Prior Exposure And Toddlers’ Sleep-Related Memory For Novel Words Part 1

Abstract

Children can easily link a novel word to a novel, unnamed object—something referred to as fast mapping. Despite the ease and speed with which children do this, their memories of novel fast-mapped words can be poor unless they receive memory supports such as further exposure to the words or sleep. Axelsson, Swinton, Winiger, and Horst (2018) found that 2.5-year-old children who napped after fast mapping had better retention of novel words than children who did not nap. Retention declined for those who did not nap. The children received no memory support and determined the word-object mappings independently. Previous studies report enhanced memories after sleeping in children and adults, but the napping children’s retention in the Axelsson et al. study remained steady across time. We report a follow-up investigation where memory supports are provided after fast mapping to test whether memories would be enhanced following napping. Children’s retention of novel words improved and remained greater than chance; however, there was no nap effect with no significant difference between the children who napped and those who did not. These findings suggest that when memory supports are provided, retention improves, and the word–object mappings remain stable over time. When memory traces are weak and labile, such as after fast mapping, without further memory support, sleeping soon after helps stabilize and prevent the decay of word–object mappings.

Keywords

word learning; sleep-related memory; napping; declarative memory; memory strength; Cistanche benefits.

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Introduction

Understanding and remembering new words is a key part of language development [1]. Children hear on average 17,000 words per day [2–4]. In the first two years of life, their vocabulary expands by up to 300%, and they learn an estimated 10 new words every two weeks [5,6]. When not explicitly told the labels for objects, children use strategies to determine what a speaker is referring to when hearing a novel word [7,8]. When a child hears a novel word (e.g., “Look at the quokka”), it could refer to any object, but when in the context of familiar objects (e.g., kangaroos, koalas, wombats), children typically map the word to a novel object (e.g., smiling marsupial). Using the mutual exclusivity assumption [8,9]—where children assume that each object can only have one label—children quickly guess the meaning of a novel word. This is often referred to as “fast mapping” [10] or “referent selection”.

Children are both fast at fast mapping and highly accurate, but when children’s memory for fast-mapped words is tested, performance drops [11–13]. Therefore, fast mapping is best described as an initial stage of the word learning process. Learning a word involves not only mapping a novel word to its referent but also retaining and recognizing the word–object associations after a delay or in different contexts [14]. While this stage is contrastingly slower and more gradual, it can prevent the retention of incorrect associations [15,16]. Retention of fast-mapped words can improve when memory supports are provided after fast mapping, such as repetition of the word with the object or increasing the salience of the object while it is renamed [17–20]. For example, Horst and Samuelson [13] found increased retention of fast-mapped words in 2-year-old children when each target object was picked up, pointed to, and explicitly named after each fast-mapping trial. The re-exposure and explicit labeling of the novel objects likely strengthened the word–object associations.

Sleep-Related Memory Consolidation

There is a large body of evidence that sleeps also supports memory, particularly in adults [21–23], and more recently, in children and infants [24,25]. Sleep-related memory consolidation is a process where the previously encoded information is strengthened and integrated into existing knowledge stores during sleep [26]. Learners typically perform better when tested on previously presented material after a period of sleep compared to the same period of wakefulness [27].

Active System Consolidation (ASC). Sleep is far from a passive state and, as indicated by electroencephalographic measures of brain activity, contains five main stages that are cycled through over 90 min in adults: four stages of non-rapid eye movement (NREM) sleep (Stage 1, Stage 2, Stage 3 slow wave sleep (SWS), Stage 4 SWS), and one rapid eye movement stage (REM) [28,29]. In toddlers, it takes around 75 min to cycle through these stages [30].

The active system consolidation (ASC) theory helps explain why sleep supports memory [31–33]. During the initial encoding of information, neural connections are activated in both the neocortex and the hippocampus. These neural connections are initially weak and susceptible to decay [34]. Once SWS begins, these neural connections repeatedly replay in the hippocampus, and this reactivates the neural connections in the neocortex. This also enables the transfer of information from the hippocampus to the relevant neocortical areas, along with an associated decay in the hippocampus. Following transfer to the neocortex, the neural connections are stabilized, and associations with those cortical areas are strengthened [22]. These neural connections are now independent of the hippocampus [33]. SWS supports largely declarative memories, namely memories for explicit information, such as facts, episodes, and semantic information, including the meaning of words [35]. Given the influential role of sleep on memory and toddlers’ difficulties retaining novel fast-mapped words, e.g., [13], the effect of sleep on children’s memory for novel words deserves attention.

