Theissen, 2007). Although both theories explain existing behavioral data, they imply that speech perception is well developed in children at this age, and that top-down factors impede it (Werker & Curtin, 2005). However, it is possible that bottom-up speech perception factors, that is, perceptual abilities
that are relevant for speech but not completely developed, may contribute to this failure. Although discrimination tasks indicate that some category boundaries are established by 1 year (e.g., Werker & Tees, 1984), there is also abundant evidence that children refine their phoneme categories well into the school years (Nittrouer, 2002; Ohde & Haley, 1997; Slawinski & Fitzgerald, 1998). LDE225 Thus, it is possible that 14-month-olds’ phonetic categories are only partially developed, and the
existing categories, while sufficient to succeed at discrimination tasks, may provide a weak platform for word learning. Rost and McMurray (2009) assessed this by examining the role of acoustic variability in learning phonologically similar words. We hypothesized that if speech categories were still developing, the small set of acoustic exemplars provided in most studies (Stager & Werker, 1997; Werker et al., 1998, 2002) might leave ambiguity about the structure of the phonetic category. Variability could provide more structure to the phonetic category, supporting word learning. Exoribonuclease Similar effects of variability on category learning find protocol have been observed in both visual categorization (Oakes, Coppage, & Dingel, 1997; Quinn, Eimas, & Rosenkrantz, 1993) and in the acquisition of phonetic categories in a second language (Lively,
Logan, & Pisoni, 1993), suggesting that this simple manipulation may be an important way to support categories that are not yet fully developed. Fourteen-month-olds were tested in the switch task (Werker et al., 1998) by habituating them to two novel objects paired with two novel, phonologically similar, words (/buk/ and /puk/, both rhyme with “luke”1). Infants were then tested on a same trial, where the word–object pairing was consistent with habituation, and a switch trial, where the word–object pairing was opposite of what it had been in habituation. If infants internalized the word–object mapping, they should dishabituate on the switch trials. Experiment 1 replicated prior work: infants hearing a small set of exemplars failed to notice the switch. However, Experiment 2 employed multiple exemplars of the words spoken by 18 speakers; infants hearing variable exemplars correctly acquired the two phonologically similar words. At face value, successful learning in the multitalker condition is surprising.