We examined code-switching (CS) in the speech of twenty-four bilingual caregivers when speaking with their 18- to 24-month-old children. All parents CS at least once in a short play session, and some code-switched quite often (over 1/3 of utterances). This CS included both inter-sentential and intra-sentential switches, suggesting that at least some children are frequently exposed to mixed-language sentences. However, we found no evidence that this exposure to CS had any detrimental effect on children's word learning: children's overall vocabulary size did not relate to parental inter-sentential CS behavior, and was positively related to within-sentence CS. Parents often repeated words across their two languages, but this did not appear to increase the likelihood of children having translation equivalents in their vocabulary. In short, parents appear to CS fairly often to young children, even within sentences, but there is no evidence that this delays child lexical acquisition.

Hearing words in sentences facilitates word recognition in monolingual children. Many children grow up receiving input in multiple languages – including exposure to sentences that ‘mix’ the languages. We explored Spanish–English bilingual toddlers’ (n = 24) ability to identify familiar words in three conditions: (i) single word (ball!); (ii) same-language sentence (Where's the ball?); or (iii) mixed-language sentence (Dónde está la ball?). Children successfully identified words across conditions; however, the advantage linked to hearing words in sentences was present only in the same-language condition. This work hence suggests that language mixing plays an important role on bilingual children's ability to recognize spoken words.

Previous work by Polka, Rvachew, and Molnar [Infancy 13(5), 421-439 (2008)] has reported that infants are poor at focusing their attention on a particular frequency range, and, as a result, are distracted by maskers that are outside of the target frequency range. The current study explores this effect of irrelevant distractors further and finds that 8-month-old infants are significantly less affected by maskers outside the frequency range (off-channel maskers) than by on-channel maskers. Thus while infants may display difficulty ignoring irrelevant distractors, they are able to do so to at least some degree, suggesting some ability to perceive speech from spectrally remote maskers, despite the demonstrated presence of greater informational masking at this age.

To investigate potential causes of L2 performance deficits that correlate with age of onset, we use a computational model to explore the individual contributions of L1 entrenchment and aspects of memory development. Since development and L1 entrenchment almost invariably coincide, studying them independently is seldom possible in humans. To avoid this confound, we study neural network models that learn to solve gender assignment and agreement tasks in Spanish and French. We model the learner as a collection of recurrent cell assemblies that subserve working memory and are facilitated by trainable long-term connections. Varying the time-course over which assemblies and connections are added allows us to compare small, growing, child-like networks to fixed-size adult-like ones. Networks undergo variable-length exposure to L1 before L2 onset to control the amount of L1 entrenchment. This model, by allowing us independent control of both variables, lends us a novel glimpse of all sides of their interaction and affords a rare test of the less-is-more hypothesis. Network comparisons suggest that final L2 proficiency declines as L2 onset delays increase relative to L1, implicating an L1 entrenchment effect. However, aspects of memory development during learning play a key role in mitigating these impairments, lending support to less-is-more as a contributor to sensitive periods.