. As such, an intervention programme that seeks to identify the population at risk of language impairments by their atypical perceptual narrowing experience can be employed. The possibility of the provision of early preventative treatment to the identified at-risk population will also be discussed.
The typical developmental trajectory of perceptual narrowing is mapped within the second half of infants’ first year of life, where early speech perception abilities develop as a function of language exposure. Werker and Tees (1984), for instance, found that English-speaking infants between 6 and 8 months of age are able to differentiate phoneme contrasts in Hindi and Nthlakapmx as proficient as native adults of the two respective languages, using the head-turn paradigm. A sharp reduction in this ability, however, was observed between 8 and 10 months of age, and by 12 months none of the subjects were able to discriminate non-native contrasts. Similar behavioural results were obtained for Japanese-speaking infants, where they showed a decline in their ability to discriminate the English consonants of /r/ and /l/ that are absent in Japanese at 10 to 12 months of age (Kuhn et al., 2006). On the other hand, age-matched American infants demonstrated significant improvement in their ability to differentiate these two consonants. These behavioural data are further supported by neurological evidence that is arguably a more sensitive measure for speech perception abilities than the head-turn preference procedure (Rivera-Gaxiola, Klarman, Garcia-Sierra & Kuhl, 2005). A longitudinal study that analysed the auditory event-related