Elsevier

Cognition

Volume 106, Issue 3, March 2008, Pages 1558-1568
Cognition

Brief article
Infants rapidly learn word-referent mappings via cross-situational statistics

https://doi.org/10.1016/j.cognition.2007.06.010Get rights and content

Abstract

First word learning should be difficult because any pairing of a word and scene presents the learner with an infinite number of possible referents. Accordingly, theorists of children’s rapid word learning have sought constraints on word-referent mappings. These constraints are thought to work by enabling learners to resolve the ambiguity inherent in any labeled scene to determine the speaker’s intended referent at that moment. The present study shows that 12- and 14-month-old infants can resolve the uncertainty problem in another way, not by unambiguously deciding the referent in a single word-scene pairing, but by rapidly evaluating the statistical evidence across many individually ambiguous words and scenes.

Introduction

The pairing of a word and a scene is not enough to determine the meaning of the word. To illustrate this point, Quine (1960) famously imagined a stranger who hears a native say “gavagai” and points to a scene. To what does “gavagai” refer – a rabbit, the grass, a tree, the rabbit’s ears, or perhaps the beauty of the whole? Even if one assumes a perceptual system that segments the scene into separate objects and an attentional system biased towards objects, the intended referent is indeterminate from this one experience. Infants are like strangers who do not know the native language, yet they solve this indeterminacy problem. This paradox – the uncertainty of the referent in word-scene associations and the fact that infants learn object names nonetheless – is a core theoretical problem in the study of early word learning. For the past 30 years most research on children’s word learning has concentrated on how the learner resolves the ambiguity at the moment the novel word is first encountered. Experimental studies leave no doubt that by the time they are 2 years old children do this at least for object names. That literature points to attentional (Smith, 2000), social (Baldwin, 1993, Tomasello, 2000), linguistic (Gleitman, 1990) and representational (Markman, 1990) constraints as crucial to children’s ability to resolve referential ambiguity and fastmap a word to its intended referent.

There are two reasons to suspect that this one-encounter solution to referential uncertainty is not the only (or even the most important) mechanism of early word learning. First, not all opportunities for word learning are as uncluttered as the experimental settings in which fast-mapping has been demonstrated. In everyday contexts, there are typically many words, many potential referents, limited cues as to which words go with which referents, and rapid attentional shifts among the many entities in the scene. It is possible that young learners just ignore the information in such highly ambiguous learning contexts and wait for contexts in which the referents of heard words are more certain (Brent & Siskind, 2001). However, a more optimal learner might be expected to make use of all the available data.

Second, the evidence indicates that 9-, 10-, and certainly 12-month-old infants are accumulating considerable receptive lexical knowledge (Fenson et al., 1994, Swingley and Aslin, 2000). Yet many studies find that children even as old as 18 months have difficulty in making the right inferences about the intended referents of novel words (e.g., Katz et al., 1974, Hirsh-Pasek et al., 1999, Moore et al., 1999, Pruden et al., 2006). There are studies showing that infants as young as 13 or 14 months (Woodward et al., 1994, Woodward and Hoyne, 1999, Schafer and Plunkett, 1998; but perhaps not younger, Werker, Cohen, Lloyd, Casasola, & Stager, 1998) can link a name to an object given repeated unambiguous pairings in a single session. Overall, however, these effects are fragile with small experimental variations often leading to no learning (see especially, Woodward and Hoyne, 1999, Werker et al., 1998; also Oviatt, 1980, Oviatt, 1982, Bloom, 2000 for a discussion). This raises the possibility that there might be some other way that young children learn word-referent mappings.

The experiment reported here shows for the first time that infants rapidly learn multiple word-referent pairs by accruing statistical evidence across multiple and individually ambiguous word-scene pairings. The indeterminacy problem is solved not in a single trial but across trials, not for a single word and its referent but for a data set of many words and referents. This learning is shown to be sufficiently rapid and robust that it could play a significant role in early lexical learning.