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Napping and Memory Consolidation

It is not only nocturnal sleep that supports memories but also naps [36]. A nap that is as short as 6 min can enhance adults’ memories for word lists [37]. Children’s naps are largely made up of SWS and can support young children’s episodic memory [37], visuospatial memory [38], and declarative memory, such as vocabulary [39,40]. In the first two years of life, most children regularly nap, but this declines in the third year [38,41]. As there are large changes in language development in the first few years of life, frequent napping might help increase vocabulary size during this period [42,43].

Evidence that napping promotes word learning is increasing. Horváth, et al. [39] trained 16-month-old infants on two novel word–object pairs. Retention tests two hours later (based on preferential looking to the correct target) revealed that those who napped in the 2-h interval from immediate testing had increased retention, whereas those who remained awake showed no change in retention.

More recently, Williams and Horst [44] presented 3-year-old children with novel words during storybook reading. Children heard the same novel words in either the same story read three times or in three different stories. Following an immediate retention test, half of the children napped before their retention was tested 2.5 h, 24 h, and one week later. Children who napped had significantly higher retention than those who remained awake in both storybook conditions. In addition, children who heard the same stories repeatedly also outperformed those who heard different stories as word repetition in the same contexts likely aided encoding. Interestingly, retention was similar across time for the children who heard different stories and napped as those who heard the same stories but did not nap. According to Williams and Horst, napping compensated for the more varied presentation of the novel words heard in different stories. The strength of association between the words and objects was perhaps weaker for the varied stories group, and napping after helped strengthen the associations.

Axelsson, et al. [45] tested the effect of napping on 2.5-year-old children’s memory for fast-mapped words. Following an immediate retention test, half of the children napped and half remained awake before a delayed retention test four hours later and another retention test the following morning (see Figure 1). Although immediate retention was the same for both conditions, the children who napped had significantly higher retention than those who did not nap on both delayed retention tests. However, unlike previous studies where performance increased after napping [39,44,46], retention scores remained stable across time [45]. For the children who did not nap, retention declined significantly by the following morning (see Figure 4B). Therefore, napping helped stabilize the memory of fast-mapped words and reduced the rate of decay.

Multiple Factors Affecting Sleep-Related Memory Consolidation

Many factors can affect sleep's support on memory such as the quality and duration of sleep, the age of the participants, and the learning material e.g., declarative or non-declarative material involving implicit or procedural memory which is less available to consciousness (27,42). Other factors include the elapse of time from exposure to the material to the onset of sleep and the strength of encoded memories, both of which will be the focus here.

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lime Interval from Encoding to Sleep. How soon sleep begins after exposure to learning material could also affect sleep-related memory consolidation. Sleep could have a greater stabilizing effect if it occurs soon after encoding (42], particularly for labile associations such as fast-mapped words. One study on infants' memory for actions (47and another on memory for word sequences (48] found that nap onsets occurring within 4 h of exposure were associated with significantly greater retention than nap onsets occurring after 4 h. However, there was no clear rationale for the 4-h cut-off. Children were grouped based on whether they slept within 4 h making it difficult to assess the effect of falling asleep shortly after 4 h. In William and Horst's (44] study on the memory of novel words in storybooks, napping started within 45 min of hearing the stories. The question is whether 45 min is a critical time point for the nap effect they found or whether it would be evident with longer sleep onset intervals. We chose to test children 4 h after exposure to the material in Hupbach, et al. (48] and Seehagen, et al. (47]. The children in the nap condition inAxelsson, et al. [45] and the current study fell asleep at a variety of times before the first delayed test 4 h later, as did the children in the wake condition for their nocturnal sleepChildren were tested the following morning to test for further delayed retention and when children in both the nap and wake conditions had slept (39]. Time of sleep onset from exposure to learning material is typically studied as a categorical variable rather than a continuous variable. It became apparent during testing for Axelsson, et al. [45] that it would be useful to assess this time as a continuous interval to allow for a more detailed exploration of the variation in sleep onset times. This may help determine optimal times to introduce children to novel material to facilitate memory retention, particularly for weaker associations.

