A case of 'sign blindness' following left occipital damage in a deaf signer

Functional and structural brain asymmetries in sign language processing

The capacity for language constitutes a cornerstone of human cognition and distinguishes our species from other animals. Research in the cognitive sciences has demonstrated that this capacity is not bound to speech but can also be externalized in the form of sign language. Sign languages are the naturally occurring languages of the deaf and rely on movements and configurations of hands, arms, face, and torso in space. This chapter reviews the functional and structural organization of the neural substrates of sign language, as identified by neuroimaging research over the past decades. Most aspects of sign language processing in adult deaf signers markedly mirror the well-known, functional left-lateralization of spoken and written language. However, both hemispheres exhibit a certain equipotentiality for processing linguistic information and the right hemisphere seems to specifically support processing of some constructions unique to the signed modality. Crucially, the so-called "core language network" in the left hemisphere constitutes a functional and structural asymmetry in typically developed deaf and hearing populations alike: This network is (i) pivotal for processing complex syntax independent of the modality of language use, (ii) matures in accordance with a genetically determined biologic matrix, and (iii) may have constituted an evolutionary prerequisite for the emergence of the human capacity for language.

Linguistic network in early deaf individuals: A neuroimaging meta-analysis

This meta-analysis summarizes evidence from 44 neuroimaging experiments and characterizes the general linguistic network in early deaf individuals. Meta-analytic comparisons with hearing individuals found that a specific set of regions (in particular the left inferior frontal gyrus and posterior middle temporal gyrus) participates in supramodal language processing. In addition to previously described modality-specific differences, the present study showed that the left calcarine gyrus and the right caudate were additionally recruited in deaf compared with hearing individuals. In addition, this study showed that the bilateral posterior superior temporal gyrus is shaped by cross-modal plasticity, whereas the left frontotemporal areas are shaped by early language experience. Although an overall left-lateralized pattern for language processing was observed in the early deaf individuals, regional lateralization was altered in the inferior frontal gyrus and anterior temporal lobe. These findings indicate that the core language network functions in a modality-independent manner, and provide a foundation for determining the contributions of sensory and linguistic experiences in shaping the neural bases of language processing.

Sign language aphasia

Signed languages are naturally occurring, fully formed linguistic systems that rely on the movement of the hands, arms, torso, and face within a sign space for production, and are perceived predominantly using visual perception. Despite stark differences in modality and linguistic structure, functional neural organization is strikingly similar to spoken language. Generally speaking, left frontal areas support sign production, and regions in the auditory cortex underlie sign comprehension-despite signers not relying on audition to process language. Given this, should a deaf or hearing signer suffer damage to the left cerebral hemisphere, language is vulnerable to impairment. Multiple cases of sign language aphasia have been documented following left hemisphere injury, and the general pattern of linguistic deficits mirrors those observed in spoken language. The right hemisphere likely plays a role in non-linguistic but critical visuospatial functions of sign language; therefore, individuals who are spared from damage to the left hemisphere but suffer injury to the right are at risk for a different set of communication deficits. In this chapter, we review the neurobiology of sign language and patterns of language deficits that follow brain injury in the deaf signing population.

Review article: Structural brain alterations in prelingually deaf

Functional studies show that our brain has a remarkable ability to reorganize itself in the absence of one or more sensory modalities. In this review, we gathered all the available articles investigating structural alterations in congenitally deaf subjects. Some concentrated only on specific regions of interest (e.g., auditory areas), while others examined the whole brain. The majority of structural alterations were observed in the auditory white matter and were more pronounced in the right hemisphere. A decreased white matter volume or fractional anisotropy in the auditory areas were the most common findings in congenitally deaf subjects. Only a few studies observed alterations in the auditory grey matter. Preservation of the grey matter might be due to the cross-modal plasticity as well as due to the lack of sensitivity of methods used for microstructural alterations of grey matter. Structural alterations were also observed in the frontal, visual, and other cerebral regions as well as in the cerebellum. The observed structural brain alterations in the deaf can probably be attributed mainly to the cross-modal plasticity in the absence of sound input and use of sign instead of spoken language.

Structural brain changes linked to delayed first language acquisition in congenitally deaf individuals

Early language experience is essential for the development of a high level of linguistic proficiency in adulthood and in a recent functional Magnetic Resonance Imaging (fMRI) experiment, we showed that a delayed acquisition of a first language results in changes in the functional organization of the adult brain (Mayberry et al., 2011). The present study extends the question to explore if delayed acquisition of a first language also modulates the structural development of the brain. To this end, we carried out anatomical MRI in the same group of congenitally deaf individuals who varied in the age of acquisition of a first language, American Sign Language -ASL (Mayberry et al., 2011) and used a neuroanatomical technique, Voxel-Based Morphometry (VBM), to explore changes in gray and white matter concentrations across the brain related to the age of first language acquisition. The results show that delayed acquisition of a first language is associated with changes in tissue concentration in the occipital cortex close to the area that has been found to show functional recruitment during language processing in these deaf individuals with a late age of acquisition. These findings suggest that a lack of early language experience affects not only the functional but also the anatomical organization of the brain.

