Effects of Onset Density in Preschool Children: Implications for Development of Phonological Awareness and Phonological Representation

Neighborhood density influences adult performance on several word processing tasks. Some studies show age-related effects of density on children’s performance, reflecting a developmental restructuring of the mental lexicon from holistic into segmental representations that may play a role in phonological awareness. To further investigate density effects and their implications for development of phonological awareness, we compared performance on dense and sparse onset words. We adapted these materials to three phonological awareness tests that were pretested on adults then administered to preschool children who were expected to vary in phonological awareness skills. For both the adults and the children who passed a phonological awareness screening task, dense onset neighborhoods were associated with slower reaction times and increased errors. A separate comparison of word repetition by the children who passed and who did not pass the phoneme awareness screening failed to provide evidence that lexical restructuring was a sufficient condition for the attainment of phonological awareness. Both groups of children more accurately repeated words from high onset density neighborhoods, regardless of the level of their phonological awareness. Thus, we find no evidence of either ageor ability-driven effects in children’s performance, contradictory to a view that the attainment of phoneme awareness relates to developmental changes in the segmental representation of words in dense neighborhoods. Phonological awareness is one of the most important predictors of successful reading development (Gottardo, Stanovich, & Siegel, 1996; Lyon, 1995; Mann, 1998; Muter & Snowling, 1998; Stanovich, 1994). Its ontogeny appears to be influenced by several factors, key among them being the status of phonological development and the extent of exposure to alphabetic instruction (for reviews, see Anthony & Francis, 2005; Foy & Mann, 2001; Mann & Foy, 2003). Obviously, a certain level of phonological processing skill is a prerequisite for phonological awareness, and © 2009 Cambridge University Press 0142-7164/09 $15.00 Applied Psycholinguistics 30:2 340 Foy & Mann: Onset density effects it should come as no surprise that children’s phonological awareness is linked with their effective and efficient perceptual and productive control of the sounds of their language (Elbro, Bostrom, & Petersen, 1998; Foy & Mann, 2001; Mann & Foy, 2003, 2007). But research suggests that something specific can be required for the attainment of phoneme awareness. Foremost among these specific factors is exposure to the alphabetic principle (i.e., learning that letter sequences are associated with specific sounds; Mann & Wimmer, 2002; Morais, Cary, Alegria, & Bertelson, 1979; Read, Zhang, Nie, & Ding, 1986). In this paper we pursue an aspect of lexical processing that has been offered as an explanation of the emergence of phoneme awareness. Metsala and Walley (e.g., 1998) have suggested that, about the time that they begin to learn to read, children’s lexicons undergo significant restructuring from holistic representations towards phoneme-sized units (lexical restructuring theory). They view this as a result of such factors as vocabulary expansion (e.g., Walley, 1993), but others note that it could arise from a changing focus from meaning to sound (Byrne & Liberman, 1999) and the learning of letter sound associations (Barron, 1998; Foy & Mann, 2006; Treiman & Bourassa, 2000; Vihman, 1981, 1996). According to Metsala and Walley, lexical restructuring is an impetus for phonological awareness, and it emerges as a consequence of spoken vocabulary growth: as children acquire more vocabulary items, their initially holistic lexical representations (e.g., “cut”) come to be represented in an increasingly segmental way (i.e., “c” “u” “t”). The addition of vocabulary items to the lexicon presumably involves increasing numbers of “high neighborhood density” words, words that are highly similar in phonological structure (Metsala & Walley, 1998). This dense similarity among words presumably forces a change from holistic representation of entire words to segmental representation of such sublexical units as phonemes. When lexical representations become more segmental, this presumably makes vocabulary growth and lexical access more efficient because words can be stored and retrieved on the basis of their shared segments, instead of as indivisible wholes. At the same time, more segmental representations can be seen as a prerequisite for children’s realization that words consist of separate phonemes. Thus, in the view of Walley, Metsala, and their colleagues, phoneme awareness follows from lexical restructuring because that restructuring provides the phoneme representations that are mapped onto alphabetic representations. Coady and Aslin (2004) recently demonstrated that very young children (2.5– 3.5 years) showed sensitivity to segmental aspects of nonwords in a repetition task varying phonotactic probability. Evidence of lexical restructuring, according to its proponents, is seen in certain differential effects of neighborhood density on the performance of preschool children compared to older children and adults. For