Is perceptual reorganization affected by statistical learning? Dutch infants’ sensitivity to lexical tone
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At least two things are known with respect to infants’ phonemic acquisition: they can track distributional information from the ambient speech input (Maye et al., 2002; Maye et al., 2008), and they experience perceptual reorganization (PR) after which their sensitivity towards non-native speech contrasts greatly decreases (Werker & Tees, 2002). However, it is largely unknown how PR may affect the ability to track sound distributions. Few previous studies have addressed the statistical learning of lexical tones in infants. It remains unclear whether non-tone-language-learning infants can discriminate tonal contrasts if only provided with some 'right' type of distributional input. Also, although previous studies suggest that tonal PR occurs between 6 and 9 months of age (Mattock & Burnham, 2006; Mattock et al., 2008), the range of tonal contrasts tested to support this suggestion is fairly small. It is likely infants acquire native phonemic inventory at different points in time due to frequency of exposure and the psycho-acoustic difficulty of individual phonemic categories. Hence, multiple tonal contrasts should be studied before concluding on a time window for tonal PR. Moreover, no previous study has combined statistical learning with PR in infants. To solve the issues above, the research questions of this study are: 1) Does statistical learning facilitate infants’ discrimination of a non-native tonal contrast? 2) If yes, is this ability affected by tonal PR? 3) When does tonal PR take place? Ninety-six Dutch infants aged at 5, 11 and 14 months were tested on their perception of a non-native tonal contrast /ta1/–/ta4/ (high-level vs. high-falling) in Mandarin Chinese. A continuum of 8 steps was created for this contrast and two conditions (uni/bimodal) were set up differing solely in the frequency distribution of the stimuli along the continuum. A bimodal distribution is known to facilitate discrimination while a unimodal distribution leads to a decrease in sensitivity. After a 3-minute familiarization phase on one of the two distributions, infants went through a habituation–dishabituation procedure in which their discrimination abilities to the test stimuli were examined. Results reveal an interesting pattern across ages and conditions. Repeated measures ANOVA shows the Dutch 5-month-old group can distinguish the Chinese tones regardless of the distribution they were exposed to (p = .020), while at 11 months only infants trained on the bimodal condition show discrimination (p = .039). By 14 months, infants trained on the bimodal condition can no longer distinguish the tonal contrast. The current study supports Mattock and his colleagues’ findings about the onset of tonal PR, yet with a slightly different offset since PR may take place later than 9 months in the current study. Future work will address the degree of influence by statistical learning and precise offset of tonal PR. Furthermore, the present study reveals not only the plasticity of PR but also the limitations of statistical learning: to answer the research questions, statistical learning does influence infants’ discrimination of the non-native tonal contrast; whereas this influence seems to be reduced before the onset and possibly after the offset of tonal PR.