Structural alterations of brain grey and white matter in early deaf adults

Functional and structural brain alterations in the absence of the auditory input have been described, but the observed structural brain changes in the deaf are not uniform. Some of the previous researchers focused only on the auditory areas, while others investigated the whole brain or other selected regions of interest. Majority of studies revealed decreased white matter (WM) volume or altered WM microstructure and preserved grey matter (GM) structure of the auditory areas in the deaf. However, preserved WM and increased or decreased GM volume of the auditory areas in the deaf have also been reported. Several structural alterations in the deaf were found also outside the auditory areas, but these regions differ between the studies. The observed differences between the studies could be due to the use of different single-analysis techniques, or the diverse population sample and its size, or possibly due to the usage of hearing aids by some participating deaf subjects. To overcome the aforementioned limitations four different image-processing techniques were used to investigate changes in the brain morphology of prelingually deaf adults who have never used hearing aids. GM and WM volume of the Heschl's gyrus (HG) were measured using manual volumetry, while whole brain GM volume, thickness and surface area were assessed by voxel-based morphometry (VBM) and surface-based analysis. The microstructural properties of the WM were evaluated by diffusion tensor imaging (DTI). The data were compared between 14 congenitally deaf adults and 14 sex- and age-matched normal hearing controls. Manual volumetry revealed preserved GM volume of the bilateral HG and significantly decreased WM volume of the left HG in the deaf. VBM showed increased cerebellar GM volume in the deaf, while no statistically significant differences were observed in the GM thickness or surface area between the groups. The results of the DTI analysis showed WM microstructural alterations between the groups in the bilateral auditory areas, including the superior temporal gyrus, the HG, the planum temporale and the planum polare, which were more extensive in the right hemisphere. Fractional anisotropy (FA) was significantly reduced in the right and axial diffusivity (AD) in the left auditory areas in the deaf. FA and AD were significantly reduced also in several other brain areas outside the auditory cortex in the deaf. The use of four different methods used in our study, although showing changes that are not directly related, provides additional information and supports the conclusion that in prelingually deaf subjects structural alterations are present both in the auditory areas and elsewhere. Our results support the findings of those studies showing that early deafness results in decreased WM volume and microstructural WM alterations in the auditory areas. As we observed WM microstructural alteration also in several other areas and increased GM volume in the cerebellum in the deaf, we can conclude that early deafness results in widespread structural brain changes. These probably reflect atrophy or degradation as well as compensatory cross-modal reorganisation in the absence of the auditory input and the use of the sign language.