Department of Psychology, University of British Columbia, Canada.
Department of Molecular and Cellular Biology, Harvard University, USA; International Research Center for Neurointelligence, University of Tokyo, Japan.
Cognition. 2021 Aug;213:104653. doi: 10.1016/j.cognition.2021.104653. Epub 2021 Mar 19.
Perceptual attunement to the native phonetic repertoire occurs over the first year of life: an infant's discrimination of non-native phonetic contrasts declines while their discrimination of native phonetic contrasts improves, with the timing of change consistent with sensitive periods. The statistics of speech sound distributions is one source of input used to collapse non-native phonetic category boundaries, while sharpening native ones. Distributional learning can be a domain-general mechanism, yet given the timing of perceptual attunement, we hypothesized that this learning mechanism may be maturationally delimited in the content domain of phonetic categories. Here, we assessed whether sensitivity to the distribution of speech sounds in the environment declines as the period of perceptual attunement closes. We used electroencephalography (EEG) to investigate whether neuronal responses to native 'ra' and 'la' phones are modulated differently in older vs young infants by exposure to either a bimodal or unimodal sound distribution spanning the [r] ~ [l] phoneme space. The native contrast, ra-la, is discriminable at all three ages, ensuring that we were testing the distributional learning mechanism, rather than confounding it with a decline in discrimination to a non-native distinction. English monolingual infants (n = 131) at 5-, 9- and 12-months-old were familiarized to either a unimodal or bimodal distribution of /ra/-/la/ speech sounds. Immediately following familiarization, an ERP oddball task was used to assess discrimination. Results showed that brief exposure to a bi- vs uni-modal distribution is sufficient to alter neuronal responses to subsequent /ra/ vs /la/ speech sounds at 5-months and 9-months, but not at 12-months. These results are the first to capture a progressive decline in sensitivity to distributional statistics in the environment. A potential mechanistic explanation based on critical period biology is discussed.
婴儿对非母语语音对比的辨别能力下降，而对母语语音对比的辨别能力提高，变化的时间与敏感期一致。语音分布的统计数据是用于缩小非母语语音范畴边界的输入源之一，同时也使母语语音范畴边界变得更加清晰。分布学习可以是一种领域通用的机制，但鉴于感知调谐的时间，我们假设这种学习机制可能在语音范畴的内容领域受到成熟的限制。在这里，我们评估了随着感知调谐期的结束，对环境中语音分布的敏感性是否会下降。我们使用脑电图 (EEG) 来研究在接触跨越 [r]~[l] 音素空间的双模态或单模态声音分布后，神经元对母语 'ra' 和 'la' 音的反应是否会因婴儿年龄的不同而不同。母语对比 'ra-la' 在所有三个年龄段都是可辨别的，这确保了我们正在测试分布学习机制，而不是将其与对非母语区别的辨别能力下降混淆。英语单语婴儿（n=131）在 5、9 和 12 个月大时，分别接触到单模态或双模态的 /ra/-/la/语音分布。在熟悉之后，立即使用 ERP oddball 任务来评估辨别力。结果表明，短暂接触双模态与单模态分布足以改变 5 个月和 9 个月大的婴儿对随后的 /ra/ 与 /la/ 语音的神经元反应，但对 12 个月大的婴儿则不然。这些结果是首次捕捉到对环境中分布统计数据的敏感性逐渐下降。讨论了基于关键期生物学的潜在机制解释。
