Ultimately, differences

in performance across phonetic contrasts may derive less from their phonological status (e.g., consonant versus vowel) and more from the statistical structure of the cues to these contrasts, particularly when we look across multiple relevant AZD1208 and irrelevant dimensions. For example, cues like VOT do vary between speakers (Allen, Miller, & de Steno, 2003), but they are largely discriminable without taking this into account; therefore, high variance in speaker cues will quickly reveal the more invariant contrastive VOT cues. However, for vowels, and to a lesser extent, fricatives and liquids, contrastive and noncontrastive acoustic dimensions overlap substantially (vowels: Hillenbrand, Daporinad chemical structure Getty, Clark, & Wheeler, 1995; fricatives: Jongman, Wayland, & Wong, 2000). These contrastive dimensions, such as formant structure, F0, and length, are also cues that vary considerably by speaker. In order to use speaker variation to detect such differences, infants may need more sophisticated ways of dealing with this variability or may simply need to learn more about those things that contribute to variance (Cole, Linebaugh, Munson, & McMurray, 2010) before vowel can be a cue to word identity. As a result, failures

in the switch task at 14 months do not represent a reversal of development, a U-shaped curve, or a discontinuity. We suggest rather that speech perception was never fully developed at 12 months, as is evidenced by studies of older children (Nittrouer, 2002; Ohde & Haley, 1997; Slawinski & Fitzgerald, 1998). Reliance largely on discrimination measures resulted in a failure to consider other factors (like dimensional weighting)

that are revealed by this task. This study hints that dimensional weighting is sensitive to the relative variation along different dimensions. This may in fact represent a general principle of learning. For example, the role of irrelevant variation suggested by this work parallels mechanisms proposed by Gómez (2002) for statistically determined grammatical dependency structures. Adults and infants learned a novel grammar with nonadjacent dependency structure. When intervening elements in the dependency were long and variable, both adults and infants detected the nonadjacent dependencies. When Loperamide intervening elements did not vary, participants were unable to learn the grammatical dependencies. Consequently, learning of grammatical dependencies in Gómez’s experiment requires high variability in those elements that are not criterial for determining the grammar. Yu and Smith’s (2007) work on learning word–object mappings via cross-situational statistics illustrates the same point. In this study, subjects learned a small set of word–object mappings solely by noticing the statistical relationship between the sound and the object: whenever a given word was heard, the referent was consistently present.