Fig. 1 illustrates how cross-trial statistics might work. The learner hears the unknown words “bat” and “ball” in the context of seeing a BAT and BALL. Without other information, the learner cannot know whether the word form “ball” refers to one or the other visual object. However, if subsequently, while viewing a scene with the potential referents of a BALL and a DOG, the learner hears the words “ball” and “dog” and if the learner can combine the co-occurrence frequencies from the two streams of data across trials, the learner could correctly map “ball” to BALL. This example represents the simplest case of cross-situational statistical learning – two words, two objects, two adjacently informative trials.

Several formal simulations of word-referent learning suggest the plausibility of cross-situational word learning in much more complex situations with many words, many possible referents, highly ambiguous individual learning trials, and the statistical resolution of the ambiguities only through the accumulation and evaluation of information over many word-referent pairings and many trials (Siskind, 1996, Yu et al., 2005). Consider the more complex case in Table 1. On trial 1, a learner could mistakenly link word A to referent b. On trial 4, the mistake could be corrected, if the system registers that word A occurred on trial 4 without possible referent b, if the cognitive system remembers the prior word-referent pairing, if it registers both co-occurrences and non co-occurrences, and if it calculates the right statistics. Can babies do this?

There is evidence in such phenomena as the mutual-exclusivity effect and contrast that 2- to 3-year-old children combine information across two adjacent naming events, using, for example, knowledge of the just-heard name of one thing to infer the object to which a subsequent name must apply (Akhtar, 2002, Akhtar and Montague, 1999, Markman, 1990, Namy and Gentner, 2002.) However, there is no evidence as to whether young learners can combine and evaluate information from highly ambiguous contexts over many trials. Until recently, there was no evidence as to whether even adult learners were capable of this, although Yu and Smith, 2007, Yu and Smith, 2007 have now shown that this form of learning is rapid and robust in adults even in situations of high uncertainty.

In the following experiment, 12- and 14-month-old infants were taught 6 word-referent pairs via a series of individually ambiguous trials. On each trial, two word forms and two potential referents were presented with no information about which word went with which referent. Although word-referent pairings were ambiguous within individual trials, they were certain across trials. For example, for a particular infant, whenever the form tobi occurred its assigned referent always occurred. After training, infants were presented with a single word and two potential referents, the cross-trial correct referent and a foil. Past research (e.g., Golinkoff et al., 1997, Swingley and Aslin, 2000) shows that within this kind of preferential looking task, infants look longer at the labeled test object. Thus if infants have calculated the statistics appropriately, despite the uncertainty on individual learning trials, they should look longer at the correct referent of the word form.

Section snippets

Participants

The participants, drawn from a working and middle-class population of a midwestern college town, were 28 12-month-old infants (range – 11 mo 17 days to 13 mo 0 days; mean – 12 mo 7 days; 13 males, 15 females) and 27 14-month-old infants (range –14 mo 2 days to 15 mo 14 days; mean –14 mo 12 days; 14 males, 13 females). Two additional children began but did not finish the experiment.

Stimuli

The 6 “words” – bosa, gasser, manu, colat, kaki and regli – followed the phonotactic probabilities of English and

Training trials

Infants were highly attentive to the training slides, looking (sum of right and left looks) at each 4 s slide on average 3.27 s (12 month olds) and 3.04 s (14 month olds). On average, infants looked at the left and right sides of each training slide for equal durations (t < 1.00 for both 12- and 14-month olds). On 87% of all training slides, the infants looked at both sides (both objects) for at least 1 s.

Test trials

On average, infants looked at each 8 s test slides for a total of 5.6 s for 12 month olds and 6.1 s

General discussion

Parents, on average, direct between 300 and 400 utterances an hour to their children (Hart & Risley, 1995). Even with social, linguistic and conceptual constraints in play, so many words in so little time seems likely to generate considerable ambiguity about intended referents. These ambiguities are most likely greater than those in this experiment. Nonetheless, the mechanisms responsible for the present results may be relevant to making use of the complexity in natural learning environments in

Acknowledgments

We thank Ling Jiang, Lisa Cantrell and Char Wozniak for collection of the data and Rich Shiffrin for insightful discussions. This research was supported by National Science Foundation Grant BCS0544995.

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