In order to build complex language from perceptual input, children must have access to a powerful information processing system that can analyze, store, and use regularities in the signal to which the child is exposed. In this article, we propose that one of the most important parts of this underlying machinery is the linked set of cognitive and language processing components that comprise the child's developing working memory (WM). To examine this hypothesis, we explore how variations in the timing, quality, and quantity of language input during the earliest stages of development are related to variations in WM, especially phonological WM (PWM), and in turn language learning outcomes. In order to tease apart the relationships between early language experience, WM, and language development, we review research findings from studies of groups of language learners who clearly differ with respect to these aspects of input. Specifically, we consider the development of PWM in children with delayed exposure to language, that is, children born profoundly deaf and exposed to oral language following cochlear implantation and internationally adopted children who have delayed exposed to the adoption language; children who experience impoverished language input, that is, children who experience early bouts of otitis media and signing deaf children born to nonsigning hearing parents; and children with enriched early language input, that is, simultaneous bilinguals and second language learners.

In their keynote article examining links between early experience, phonological working memory, and language outcomes, Pierce, Genessee, Delcenserie, and Morgan (2017) present, what I argue, is a two-pronged hypothesis. In brief, the thesis is that the timing of language exposure and the quality and quantity of language input during an early sensitive period of phonological development shape the quality of phonological representations later used by the phonological working memory system to support short- and long-term language learning. The hypothesis is two-pronged because it hinges on (a) qualitative differences in the development of phonological representations, resulting in (b) variations in the efficiency of working memory as the key constraint on language learning. In this commentary, I examine support for these two prongs in detail.

Children with bilingual typical language development (BITLD) and children with specific language impairment (SLI) present similarities in their linguistic performance (e.g., vocabulary and verb inflections). These observed similarities (Paradis 2010) have led to an influx of studies exploring the sources of these similarities and pointing to areas of disparity aiming to dissociate the effects of SLI and bilingualism in bilingual children with SLI (BISLI). The intriguing link, explored in the Pierce et al. (2017) keynote, between early language experience and the development of phonological working memory (PWM) seamlessly ties into this discussion. Bilinguals, and particularly simultaneous bilinguals, it is claimed, experience an enriched linguistic environment in early childhood that is expected to enhance their PWM. By contrast, children with SLI are claimed to have a neurocognitive impairment that negatively affects their PWM. This commentary explores, first, how both age of onset (AoO) of bilingual exposure and the neurocognitive impairment associated with SLI impact PWM and, second, how this impact might be related to the observed similarities as well as evident differences that emerge from studies comparing children with BITLD to children with SLI (monolingual and bilingual).

In line with the recent trend in comparative analysis of different populations (see Friedmann & Rusou, 2015, as an example), Pierce, Genesee, Delcenserie, and Morgan (2017) present a comprehensive review of different language outcomes in populations that have received qualitatively and quantitatively different input during the first year of life, from enriched stimuli in bilingual situations to no stimuli at all in the case of children with profound hearing impairment. The claims derived from these data deserve some comment, however, and need some caveats about the measures used, which I will provide in the following with a brief discussion of complementing research and the presentation of some new data derived from the Bilingual Language Development (BiLaD) Project, a recent French/German collaboration studying bilingual populations with and without specific language impairment (SLI).

Looking to atypical populations to illuminate the functioning of core cognitive systems has a long history in the behavioral sciences (e.g., Caramazza, 1986; McCloskey, 1992). Pierce, Genesee, Delcenserie, and Morgan (2017) continue that tradition in their keynote article, here by shedding light on how the development of phonological working memory (PWM) may be shaped not only by individual genetic variation but also by various features of the language acquisition context, whether that context is enriched, as in bilingualism; delayed, as with cochlear implantation (CI) or international adoption (IA); or impoverished, as with deaf children of hearing parents or repeat cases of otitis media. Their review of the literature demonstrates that a sensitive (/critical) period for phonological development, encompassing variation in quality, quantity, and timing of early exposure, has crucial consequences for an individual's level of PWM attainment and, by a compelling extension, for aspects of ongoing language development such as vocabulary learning and reading skills.

Taking as a premise that phonological working memory (PWM) influences later language development, in their keynote article, Pierce, Genesee, Delcenserie, and Morgan aim to specify the relations between early language input and the development of PWM in terms of separable influences of timing, quantity, and quality of early language input. We concur that prior work has established that PWM and language development have reciprocal influences on one another during development (e.g., Baddeley, Gathercole, & Papagno, 1998; Gathercole, 2006; Gathercole, Hitch, Service, & Martin, 1997; Metsala & Chisolm, 2010). The goal of the keynote article was to describe how early language experience may influence the development of PWM. Pierce et al. argue that this can be done by comparing the development of PWM across groups of children with differing language experiences during early childhood, specifically (a) delayed exposure to language, (b) impoverished language input, or (c) enriched language input. The authors suggest that this comparison may contribute to establishing that individual differences in PWM are due, in part, to early language experience. Sensitive periods for phonological development that are open roughly in the first year of life are discussed, and it is suggested that the quantity and quality of early language input shapes the quality of phonological representations. Efforts to specify mechanisms by which early language input may influence the development of PWM have both theoretical and, potentially, clinical importance. Considering this, Pierce et al.’s article, which aims to create a platform for future research in terms of the timing, quantity, and quality of early language input, is a valuable contribution.

