Communication and Categorization: New Insights into the Relation Between Speech, Labels and Concepts for Infants

Communication and Categorization: New Insights into the Relation Between Speech, Labels and Concepts for Infants Brock Ferguson (brock@u.northwestern.edu) Sandra R. Waxman (s-waxman@northwestern.edu) Department of Psychology, Northwestern University, 2029 Sheridan Rd. Evanston, IL 60208 USA Abstract Almost two decades of research has demonstrated that labels facilitate infants’ categorization of novel objects. Some interpret this as evidence of an early link between infants’ linguistic and conceptual systems. Others suggest that these effects stem exclusively from lower-level processing mechanisms in cross-modal perception, and that words promote categorization only because they are more familiar to infants than non-linguistic acoustic stimuli and therefore easier to process. Here we address these discrepant interpretations using a novel approach. We expose infants to unfamiliar non-linguistic stimuli (sine-wave tone sequences), manipulating the exposure conditions. For 6-month-olds, if the novel acoustic stimuli were embedded within a communicative episode, they subsequently facilitated categorization (Experiment 1), but if they were presented in a non-communicative episode, they had no such effect (Experiment 2). We propose a developmental model that takes infants’ burgeoning perceptual and conceptual capacities into account in identifying how communication and words are linked to concepts. Keywords: language development; words; categorization; auditory overshadowing; infancy concepts; Introduction The nature of word learning has been the focus of a noteworthy debate in recent years. At stake is the relationship between words and concepts: Are words merely associated with objects by infants, as any percept might be associated with another (e.g., Sloutsky & Fisher, 2012)? Or might even the youngest word learners appreciate words as symbols that refer to concepts (e.g., Waxman & Gelman, 2009)? Further, if there is a privileged link between words and concepts in infancy, how is it established? Evidence for this latter position, positing an early and unique link between words and concepts, comes from numerous studies demonstrating that infants integrate domain-specific knowledge about words when they map novel words to objects (Fennell & Waxman, 2010; Namy & Waxman, 2000; Woodward & Hoyne, 1999), generalize words to object concepts (Booth & Waxman, 2009; Booth, Waxman, & Huang, 2005), make inferences about hidden properties of named objects (Diesendruck & Graham, 2010; Gelman & Heyman, 1999; Graham, Booth, & Waxman, 2012), and individuate named objects (Dewar & Xu, 2007; There is also evidence for a developmental cascade underlying infants’ establishment of a link between words and concepts. Initially, infants appear to hold a broad expectation that words refer to commonalities amongst objects (Waxman, 2003). With development, they refine this broad expectation to link particular types of words (e.g., nouns, adjectives) to particular types of categories (e.g., object categories, property categories) (Booth & Waxman, 2009). This increasingly precise relation between words and concepts can be observed over the first year in object categorization tasks. Infants hearing human language successfully form categories, but other matched acoustic stimuli (e.g., sine-wave tone sequences) do not (Balaban & Waxman, 1997; Fulkerson & Haaf, 2003; Fulkerson & Waxman, 2007; Waxman & Markow, 1995). More recent evidence reveals that infants as young as 3- and 4-months (who do not yet segment distinct words from fluent speech) form object categories in the context of human speech, but not in the context of sine-wave tones (Ferry, Hespos & Waxman, 2010). Thus over the first year, infants’ response to words may be a refinement of a broader and earlier response to communicative signals. Some researchers have argued that the influence of language in these studies reflects cross-modal perceptual processing alone (Robinson & Sloutsky, 2007; Sloutsky & Robinson, 2008). Their claims are clear: (1) object categorization tasks with paired acoustic stimuli recruit infants’ cross-modal processing abilities, (2) unfamiliar auditory stimuli impede visual processing through “auditory overshadowing”, and (3) verbal labels are more familiar to infants than the acoustic stimuli (e.g., tone sequences) to which they are typically compared (Sloutsky & Robinson, 2008). On this account, words benefit category formation only insofar as they are acoustically familiar. Here we take a novel empirical approach to tease apart these two accounts. In each experiment, infants participated in a standard object categorization task. But instead of pitting human language against unfamiliar sounds, all infants heard the same unfamiliar sounds: sine-wave tone sequences. Crucially, we introduced infants to these novel sounds in a video before they were presented within an object categorization task. This gave us full control over infants’ prior exposure to these novel stimuli, which in turn permits us to ascertain the precise exposure conditions that enable an auditory stimulus to facilitate visual categorization. In Experiment 1, we ask whether embedding