Two-year-olds consolidate verb meanings during a nap

Abstract

Successful word learning requires establishing an initial representation that is sufficiently robust to be retained in memory. Sleep has profound advantages for memory consolidation, but evidence concerning the effects of sleep in young children's word learning is slim and focuses almost exclusively on learning nouns. Verbs are representationally more complex and are often learned from non-concurrent linguistic and observational information (e.g., hearing “let's pour your milk” before the pouring event takes place). What remains unknown is whether initial representations built this way are robust enough to sustain a delay, and how these representations are affected by sleep. We presented two-year-olds with non-concurrent linguistic and observational information about novel verbs and immediately tested their knowledge of the verbs' meanings by evaluating their eye gaze as they looked at potential referents. Then, after a 4-hour delay during which half of the children napped and half remained awake, we retested them to see if they remembered the verbs' meanings. The results demonstrate differences in two-year-olds' representations of a novel verb before and after the delay; specifically, their verb representations withstood the 4-hour delay if they had napped, but decayed if they had remained awake.

Introduction

Young children learn new words quickly and then expand and refine their representations of meaning with increasing exposure (see He & Arunachalam, 2017, for a review on word learning). Children's initial representations of a novel word's meaning must therefore be sufficiently robust to be retained in memory until the next encounter. The evidence, thus far derived almost exclusively from the acquisition of novel nouns, suggests that initial representations are indeed retained. At issue, however, is whether young word learners encounter the same success in acquiring the meaning of novel verbs. Verbs pose an extra challenge because the initial representations are often gleaned from “fragmented” input. Caregivers rarely label events while they are ongoing, and so the learner cannot observe the referent event while hearing the verb (e.g., Gleitman & Gleitman, 1992; Tomasello & Kruger, 1992). In situations like this, when linguistic and observational information become available at different time points, are young word learners' initial representations sufficiently robust to sustain a delay?

Years of research have established that both linguistic and observational context provide important information about verb meaning (e.g., Gillette, Gleitman, Gleitman, & Lederer, 1999; Gleitman, 1990; Landau & Gleitman, 1985). The linguistic context provides structural information; for example, blick in “the boy blicked the girl” denotes a causative event whereas in “the boy and the girl blicked,” it denotes a non-causative event. The observational context further instantiates the semantic content of the event (e.g., how a blicking event unfolds). These two sources of information, however, are often temporally “fragmented.” Over half of the verbs in child-directed speech refer to events that occur after the verb has been uttered (e.g., “Let's pour some milk into the glass”) (Tomasello & Kruger, 1992). Nevertheless, 2-year-old children successfully harvest this fragmented input, integrating linguistic information about a novel verb (e.g., a conversation between two actors who use the verb in informative sentences) with observational evidence about its meaning (e.g., Arunachalam, 2013; Arunachalam, Escovar, Hansen, & Waxman, 2013; Arunachalam & Waxman, 2010; Dautriche et al., 2014; Messenger, Yuan, & Fisher, 2015; Scott & Fisher, 2009; Yuan & Fisher, 2009). For example, Arunachalam and Waxman (2010) and Arunachalam et al. (2013) introduced children to novel verbs, presented in either transitive or intransitive syntactic frames in a dialogue between two actors, without providing any visual cues to the verb's meanings. Children were subsequently shown two candidate referent scenes, one in which one actor performed a causative action on the other (e.g., spinning) and another in which the two actors engaged in independent non-causative actions (e.g., waving). Children mapped novel verbs presented in transitive frames, but not verbs in intransitive frames, to the causative scene. Because these representations were established from temporally fragmented input, they may well be fragile. Our goal in the current investigation is to test children's ability to retain such fragmented representations over a delay.

Although the evidence for retaining initial representations of novel nouns over delays is promising (e.g., Carey & Bartlett, 1978; Dollaghan, 1985; Golinkoff, Hirsh-Pasek, Bailey, & Wenger, 1992; Goodman, McDonough, & Brown, 1998; Heibeck & Markman, 1987; Jaswal and Markman, 2001, Jaswal and Markman, 2003; Markson & Bloom, 1997; Mervis & Bertrand, 1994; Waxman & Booth, 2000; Wilkinson & Mazzitelli, 2003; Wilkinson, Ross, & Diamond, 2003; Woodward, Markman, & Fitzsimmons, 1994),¹ the evidence concerning verb learning is considerably sparser. Yuan and Fisher (2009) reported that 28-month-olds retained an initial verb representation after a delay of one or two days. This is impressive, but because delays of this duration necessarily include sleep, it raises a compelling question: Is sleep an essential ingredient in maintaining verb representations over delays? In the current investigation, we address this directly, introducing a shorter delay period during which we manipulate whether or not the child napped.

There is substantial evidence that new memories are consolidated during sleep–– stabilized, strengthened, and integrated into long-term memory (Diekelmann & Born, 2010; Rasch, Büchel, Gais, & Born, 2007), and that short naps also have a consolidation effect (e.g., Lahl, Wispel, Willigens, & Pietrowsky, 2008). Young children tend to sleep longer at night than adults and typically take daytime naps (Iglowstein, Jenni, Molinari, & Largo, 2003; Ohayon, Carskadon, Guilleminault, & Vitiello, 2004; Weissbluth, 1995). But what remains unclear is whether young word learners' retention and retrieval of initial representations of novel words are affected by sleep. Some studies report a sleep advantage (e.g., Friedrich, Wilhelm, Born, & Friederici, 2015; Horváth, Myers, Foster, & Plunkett, 2015; Williams & Horst, 2014), but others do not (Werchan & Gómez, 2014). And again, most existing evidence is on nouns.

To our knowledge, there is only a single report of the effect of sleep in learning verbs. Sandoval, Leclerc, and Gómez (2017) found that only children who napped shortly after learning, but not those who stayed awake, retained and generalized the meaning of novel verbs. However, because the novel verbs in their design were presented concurrently with a referent scene, this work cannot address the acquisition of meaning when they must lay down an initial representation based on the linguistic context alone, and then integrate it with observational information when it later becomes available.

To assess how fragmented representations of verb meaning fare over a delay, we invited 2-year-old children and their parents to participate in a study with the two distinct visits (Visit 1 and Visit 2) separated by a 4-h delay during which the child either slept (Nap Condition) or remained awake (Wake Condition). We compared performance across the two visits to ascertain whether children's initial representations of novel verb meanings were sufficiently robust to withstand a delay and whether their representations were enhanced with sleep. Specifically, we predicted that children would retain initial representations if the delay included sleep, but that without sleep, the representations would decay.

We take as a starting point the robust evidence that children successfully establish an initial representation of a novel verb even from fragmented input (e.g., Arunachalam, 2013; Arunachalam & Waxman, 2010; Yuan & Fisher, 2009). Adopting the stimuli and design of Arunachalam and Waxman (2010), we focus specifically on learning novel transitive verbs, asking whether and how children's verb learning is affected by a delay with or without sleep. We target 27-month-olds, children in an active phase of acquiring new verbs.