Pierce, Genesee, Delcenserie, and Morgan have provided an innovative and thought-provoking juxtaposition of the putative role played by phonological working memory (PWM) in explaining individual and group differences among early internationally adopted (IA) children, deaf children with cochlear implants, simultaneous and sequential bilinguals, children who are learning sign languages, and children with otitis media. This novel comparison suggests that adequate exposure to phonology before 12 months of age is key in the development of improved PWM. Consequently, long-term linguistic advantages in vocabulary and learning of morphosyntax arise, but not advantages in other areas of higher cognition. One significant implication of the review is that although many intriguing links exist between language development and PWM, irrefutable conclusions elude the field as to the directionality of a causal relationship between phonological development, other linguistic development, and PWM. In this commentary, I should like to make the somewhat controversial proposal (Gathercole, 2006, and commentaries) that the evidence presented points to PWM being an epiphenomenon arising out of individual differences in the robustness and richness of phonological representations themselves. The authors hinted several times at the tantalizing relationship between phonology and PWM, but they do not articulate explicitly that PWM could be a redundant construct. I also offer some proposals as to how one might test this suggestion experimentally or in a corpus of child language.

The central hypothesis in the Pierce, Genesee, Delcenserie, and Morgan article is that phonological memory is key to explaining the relationship between early language experience (more specifically, less and more optimal ends of the language-experience continuum) and language learning outcomes. One piece of evidence offered is that phonological memory skills are enhanced by bilingualism, with bilingualism representing enriched experience. Here, I propose that data from bilingual children may contradict Pierce et al.’s central hypothesis, rather than support it.

Infants are exposed to the language of the environment in which they are born and, in most instances, become native speakers of that language. Although the history of research on language acquisition provides a colorful debate on the specific ways that nature and nurture shape this process (e.g., MacWhinney, 1999; Pinker, 1995), its primary focus has been on typically developing children exposed to a single language from birth. Pierce, Genesee, Delcenserie, and Morgan (2017) turn the table on this discussion to argue that critically important lessons can be learned by shifting the focus from typically developing children to children for whom the trajectory of language learning follows a different course. Some of the variation in language development reflects attributes of child learners themselves, such as whether they are born hearing or deaf and whether they have conditions that disrupt their ability to fully perceive the speech input to which they are exposed. Other variations reflect attributes of the external conditions in which learners develop, including whether they remain in their country of birth or move to a location in which another language is spoken, whether exposure to the native language is continuous or disrupted, and whether they are exposed to a second language (L2) early or late in development. For deaf children, there is also variation in whether their parents or caregivers are themselves deaf or hearing and able to expose them to sign language during infancy. Pierce et al. use the diversity of early language experience as a tool to examine the relation between phonological working memory and language development and to begin to suggest how conditions that may produce costs or benefits in language learning may be related to one another.

Pierce, Genesee, Delcenserie, and Morgan (2017) are right to suggest that working memory is a crucial part of the machinery underlying linguistic development. In this brief commentary, I will move beyond the emergence of phonological representations, on which Pierce et al.’s essay focuses, and consider ways in which working memory shapes the character and acquisition of grammatical phenomena, a topic that has been explored in various ways in the recent literature (e.g., Chater & Christiansen, 2010; Hawkins, 2014; O'Grady, 2005, 2015).

Early linguistic experiences are intimately tied to later language learning outcomes (e.g., Chilosi et al., 2013; Kaushanskaya & Marian, 2009b). The underlying neural and cognitive processes mediating this relationship remain unclear. Pierce, Genesee, Delcenserie, and Morgan (2017) propose that the phonological working memory (PWM) system is the critical component responsible for linking early linguistic experiences to later language development. Their argument arises from research demonstrating that exposure to linguistic input early in life shapes the kinds of phonological representations that are formed about the sounds within one's native language (Kuhl, 2004; Majerus et al., 2005; Mody, Schwartz, Gravel, & Ruben, 1999; Nittrouer & Burton, 2005). These phonological representations, which the authors believe to be created and stored using the PWM system (Gathercole & Baddeley, 1989), are highly predictive of later language outcomes (Bernhardt, Kemp, & Werker, 2007; Kuhl, 2010; Molfese & Molfese, 1985; Tsao, Liu, & Kuhl, 2004). Explaining language development via memory processes has many benefits. For instance, drawing on a large extant literature on memory-related processes allows researchers to make novel predictions about language learning, especially in regard to its connections to other kinds of learning (e.g., Gathercole, 2006). Identifying PWM as a key driver to language development is a useful insight likely to generate much further research, but it may not go far enough in tying language acquisition to more domain-general aspects of the human cognitive architecture. We believe that a complete account of language will require consideration of additional aspects of the human cognitive architecture working alongside the PWM system.

Language learning, while seemingly effortless for young learners, is a complex process involving many interacting pieces, both within the child and in their language-learning environments, which can result in unique language learning trajectories and outcomes. How does the brain adjust to or accommodate the myriad variations that occur during this developmental process. How does it adapt and change over time? In our review, we proposed that the timing, quantity, and quality of children's early language experiences, particularly during an early sensitive period for the acquisition of phonology, shape the establishment of neural phonological representations that are used to establish and support phonological working memory (PWM). The efficiency of the PWM system in turn, we argued, influences the acquisition and processing of more complex aspects of language. In brief, we proposed that experience modulates later language outcomes through its early effects on PWM. We supported this claim by reviewing research from several unique groups of language learners who experience delayed exposure to language (children with cochlear implants [CI] or internationally adopted [IA] children, and children with either impoverished [signing deaf children with hearing parents)] or enriched [bilingual] early language experiences). By comparing PWM and language outcomes in these groups, we sought to highlight general patterns in language development that emerge based on variation in early language exposure. Moving forward, we also proposed that the language acquisition patterns in these groups, and others, can be used to understand how variability in early language input might affect the neural systems supporting language development and how this might affect language learning itself.