WORD LEARNING : ASSIGNING MEANING TO WORDS

Another important step in learning the inventory of words of the mother tongue following the segmentation of words from fluent speech, is mapping meaning to these newly extracted word forms. The ability to link a word to a particular referent seems to begin relatively early. At the age of 6 months, infants have demonstrated that they know the meaning of the words “mommy” and “daddy” as infants looked significantly

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longer to the named parent than to the unnamed one (Tincoff & Jusczyk, 1999).

Further research supported the claim of this early referent learning: Infants at 6 months also identify the referents of common names and body parts. Notably, Bergelson and Swingley (2012) had infants’ own mothers produce the target words of the body part and food category during the eye-tracking task which might have facilitated word recognition (cf., Barker & Newman, 2004).

In addition to the studies on the associations of words and their referents that have already been acquired by infants, a large amount of research has explored infants’ abilities to learn new meanings of words (Schafer & Plunkett, 1998; Smith &

Yu, 2008; Swingley, 2007). Accordingly, infants are also able to “fast map”, meaning that they are able to associate a word and its referent after just a few exposures (Carey & Bartlett, 1978). Furthermore, infants manage to learn novel words and extend the meaning of these words to new exemplars even before the end of their first year of life (Schafer, 2005).

1.3.1. Word learning from fluent IDS and ADS

Isolated tokens are typically used in most research relating to word learning from fluent IDS and ADS, which exempts infants from managing the difficult task of isolating the target word (e.g., Schafer & Plunkett, 1998). The few studies that employ fluent speech rather than isolated tokens all tend to present the stimuli in the same exaggerated way. Although this method might facilitate learning, it may fail to represent the majority of natural language learning situations (e.g., Mani & Plunkett, 2008; Swingley, 2007).

Only a very limited number of word learning studies have addressed learning from speech registers other than IDS (Graf-Estes & Hurley, 2013; Ma, Golinkoff, Houston, & Hirsh-Pasek, 2011; Schafer & Plunkett, 1998). Using the switch task,

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either in IDS or ADS during the training phase (Graf Estes & Hurley, 2013). In the test phase, infants watched same trials, where the object and the auditory stimulus were combined as in the training phase, and switch trials, where the object was presented with the auditory stimulus of a different word-object-association to that of the training phase. The difference in looking times between the same trials and switch trials was taken as an index of successful learning and was found only in the IDS group, but not in the ADS group. Similarly, Schafer and Plunkett (1998) investigated word learning from ADS in 15-month-old infants. Here, infants successfully learned word-object associations from this adult-directed register. However, this finding is constrained by the invariability of the input. Infants were only presented with isolated tokens and hence, were not required to demonstrate any word segmentation abilities. In addition, these isolated tokens were used in both the training and test phases, and exempted infants from generalizing across different stimuli.

The only study to date that has addressed infants’ word learning from fluent IDS and ADS, suggests that infants develop the capability to learn words from adult-directed input between 21 and 27 months of age (Ma, Golinkoff, Houston, & Hirsh-Pasek, 2011). As already mentioned, an onset of word learning from ADS at this late stage would imply severe limitations on infants’ early language acquisition and conflicts with research findings on word learning from overheard speech (Akhtar, Jipson, & Callanan, 2001; Gampe, Liebal, & Tomasello, 2012).

Moreover, the common trait of all the word learning studies presented is that the register at test is always identical to the register during training. If the role of IDS is attracting and maintaining infants’ attention, being tested in ADS, a register that infants typically only overhear, might not grasp infants’ attention enough to respond to the task. Thus, it remains open whether the failure in learning from ADS reflects the true inability of infants to learn words from this register or whether infants are just

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unable to demonstrate the successful learning in this particular speech register.

Studies on infants’ word learning abilities from overheard speech between two adults support the latter possibility (Akhtar, Jipson, & Callanan, 2001; Gampe, Liebal, &

Tomasello, 2012). Infants of 24 months have been identified as succeeding in learning from overheard ADS. However, whether infants were also tested in ADS in these studies remains questionable. Adults directly addressed the infant during life interaction which might have increased the likelihood of the use of an IDS register.

Therefore, the fourth study of the current thesis, Schreiner and Mani (in revision), systematically addresses the role of speech register during both the training and test phases by exposing infants to both IDS and ADS during tests in order to additionally explore learning from ADS separately from the register in the test.

1.3.2. Pre-exposure and word learning

As words are rarely presented simultaneously and unambiguously with their respective referents (Brent & Siskind, 2001), various studies have explored how pre-exposure to the phonological form of the target word and the object affects infants’

word learning abilities, with varying results. After investigating pre-exposure to solely the phonological form of a target word used later, Swingley (2007) reported that 19-month-old infants’ word recognition improved if they had been pre-exposed to the target label. In investigating pre-exposure to the object, Fennell (2012) similarly found that only those 14-month-old infants who had been pre-exposed to the object were able to demonstrate successful learning. Through the manipulation of pre-exposure to both label and object, the findings of Kucker and Samuelson (2012) suggest that in order to retain previously learned word-object associations after a five-minute delay, 24-month-old infants require pre-exposure to the object and that pre-exposure to the label is not sufficient. At a younger age, 16-month-old infants require both

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exposure to label and object in order to demonstrate successful learning of the word-object associations (Altvater-Mackensen & Mani, 2013).

Against this background, the referents of the word-object associations to be learned in the fourth study by Schreiner and Mani (in revision) reported in this thesis are presented individually first, without any label. This allows the infants to have a real-world experience with the objects and increases the infants’ engagement in the task whilst simultaneously incorporating the findings on essential pre-exposure to objects for word learning.

As words are commonly not presented to infants as isolated tokens, Graf Estes, Evans, Alibali, and Saffran (2007) explored the effects that pre-exposure to words in fluent artificial ADS have on word learning. The 17-month-old infants of the study successfully differentiated between trials that represented the original word-object association and trials that represented word-word-object associations that infants had not been familiarized with, which is taken as an indicator of successful learning in switch paradigms. However, the word learning phase of this experiment only contained isolated tokens thus exempting infants from extracting the individual words.

Hence, the fourth paper of the current dissertation by Schreiner and Mani (in revision) attempts to explore the interaction of word segmentation and word learning using the natural language stimuli of IDS and ADS in both the segmentation and the word learning tasks. While this approach allows for comparisons of infants’ word segmentation and word learning skills in multiple speech registers, it also enables us to investigate the effect of pre-exposure to a label during word learning.

The current thesis therefore provides important insights into three different aspects of word learning. First, it systematically explores the relationship of infants’

word segmentation and word learning skills. Second, it investigates the impact of different kinds of speech input on infants’ word segmentation and word learning

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abilities. Third, it explores infants’ flexibility in recognizing previously learned words in a register other than the one presented during training.