45) and switch (M = 5.16 5-Fluoracil sec, SD = 3.45) trials (F < 1). In addition, there was no effect of word used at switch (F < 1) or test order, F(2, 24) = 1.08, p = .36, and no two- or three-way interaction (trial × word, F[2, 11] = 1.1, p = .36; trial × test order, F < 1; trial × word × test order, F[2, 11] = 2.1,
p = .17), indicating that children responded without preference for either word, and order of test trials did not affect responses. The null result was unexpected, as work in infant speech perception has shown robustly that infants use variability in contrastive acoustic dimensions to learn phonemic contrasts (Maye et al., 2002, 2008), phonetic analyses support such structure in the input (Kuhl et al., 2007), and a number of computational models have shown that such processes can account for a range of behavioral data (McMurray et al.,
2009; Toscano & McMurray, 2010a; Vallabha, McClelland, Pons, Werker, & Amano, 2007). One possible explanation for this failure Bortezomib clinical trial could be the method used to construct the stimuli. This method of continuum construction has the disadvantage of producing voiceless tokens without the F0 pitch-onset rise in naturally produced speech. Younger infants in previous experiments have responded to voice distinctions in continua constructed this way (McMurray & Aslin, 2005), and data indicate that children do not perceive F0 as a cue before 4 years of age (Bernstein, 1983), yet it remains possible that the infants in Experiment 1 heptaminol might have responded poorly to the /puk/ stimuli because of the unnatural properties of the continuum. In fact, beyond F0, many cues to voicing are simultaneously
present in natural speech (e.g., pitch, burst amplitude, vowel length, first formant frequency, Burton, Baum, & Blumstein, 1989; Burton & Blumstein, 1995; Ohde & Haley, 1997). It is possible that variability in additional acoustic cues may be needed to establish a robust voicing contrast, cues that were likely to vary in Rost and McMurray (2009) within and across speakers. Experiment 2 therefore tested infants’ use of variability in these additional contrastive cues by using a continuum that covaried in VOT, pitch, and burst amplitude. Recruitment and exclusion criteria were the same as in Experiment 1. Twenty-two infants participated and data from six were excluded for failing to habituate (2), having ear infections (2), fussiness (1), and experimenter error (1). Analyses were run on data from the 16 remaining infants (10 boys; M age = 14 months 13 days, range = 13 months 10 days to 15 months 0 days). In Experiment 2 we modified the continuum from Experiment 1 to include additional covariation between VOT and two secondary voicing cues (burst amplitude and F0). Figure 3 details this process. The amplitude of the burst and aspiration was manipulated by excising the burst (including the entire VOT) from the voiced tokens and multiplying the waveform.