Neuroanatomical profiles of deafness in the context of native language experience.

The study of congenitally deaf adult humans provides an opportunity to examine neuroanatomical plasticity resulting from altered sensory experience. However, attributing the source of the brain's structural variance in the deaf is complicated by the fact that deaf individuals also differ in their language experiences (e.g., sign vs spoken), which likely influence brain anatomy independently. Although the majority of deaf individuals in the United States are born to hearing parents and are exposed to English, not American Sign Language (ASL) as their first language, most studies on deafness have been conducted with deaf native users of ASL (deaf signers). This raises the question of whether observations made in deaf signers can be generalized. Using a factorial design, we compared gray (GMV) and white (WMV) matter volume in deaf and hearing native users of ASL, as well as deaf and hearing native users of English. Main effects analysis of sensory experience revealed less GMV in the deaf groups combined (compared with hearing groups combined) in early visual areas and less WMV in a left early auditory region. The interaction of sensory experience and language experience revealed that deaf native users of English had fewer areas of anatomical differences than did deaf native users of ASL (each compared with their hearing counterparts). For deaf users of ASL specifically, WMV differences resided in language areas such as the left superior temporal and inferior frontal regions. Our results demonstrate that cortical plasticity resulting from deafness depends on language experience and that findings from native signers cannot be generalized.

Cortical plasticity for visuospatial processing and object recognition in deaf and hearing signers.

Experience-dependent plasticity in deaf participants has been shown in a variety of studies focused on either the dorsal or ventral aspects of the visual system, but both systems have never been investigated in concert. Using functional magnetic resonance imaging (fMRI), we investigated functional plasticity for spatial processing (a dorsal visual pathway function) and for object processing (a ventral visual pathway function) concurrently, in the context of differing sensory (auditory deprivation) and language (use of a signed language) experience. During scanning, deaf native users of American Sign Language (ASL), hearing native ASL users, and hearing participants without ASL experience attended to either the spatial arrangement of frames containing objects or the identity of the objects themselves. These two tasks revealed the expected dorsal/ventral dichotomy for spatial versus object processing in all groups. In addition, the object identity matching task contained both face and house stimuli, allowing us to examine category-selectivity in the ventral pathway in all three participant groups. When contrasting the groups we found that deaf signers differed from the two hearing groups in dorsal pathway parietal regions involved in spatial cognition, suggesting sensory experience-driven plasticity. Group differences in the object processing system indicated that responses in the face-selective right lateral fusiform gyrus and anterior superior temporal cortex were sensitive to a combination of altered sensory and language experience, whereas responses in the amygdala were more closely tied to sensory experience. By selectively engaging the dorsal and ventral visual pathways within participants in groups with different sensory and language experiences, we have demonstrated that these experiences affect the function of both of these systems, and that certain changes are more closely tied to sensory experience, while others are driven by the combination of sensory and language experience.