Center for Brain Plasticity and Recovery, Georgetown University Medical Center and MedStar National Rehabilitation Hospital, Washington, DC 20057.
Center for Neuroscience and Behavioral Health, Children's National Hospital, Washington, DC 20010.
Proc Natl Acad Sci U S A. 2020 Sep 22;117(38):23477-23483. doi: 10.1073/pnas.1905590117. Epub 2020 Sep 8.
We have long known that language is lateralized to the left hemisphere (LH) in most neurologically healthy adults. In contrast, findings on lateralization of function during development are more complex. As in adults, anatomical, electrophysiological, and neuroimaging studies in infants and children indicate LH lateralization for language. However, in very young children, lesions to either hemisphere are equally likely to result in language deficits, suggesting that language is distributed symmetrically early in life. We address this apparent contradiction by examining patterns of functional MRI (fMRI) language activation in children (ages 4 through 13) and adults (ages 18 through 29). In contrast to previous studies, we focus not on lateralization per se but rather on patterns of left-hemisphere (LH) and right-hemisphere (RH) activation across individual participants over age. Our analyses show significant activation not only in the LH language network but also in their RH homologs in all of the youngest children (ages 4 through 6). The proportion of participants showing significant RH activation decreases over age, with over 60% of adults lacking any significant RH activation. A whole-brain correlation analysis revealed an age-related decrease in language activation only in the RH homolog of Broca's area. This correlation was independent of task difficulty. We conclude that, while language is left-lateralized throughout life, the RH contribution to language processing is also strong early in life and decreases through childhood. Importantly, this early RH language activation may represent a developmental mechanism for recovery following early LH injury.
我们早就知道，在大多数神经健康的成年人中，语言是偏向于左半球（LH）的。相比之下，在发展过程中对功能偏侧化的研究结果则更为复杂。与成年人一样，对婴儿和儿童的解剖学、电生理学和神经影像学研究表明，语言功能偏向于左半球。然而，在非常年幼的儿童中，左、右半球的病变都同样可能导致语言缺陷，这表明语言在生命早期是对称分布的。我们通过检查儿童（4 至 13 岁）和成人（18 至 29 岁）的功能磁共振成像（fMRI）语言激活模式来解决这个看似矛盾的问题。与之前的研究不同，我们关注的不是偏侧性本身，而是个体参与者在年龄上的左半球（LH）和右半球（RH）激活模式。我们的分析表明，不仅在所有最年幼的儿童（4 至 6 岁）中都存在明显的 LH 语言网络激活，而且在其 RH 同源物中也存在明显的激活。随着年龄的增长，表现出明显 RH 激活的参与者比例下降，超过 60%的成年人没有任何明显的 RH 激活。全脑相关性分析显示，只有 Broca 区 RH 同源物的语言激活随年龄相关而下降。这种相关性与任务难度无关。我们的结论是，尽管语言在整个生命周期中都是偏向于左半球的，但 RH 对语言处理的贡献在生命早期也很强，并随着儿童期的发展而下降。重要的是，这种早期的 RH 语言激活可能代表了早期 LH 损伤后恢复的一种发育机制。
