Abstract
Previous research has identified ventral and dorsal white matter tracts as being crucial for language processing; their maturation correlates with increased language processing capacity. Unknown is whether the growth or maintenance of these language-relevant pathways is shaped by language experience in early life. To investigate the effects of early language deprivation and the sensory-motor modality of language on white matter tracts, we examined the white matter connectivity of language-relevant pathways in congenitally deaf people with or without early access to language. We acquired diffusion tensor imaging (DTI) data from two groups of individuals who experienced language from birth, twelve deaf native signers of American Sign Language, and twelve hearing L2 signers of ASL (native English speakers), and from three, well-studied individual cases who experienced minimal language during childhood. The results indicate that the sensory-motor modality of early language experience does not affect the white matter microstructure between crucial language regions. Both groups with early language experience, deaf and hearing, show leftward laterality in the two language-related tracts. However, all three cases with early language deprivation showed altered white matter microstructure, especially in the left dorsal arcuate fasciculus (AF) pathway.
Highlights
Human language is a highly complex cognitive system that relies on a distributed neural network
We report the results of a diffusion tensor imaging (DTI) study with 12 hearing native speakers of English who were L2 learners of American Sign Language (ASL), 12 deaf native signers of ASL who were L2 learners of English, and 3 deaf individuals who experienced extreme language deprivation throughout childhood and who experienced ASL as their first language in adolescence or early twenties
After controlling for gender (F(1, 174) = 2.596, p = 0.108) and age (F(1, 174) = 2.924, p = 0.089) effects, the results showed a significant difference among fiber tracts (F(3, 174) = 60.770, p < 0.001), a difference between hemispheres (F(1,174) = 14.689, p < 0.001) with lower fractional anisotropy (FA) in the right hemisphere, a trend toward interaction between fiber tract and hemisphere (F(3, 174) = 2.389, p = 0.070), but no difference between the groups (F(1, 22) = 0.094, p = 0.759), and no interactions between group and fiber tract (F(3,174) = 0.261, p = 0.853), group and hemisphere (F(1, 174) = 0.036, p = 0.848), or between group, fiber tract and hemisphere (F(3,174) = 0.173, p = 0.914)
Summary
Human language is a highly complex cognitive system that relies on a distributed neural network. One crucial question regarding the neurobiology of human language is the role of language experience during development. Neural plasticity allows environmental experience and learning to shape postnatal brain development (Huttenlocher, 2002) and is often limited to a critical period (Hensch, 2005). A similar critical period has been suggested for language development (Lenneberg et al, 1967), it remains unclear how experience within a critical time window contributes to language acquisition. The question is difficult to investigate because neural changes at different levels occur simultaneously during the first few years of postnatal development in typically developing children. One approach to the question is to compare populations with different early language experience. Spoken language is ubiquitous for children who hear normally, congenitally
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