Strength of Memory Prior to Sleep. Napping children’s memory for fast-mapped words did not increase in Axelsson, et al. [45]. One possible reason is that the fast-mapped words were weakly encoded. Children’s retention of fast-mapped words is typically poor, and the initial memory traces are too weak to support retention unless children receive memory supports such as ostensive naming after fast mapping [12,13,17]. Axelsson et al.’s [45] findings suggest napping can stabilize and maintain children’s retention for several hours and into the next day. Previous research indicates that the strength of associations prior to sleep can affect the ability to see sleep-related effects [22,42]. For example, Drosopoulos, et al. [49] trained one group of adults with lists of word pairs with 90% accuracy in cued recall. Another group was trained with 60% accuracy. Improvement in cued recall following nocturnal sleep was seen only in the lower accuracy condition, suggesting that the benefits of sleep on memory were only evident for weakly encoded declarative information. Similarly, in a study on procedural motor learning (button press sequences), Wilhelm, et al. [50] found that napping only supported the memories of children (4–6 years) trained to a lower level than a higher level.

Sleep also supports memories subjected to interference, such as when participants are trained on two lists of word pairs; those who sleep shortly after training have better recall for the first list than those who remain awake [49,51]. Sleep likely helps make the original list more resistant to interference. These examples highlight where sleep supports the consolidation of weaker memories. Contrastingly, Tucker, and Fishbein [52] split participants into high- and low-scoring groups based on recall of word pairs relative to training performance. A nap effect was only evident in the high-performing group rather than the low-performing group, suggesting naps only support stronger memories. However, it is difficult to compare to other studies where participants were trained to a pre-determined level [49,51], and recall was quantified differently (difference scores from training).

Stickgold [23] further argued that the effect of pre-sleep memory strength follows an inverted-U-shaped curve, with strong and weak memories less supported by sleep, but those encoded to an intermediate level, benefiting most from sleep. There is likely little capacity for neural reactivation during sleep with excessively weakly encoded material, and heavily encoded material would already be integrated into cortical stores. Sleep in both cases would offer little consolidative benefit. However, these studies involved different learning materials, methods, age groups, and definitions for memory strength and learning level. This makes it difficult to compare the level of memory strength across the studies.

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Current study. Here, the effects of sleep onset and the strength of memory for fast-mapped words prior to napping were tested. The fast-mapped words in Axelsson, et al. [45] were possibly weakly encoded as they only fast-mapped and retention only stabilized following napping rather than increased as is typically seen in other studies [44,47,48]. Ostensive naming can increase children’s retention of fast-mapped words [17,20]. What remains unknown is whether ostensive naming would be associated with enhanced retention particularly in those who nap due to stronger pre-nap memory strength [49].

Using the same methods as the Axelsson, et al. [45] study, we exposed 40 toddlers to four novel words and novel objects using a fast mapping task presented on a computer. The only difference to the Axelsson et al. study was that after every referent selection trial, the correct target moved up the screen away from the competitors, and children heard the name repeated (see Figure 2). Horst and Samuelson [13] argued that ostensive naming benefited children’s memory for new words because the target object was distanced from the competitors when it was explicitly named. Retention was tested immediately after fast mapping, after a 4-h interval during which half the children napped, and after nocturnal sleep roughly 24 h later (see Figure 1). We predicted a nap-related memory effect: the children who napped after fast mapping would have better-delayed retention than those who remained awake, as in Axelsson, et al. [45]. As the children here saw the objects ostensively named, we also predicted that retention would increase in the children who napped rather than only stabilize. However, if sleep-related memory effects are only found with weaker memories [49,51], then the inclusion of ostensive naming would lead to smaller differences between the nap and wake conditions than what was seen in Axelsson, et al. [45]. To test for the effects of prior memory strength, a comparison to retention in Axelsson et al. [46] was performed as children in that study did not receive ostensive naming. This comparison was expected to reveal better retention in the current study with ostensive naming. When it comes to time intervals from fast mapping, sleep onsets occurred at a variety of times prior to the afternoon test for those who napped and prior to nocturnal sleep for those who did not nap. We aggregated the nap and wake conditions into one group to assess sleep onset intervals as a continuous variable to determine the specific times associated with better retention the following morning. Sleep onsets occurring sooner after novel word exposure was expected to be associated with better post-nocturnal retention, e.g., [48].

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Emma L. Axelsson 1 , Jaclyn Swinton 2 , Isabel Y. Jiang 2 , Emma V. Parker 2 and Jessica S. Horst 3

1 School of Psychological Sciences, University of Newcastle, Callaghan 2308, Australia

2 Research School of Psychology, The Australian National University, Canberra 2601, Australia; jacis7@outlook.com (J.S.); yingxiv@gmail.com (I.Y.J.); emma.vparker@hotmail.com (E.V.P.)

3 School of Psychology, University of Sussex, Brighton BN1 9RH, UK; jessica@sussex.ac.uk