example, in speech gating tasks (e.g., Walley, 1993), where increasingly larger segments of speech are added on successive trials, preliterate children responded to words from dense neighborhoods more segmentally than they did to words from shallow neighborhoods (Garlock, Walley, & Metsala, 2001), whereas adults responded to all words segmentally. The fact that density effects on speech processing depend on age, age of acquisition (Garlock et al., 2001), task (Metsala, 1999; Vitevitch & Sommers, 2003), and vocabulary size (De Cara & Goswami, 2003) has Applied Psycholinguistics 30:2 341 Foy & Mann: Onset density effects been a primary basis of support for the lexical restructuring theory. For example, using a word repetition task, Garlock et al. (2001) found dense neighborhoods associated with poorer performance in preschoolers, but only for early-acquired words. In contrast, for both earlyand late-acquired words elementary schoolaged children (first and second graders) and adults showed poorer performance on words from dense neighborhoods. These findings were taken to suggest that for very young children, with presumably small vocabularies, neighborhood density facilitates segmental effects in speech-processing performance. We note that a link between vocabulary size and density effects echoes findings of a link between vocabulary size, reading ability, and such prereading skills as phonological awareness in young children (e.g., Bowers & Wolf, 1993; Foy & Mann, 2001, 2003; Mann & Foy, 2003; Wolf, 1999). Our interest in neighborhood density effects and in the lexical restructuring theory concerns their ability to explain why young children do poorly on phoneme awareness tasks. A few studies have explicitly examined effects of neighborhood density on phonological awareness. They illustrate a mixed pattern of results. For example, one study shows facilitative effects of neighborhood density on the phoneme blending performance of 3to 4-year-old children but no effect in an onset-rime blending task (Metsala, 1999). Density has also been reported to facilitate rhyme awareness in a study of 5year-olds (De Cara & Goswami, 2003); children with large vocabularies had more accurate responses in a rhyme oddity task for words from dense neighborhoods than for words from sparse neighborhoods. In the experiment reported below, we have continued to study the effects of neighborhood density on phonological awareness. However, rather than using overall neighborhood density, we have adapted a set of materials from Vitevitch (Vitevitch, 2002a; Vitevitch, Armbruster, & Chu, 2004), who manipulated the relative onset density of words from highly dense neighborhoods. By showing performance differences between words from neighborhoods that share the initial consonant (e.g., the onset) and those whose neighborhoods involve a diversity of initial consonants, these researchers provide evidence of sublexical (e.g., segmental) processes in lexical decision and repetition. Our reasoning was that (a) if the lexical representations of preschool children are predominantly holistic, then we should find less evidence of sublexical processes in their processing of spoken words, especially when the children are unable to perform phonological awareness tasks, and (b) if phoneme awareness follows from a shift to segmental representation, then we would observe a contrast between onset density effects on the speech processing performance of children who demonstrate phoneme awareness and those who do not. The concept of “neighborhood” has become increasingly important to models of the lexicon. A word’s neighborhood is the set of words that share the majority of phonemes and their sequence. Members of a consonant–vowel–consonant (CVC) word’s neighborhood, for example, can be produced by systematically changing one phoneme in the word (by adding, substituting, or deleting a phoneme) and then noting whether the item that results is also a word. For example, “mad” is in the neighborhood of “bad” (“b” changed to “m”) and “mud” (“u” changed to “a”). Neighborhood density, as calculated in the present study, refers to the number of Applied Psycholinguistics 30:2 342 Foy & Mann: Onset density effects words that can be so derived for a target word (Coltheart, Davelaar, Jonasson, & Besner, 1977). Words that have many neighbors share a highly similar phonological structure with many other words, and are said to “reside” in “dense neighborhoods” (Logan, 1992; Luce, 1986; Metsala & Walley, 1998). Most typically, in lexical recognition studies, density shows a competition effect: words from dense neighborhoods evoke longer response times than words from sparse neighborhoods (Garlock et al., 2001; Luce & Pisoni, 1998; Motley & Baars, 1975; Vitevitch, 2002a, 2002b; Vitevitch et al., 2004; Vitevitch & Luce, 1998, 1999; Vitevitch, Luce, CharlesLuce, & Kemmerer, 1997). Vitevitch and colleagues (Vitevitch, 2002a; Vitevitch et al., 2004) used a refinement of neighborhood density, namely, onset density to provide a more direct form of evidence about sublexical processes in speech perception and production. When words from equally “dense” neighborhoods were ranked in terms of the density of shared onset (e.g., the number of neighbors that shared the initial consonant), the reaction time (RT) increased as onset density increased (Vitevitch, 