L EARNING WORDS IN IDS AND ADS

Our finding that infants are able to learn words from IDS and ADS so long as their word knowledge is tested in IDS has important implications for our understanding of the kinds of input that infants are able to learn from. First, as noted above, this finding is consistent with studies showing that young children can even learn from overhearing speech between two adults (Akhtar, Jipson & Callanan, 2001;

Gampe, Liebal, & Tomasello, 2012). In general, it appears that even young toddlers at 18-months of age are able to learn from both infant- and adult-directed input at least with regard to the acoustic characteristics of such input. In conjunction with the results of the current study, these findings suggest that infants are able to learn from a much greater range of input than was previously assumed and might go some way to explaining the explosion in children’s vocabulary that takes place during the second year of life.

Our results also speak to models of language processing that, for instance, suggest that the child needs to be interested and attracted to speech in order to learn (Kuhl, 2007). Typically, children tend to be less interested and attracted to ADS relative to IDS (Cooper & Aslin, 1990; Pegg, Werker, McLeod, 1992). The fact that we found successful learning from ADS suggests either, that children can learn from

CHAPTER 5: EARLY WORD LEARNING FROM ADULT-DIRECTED SPEECH interactions they may not be as interested in, or that older children may begin to attend to even such less engaging interactions. At least by 18-months, learning is not merely restricted to child-oriented interactions carried out in an infant-directed manner.

Indeed, this finding qualifies much of the previous literature on the benefits of IDS – suggesting an important distinction that has been missed to-date. It appears not to be the case that infants learn less robustly from less engaging speech registers but rather that they do not demonstrate such learning unless tested in a more engaging register of speech. In other words, while there are obvious benefits associated with IDS being addressed to infants, young children are able to learn with considerable ease from different kinds of input that have typically attracted less attention in the infant learning literature.

Nevertheless, the finding that the register at test is critical to demonstration of learning success highlights the benefits of such child-directed interactions. On the one hand, this finding is in keeping with research arguing for an important role for IDS in early language acquisition (Graf-Estes & Hurley, 2013; Ma et al., 2011; Singh, Nestor, Parikh, & Yull, 2009; Song, Demuth, & Morgan, 2010; Thiessen, Hill, &

Saffran, 2005). Thus, while children may learn from speech presented in engaging and less engaging registers, children respond better to more engaging speech. One reason for improved performance when tested in IDS may be that the prosodic characteristics and simple, repetitive structure of IDS elicited infants’ attention more than ADS (Kuhl, 2007), driving infants to respond to the task. Indeed, the fact that we found longer looking times in the familiarization phase for IDS relative to ADS in the segmentation task of the study supports this explanation for the results of the word learning study. Taken together, the increased attention to IDS across the two tasks of

CHAPTER 5: EARLY WORD LEARNING FROM ADULT-DIRECTED SPEECH the study suggest that it remains critical to parent-child interactions to employ a more exaggerated register of speech in communications with young children.

Furthermore, we note that IDS is routinely directed to the infant while ADS is not (Cristia, 2013). While infants are frequently exposed to ADS (van de Weijer, 1998), and appear also to be able to learn from this register, they are rarely directly addressed using ADS. Thus, infants may learn over time that they are being directly spoken to in IDS and may consequently respond to this input with greater attention and be more engaged in responding to a conversational partner according to the register of speech they employ (Schachner & Hannon, 2011).

We also note that infants in the current study were able to generalize across speech registers, in showing recognition of a word they had been exposed to in one register when they heard it in a different register. Thus, infants who had only heard a word in ADS before, showed recognition of this same word in IDS and also recognition of the object association for this word in this new register. This finding is particularly remarkable especially given that research on infants’ word learning abilities finds that, at 17 months, infants have difficulties in recognizing a word in unfamiliar sentence contexts because of coarticulation with the adjacent sounds (Plunkett, 1997). Indeed, previous work from our laboratory suggests that even younger infants, at 9-months of age, show similar flexibility in word recognition by generalizing across speaker-specific attributes in word recognition, by recognizing the same word spoken by two different speakers in a word segmentation task. Taken together, these findings highlight infants’ flexibility with phonological representations of words going from word segmentation (Schreiner et al., 2016) to word learning.

Register overlap does appear to have its benefits for word recognition, however. Thus, we note that infants of the ADS condition, who attended longer to the familiarized test words relative to the novel control words in the segmentation task,

CHAPTER 5: EARLY WORD LEARNING FROM ADULT-DIRECTED SPEECH were also better at learning and recognizing the word-object association of this familiarized word, when assessed in ADS. This finding would suggest that – for those infants who were more able to learn from ADS – familiarity with a word in ADS positively and exclusively impacts recognition of this word in the same register later.

Taking this finding further, it would appear that these infants might also have greater opportunities to acquire novel lexical items given that they show increased dexterity with different kinds of input in language acquisition. We note, however, that this finding must be treated with caution – given that we found neither successful segmentation from ADS nor word recognition when tested in ADS.