Children’s Use of Disfluencies for Pragmatic Inference in Lexical Development

Children’s Use of Disfluencies for Pragmatic Inference in Lexical Development Celeste Kidd, Katherine S. White, and Richard N. Aslin {ckidd, kwhite, aslin}@bcs.rochester. edu Department of Brain & Cognitive Sciences, Meliora Hall Rochester, NY 14627 USA Abstract In early lexical development, children must learn to map spoken words onto their respective referents. Since multiple objects are typically present when any word is used, a child is charged with the difficult task of inferring the speaker’s intended referent. Previous research has uncovered various cues children may use in this task, including contextual and social cues. We investigate a previously unexplored cue for inferring speaker intention during word learning: speech disfluencies. Disfluencies (such as “uh” and “um”) occur in predictable locations, such as before words that are infrequent and words that have not been previously mentioned. We conducted an eye-tracking study to investigate whether young children can make use of the information contained in disfluencies to infer a speaker’s intended referent. Our results demonstrate that young children (ages 2;4 to 2;8) are sensitive to disfluencies. More critically, they show that children appear to use disfluencies predictively as a cue to reference and to speaker intention as the disfluency is occurring. We also examined potential sources of learning about disfluencies in CHILDES (MacWhinney, 2000) and found that disfluencies, though rare, occur regularly and with increasing frequency over time in child-directed and child- produced speech. These results reveal that young children attend to speech disfluencies relatively early in lexical development and are able to use the disfluencies to infer speaker intention during online comprehension. Keywords: Language development; lexical development; word learning; pragmatic inference; speech disfluency. Introduction Word learning is fundamental to language acquisition, but the successful mapping of auditory events (spoken words) onto referents (objects in the world) is not always transparent. While some labeling contexts are unambiguous (e.g., holding a cookie and saying “cookie”), most contexts involve multi-word utterances and multiple objects in the child’s visual field. Thus, extra-linguistic cues, such as inferring speaker intention, can play a crucial role in word learning. The inability to correctly infer speaker intent, as in autism, can lead to catastrophic failures of communication (Preissler & Carey, 2005). Previous work has explored various cues available to learners that could aid in determining speaker intention. Social cues, such as parent-child joint visual attention and pointing, provide cues to word learners about the intended referent (Butterworth, 1980; Southgate, 2002). By 18 months, children are even able to make use of information conveyed by the speaker’s eye-gaze (Baldwin, 1991). Discourse context may also aid in determining a speaker’s intended referent (Frank et al., 2009). In addition to these externally available cues, young children appear to make certain assumptions that facilitate rapid lexical development. One assumption of particular relevance is that of mutual exclusivity (Markman, 1990). Experimental evidence suggests that young word learners assume a one-to-one mapping between words and referents, starting as young as 15 months of age (Halberda, 2003; Markman, Wasow, & Hansen, 2003). Here we investigate a previously unexplored cue for inferring speaker intention: speech disfluencies. Disfluencies (e.g., “uh” and “um”) could be a potentially powerful cue to reference for several reasons. First, disfluencies occur in highly predictable locations— specifically, before unfamiliar or infrequent words, and before words that have not previously been mentioned in the prior discourse. Thus, rather than being noise, disfluencies provide information which is potentially useful for discovering what a speaker intends to refer to. Since disfluencies occur before an object is labeled, they could enable a child to anticipate upcoming referents and therefore enhance the speed of spoken word recognition. Disfluencies are known to be a reliable property of speech between adults. Fox Tree (1995) estimated that about 6 disfluencies occur per 100 words, excluding pauses (which are not necessarily disfluencies). Shriberg (1996) found that disfluencies occur every 7 to 15 words in conversation between adults, depending upon which corpus (SWBD or AMEX) was used for the analysis. The rate of disfluency varies as a function of several factors: speaker familiarity, utterance length, and speech rate (Shriberg, 1996). Disfluencies come in a variety of different types, including repetitions, substitutions, insertions, deletions, and speech errors. We focus here on the most common type of disfluency, the filled pause—“uh” and “um” in English (Shriberg, 1996). This type of disfluency is particularly common before infrequent and previously unmentioned words (Arnold & Tanenhaus, 2007). Consider the following example of a filled pause from the Sachs corpus in CHILDES (MacWhinney, 2000): *CHI: Telephone ? *MOT: No, that wasn't the telephone, honey. That was the, uh, timer. The disfluency occurs before the less frequent word and previously unmentioned object, “timer”. Such disfluencies are thought to result from a delay in lexical retrieval (Clark & Fox Tree, 2002; Fox Tree & Clark, 1997).