2002a; Vitevitch et al., 2004), and Vitevitch and colleagues attributed the penalizing effect of onset density to interactive processes between words and phonemes. In their view, if a word contains phonemes that occur in the same position in many other words then it receives additional activation because of reverberation between the words and the phonemes. High onset density words like “mass” activate relatively many neighbors that share the same initial phoneme “m” but differ in medial or final segments. This activation of words that start with “m” leads to competition for the medial and final phonemes of the target word (e.g., neighbors like “mad” and “miss” compete with the target word “mass”), a slowing of response time in lexical decision and a loss of accuracy in picture naming. Low onset density words like “sad” activate relatively few words that start with “s,” as fewer such words exist, but there are relatively more words that share medial and/or final phonemes (e.g., “bad,” “mad”), so the spreading activation occurs at a later point in word processing and thus causes less competition. Vitevitch et al.’s primary interest is in distinguishing between lexical and sublexical levels of speech representation (for a review, see Vitevitch et al., 2004) and in contrasting the merits of interactive versus feedforward models of speech processing. For our purposes, their research can provide an important tool for studying any developmental changes in sublexical, segmental representation. If onset density effects imply an interaction between sublexical and lexical units, then their existence presumes that units such as onsets and rimes or phonemes are an active part of lexical representation. Children who show onset density can be presumed to be representing words segmentally. In the present experiment, our strategy is to explore onset density effects on phoneme awareness and on speech processing. Vitevitch and colleagues have observed competition effects of onset density on shadowing, lexical decision, and naming tasks in the case of literate adults, and they have used this as evidence for the role of phonemic levels of representation in speech processing. We now ask if there are parallel effects of onset density on phoneme awareness tasks among preschool-aged children, and we ask if onset density effects among preschool children are limited to children who have attained phoneme awareness. Applied Psycholinguistics 30:2 343 Foy & Mann: Onset density effects According to the lexical restructuring account, effects of a reduced vocabulary size should be limited to children close to the level at which holistic representation gives way to segmental representation. In the Vitevitch account, however, vocabulary size could relate to the effects of onset density, as larger vocabularies could associate with greater activation and a greater number of competing responses. Extrapolating from the lexical restructuring hypothesis, we hypothesized the following: (a) for children who are aware of phonemes, we should see effects of onset density in both phoneme awareness and in word repetition, consistent with the operation of sublexical processes; (b) the effects of onset density on the word repetition of children who lack phoneme awareness should contrast with that of children who demonstrate phoneme awareness, if lexical restructuring is the impetus for attainment of phoneme awareness; and (c) onset density effects should be stronger in children with large vocabularies than in children with smaller vocabularies if vocabulary size is the primary impetus for lexical restructuring. Children received a phonological awareness screening, the onset density phonological awareness tasks, and a subset of the items from Vitevitch’s study of onset density effects on repetition. We also included vocabulary measures and a nonsense word repetition task and a test of working memory test. These choices were driven by the work of Gathercole and colleagues (Adams & Gathercole, 1995; Gathercole, Willis, Emslie, & Baddeley, 1992), who have consistently shown that vocabulary and working memory are interrelated. To test these predictions regarding onset density and phonological awareness requires that we can, in principle, obtain onset density effects in phonological awareness. Prior to conducting our experiment we used a subset of Vitevitch et al.’s materials to examine the effects of onset density on the phonological awareness of a group of adult subjects. We found that adults were significantly faster and more accurate in rhyme production, phoneme deletion, and phoneme substitution for words from sparse onset neighborhoods compared to words from dense onset neighborhoods. This is consistent with our previous findings of competition effects in adults with nonwords (Foy & Mann, 2004). This effect of onset density on phonological awareness was stronger among participants with larger receptive vocabularies, as would be expected from Vitevitch et al.’s account of onset density effects (Vitevitch, 2002a; Vitevitch et al., 2004) as the reflection of sublexical processes and response competition. It was also stronger among those with greater working memory performance, consistent with Gathercole and Baddeley’s model of the connection between vocabulary and working memory (see the Appendix A for details of the adult data).