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.
The extent to which development of the brain language system is modulated by the temporal onset of linguistic experience relative to post-natal brain maturation is unknown. This crucial question cannot be investigated with the hearing population because spoken language is ubiquitous in the environment of newborns. Deafness blocks infants' language experience in a spoken form, and in a signed form when it is absent from the environment. Using anatomically constrained magnetoencephalography, aMEG, we neuroimaged lexico-semantic processing in a deaf adult whose linguistic experience began in young adulthood. Despite using language for 30 years after initially learning it, this individual exhibited limited neural response in the perisylvian language areas to signed words during the 300-400 ms temporal window, suggesting that the brain language system requires linguistic experience during brain growth to achieve functionality. The present case study primarily exhibited neural activations in response to signed words in dorsolateral superior parietal and occipital areas bilaterally, replicating the neural patterns exhibited by two previously case studies who matured without language until early adolescence (Ferjan Ramirez N, Leonard MK, Torres C, Hatrak M, Halgren E, Mayberry RI. 2014). The dorsal pathway appears to assume the task of processing words when the brain matures without experiencing the form-meaning network of a language.
Areas within the left-lateralized neural network for language have been found to be sensitive to syntactic complexity in spoken and written language. Previous research has revealed that these areas are active for sign language as well, but whether these areas are specifically responsive to syntactic complexity in sign language independent of lexical processing has yet to be found. To investigate the question, we used fMRI to neuroimage deaf native signers' comprehension of 180 sign strings in American Sign Language (ASL) with a picture-probe recognition task. The ASL strings were all six signs in length but varied at three levels of syntactic complexity: sign lists, two-word sentences, and complex sentences. Syntactic complexity significantly affected comprehension and memory, both behaviorally and neurally, by facilitating accuracy and response time on the picture-probe recognition task and eliciting a left lateralized activation response pattern in anterior and posterior superior temporal sulcus (aSTS and pSTS). Minimal or absent syntactic structure reduced picture-probe recognition and elicited activation in bilateral pSTS and occipital-temporal cortex. These results provide evidence from a sign language, ASL, that the combinatorial processing of anterior STS and pSTS is supramodal in nature. The results further suggest that the neurolinguistic processing of ASL is characterized by overlapping and separable neural systems for syntactic and lexical processing.
Due to the ubiquitous nature of language in the environment of infants, how it affects the anatomical structure of the brain language system over the lifespan is not well understood. In this study, we investigated the effects of early language experience on the adult brain by examining anatomical features of individuals born deaf with typical or restricted language experience in early childhood. Twenty-two deaf adults whose primary language was American Sign Language and were first immersed in it at ages ranging from birth to 14 y participated. The control group was 21 hearing non-signers. We acquired T1-weighted magnetic resonance images and used FreeSurfer [B. Fischl, Neuroimage 62 , 774–781(2012)] to reconstruct the brain surface. Using an a priori regions of interest (ROI) approach, we identified 17 language and 19 somatomotor ROIs in each hemisphere from the Human Connectome Project parcellation map [M. F. Glasser et al. , Nature 536 , 171–178 (2016)]. Restricted language experience in early childhood was associated with negative changes in adjusted grey matter volume and/or cortical thickness in bilateral fronto-temporal regions. No evidence of anatomical differences was observed in any of these regions when deaf signers with infant sign language experience were compared with hearing speakers with infant spoken language experience, showing that the effects of early language experience on the brain language system are supramodal.
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