Humans appear to have a critical period (CP) for language acquisition: Second language (L2) acquisition becomes harder after early childhood, and ceasing exposure to a first language (L1) after this period (but not before) typically does not lead to substantial loss of L1 proficiency. It is unknown whether these CP effects result from innately determined brain maturation or as a stabilization of neural connections naturally induced by experience. In this study, we use language models (LMs) to test the extent to which these phenomena are peculiar to humans, or shared by a broader class of language learners. We vary the age of exposure by training LMs on language pairs in various experimental conditions, and find that LMs, which lack any direct analog to innate maturational stages, do not show CP effects when trained sequentially on L1 and L2. Our results contradict the claim that CP effects are an inevitable result of learning in statistical learners, and they are consistent with an innate mechanism for CP effects. We show that we can reverse-engineer the CP by introducing a regularizer partway through training to simulate a maturational decrease in plasticity. All in all, our results suggest that L1 learning on its own may not be enough to induce a CP, and additional engineering is necessary to make language models more cognitively plausible.

翻译：人类似乎存在语言习得的关键期（CP）：第二语言（L2）习得在幼儿期后变得困难，且在此关键期后（而非之前）停止接触第一语言（L1）通常不会导致L1能力的显著丧失。目前尚不清楚这些关键期效应是由先天决定的大脑成熟所致，还是由经验自然诱导的神经连接稳定化所引发。本研究利用语言模型（LMs）来检验这些现象在多大程度上是人类特有的，还是更广泛语言学习者所共有的。我们通过在多种实验条件下训练语言模型处理语言对来改变其接触语言的年龄，发现缺乏任何与先天成熟阶段直接类似机制的语言模型，在顺序训练L1和L2时并未表现出关键期效应。我们的结果反驳了关键期效应是统计学习者学习过程中必然产物的观点，且与关键期效应的先天机制假说相一致。我们证明可以通过在训练中途引入正则化器来模拟可塑性随成熟度下降的现象，从而逆向构建关键期效应。总体而言，我们的研究结果表明，仅靠L1学习本身可能不足以诱发关键期效应，需要额外的工程化设计才能使语言模型更具认知合理性。