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Abstract

A surprising discovery in recent years is that the structure of the adult human brain changes when a new cognitive or motor skill is
learned. This effect is seen as a change in local gray or white matter density that correlates with behavioral measures. Critically, however,
the cognitive and anatomical mechanisms underlying these learning-related structural brain changes remain unknown. Here, we combined
brain imaging, detailed behavioral analyses, and white matter tractography in English-speaking monolingual adolescents to show
that a critical linguistic prerequisite (namely, knowledge of vocabulary) is proportionately related to relative gray matter density in
bilateral posterior supramarginal gyri. The effect was specific to the number of words learned, regardless of verbal fluency or other
cognitive abilities. The identified region was found to have direct connections to other inferior parietal areas that separately process
either the sounds of words or their meanings, suggesting that the posterior supramarginal gyrus plays a role in linking the basic
components of vocabulary knowledge. Together, these analyses highlight the cognitive and anatomical mechanisms that mediate an
essential language skill.