Age of acquisition effects on the functional organization of language in the adult brain

Using functional magnetic resonance imaging (fMRI), we neuroimaged deaf adults as they performed two linguistic tasks with sentences in American Sign Language, grammatical judgment and phonemic-hand judgment. Participants' age-onset of sign language acquisition ranged from birth to 14 years; length of sign language experience was substantial and did not vary in relation to age of acquisition. For both tasks, a more left lateralized pattern of activation was observed, with activity for grammatical judgment being more anterior than that observed for phonemic-hand judgment, which was more posterior by comparison. Age of acquisition was linearly and negatively related to activation levels in anterior language regions and positively related to activation levels in posterior visual regions for both tasks.

Neural reorganization following sensory loss: the opportunity of change

There is growing evidence that sensory deprivation is associated with crossmodal neuroplastic changes in the brain. After visual or auditory deprivation, brain areas that are normally associated with the lost sense are recruited by spared sensory modalities. These changes underlie adaptive and compensatory behaviours in blind and deaf individuals. Although there are differences between these populations owing to the nature of the deprived sensory modality, there seem to be common principles regarding how the brain copes with sensory loss and the factors that influence neuroplastic changes. Here, we discuss crossmodal neuroplasticity with regards to behavioural adaptation after sensory deprivation and highlight the possibility of maladaptive consequences within the context of rehabilitation.

Role of the left hemisphere in sign language comprehension

We investigated the relative role of the left versus right hemisphere in the comprehension of American Sign Language (ASL). Nineteen lifelong signers with unilateral brain lesions [11 left hemisphere damaged (LHD) and 8 right hemisphere damaged (RHD)] performed three tasks, an isolated single-sign comprehension task, a sentence-level comprehension task involving simple one-step commands, and a sentence-level comprehension task involving more complex multiclause/multistep commands. Eighteen of the participants were deaf, one RHD subject was hearing and bilingual (ASL and English). Performance was examined in relation to two factors: whether the lesion was in the right or left hemisphere and whether the temporal lobe was involved. The LHD group performed significantly worse than the RHD group on all three tasks, confirming left hemisphere dominance for sign language comprehension. The group with left temporal lobe involvement was significantly impaired on all tasks, whereas each of the other three groups performed at better than 95% correct on the single sign and simple sentence comprehension tasks, with performance falling off only on the complex sentence comprehension items. A comparison with previously published data suggests that the degree of difficulty exhibited by the deaf RHD group on the complex sentences is comparable to that observed in hearing RHD subjects. Based on these findings we hypothesize (i) that deaf and hearing individuals have a similar degree of lateralization of language comprehension processes and (ii) that language comprehension depends primarily on the integrity of the left temporal lobe.

Discourse deficits following right hemisphere damage in deaf signers

Previous findings have demonstrated that hemispheric organization in deaf users of American Sign Language (ASL) parallels that of the hearing population, with the left hemisphere showing dominance for grammatical linguistic functions and the right hemisphere showing specialization for non-linguistic spatial functions. The present study addresses two further questions: first, do extra-grammatical discourse functions in deaf signers show the same right-hemisphere dominance observed for discourse functions in hearing subjects; and second, do discourse functions in ASL that employ spatial relations depend upon more general intact spatial cognitive abilities? We report findings from two right-hemisphere damaged deaf signers, both of whom show disruption of discourse functions in absence of any disruption of grammatical functions. The exact nature of the disruption differs for the two subjects, however. Subject AR shows difficulty in maintaining topical coherence, while SJ shows difficulty in employing spatial discourse devices. Further, the two subjects are equally impaired on non-linguistic spatial tasks, indicating that spared spatial discourse functions can occur even when more general spatial cognition is disrupted. We conclude that, as in the hearing population, discourse functions involve the right hemisphere; that distinct discourse functions can be dissociated from one another in ASL; and that brain organization for linguistic spatial devices is driven by its functional role in language processing, rather than by its surface, spatial characteristics.

Hemispheric organization of local- and global-level visuospatial processes in deaf signers and its relation to sign language aphasia

Previous work has shown that deficits in the production and perception of signed language are linked to left hemisphere damage but not right hemisphere damage in deaf lifelong signers, whereas severe deficits in nonlinguistic visuospatial abilities are more frequent following right hemisphere damage than left hemisphere damage in this population. In the present study we investigated the extent to which sign language deficits in deaf individuals can be dissociated from more subtle visuospatial deficits commonly associated with left hemisphere damage in the hearing/speaking population. A group of left- or right-lesioned deaf signers were asked to reproduce (1) two line drawings (a house and an elephant) and (2) four hierarchical figures. Drawings were scored separately for the presence of local vs global features. Consistent with data from hearing patients, left hemisphere-damaged deaf subjects were significantly better at reproducing global-level features, whereas right hemisphere-damaged deaf subjects were significantly better at reproducing local-level features. This effect held for both types of stimuli. Local-level performance in the LHD group did not correlate with performance on sign language tasks, suggesting that language deficits in LHD deaf signers are in fact linguistic specific.