|Title:||How can prior visual experience modulate cross auditory-spatial learning in blind individuals?|
|Advisors:||Chan, Chetwyn (RS)|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Rehabilitation Sciences|
|Pages:||xvii, 198 leaves : color illustrations ; 30 cm|
|Abstract:||Over the last several decades, the topic of "neural plasticity" has become the focus for investigation in neuroscience research. Neural plasticity has been found to be experience-dependent. This thesis is aimed at investigating visual experience modulation on neural plasticity in a blind model of cross auditory-spatial learning. The behavioral marker used was the participants' performances on a sound localization task, which relies on cross auditory-spatial processing. The stimuli used were recorded sounds from the "Bat-ears," which is a navigation device developed for the blind. The "Bat-ears" stimuli contain spatial information with varied locations within a fan-shape space. The cross auditory-spatial processing is enabled by associating the sound properties with the spatial locations, such as pitch is associated with azimuth and intensity is associated with distance. Functional brain imaging was used to capture the BOLD responses, which reflect neural processes and plastic changes in the participants before and after training on the auditory-spatial stimuli. In the first study, a cross-sectional functional imaging experiment was used to investigate visual experience modulation in blind individuals in processing cross auditory-spatial stimuli. The second study used longitudinal functional imaging to explore specific neural changes due to training on cross auditory-spatial processing. The role of visual experience was manipulated by recruiting blind individuals with varied onset ages of blindness, early-onset and late-onset. Study 1 recruited 17 late- (mean age=32.4, mean onset age=20.4) and 15 early-onset blind participants (mean age=28.9). Study 2 recruited 11 late- (mean age=34.1, mean onset age=21.8) and 11 early-onset blind participants (mean age=26.4). In addition to sound localization, the participants were assessed on their visuospatial working memory and general intellectual abilities.In Study 1, the results revealed a common occipito-parieto-frontal network, including the middle occipital gyrus, superior frontal gyrus, precuneus, and precentral gyrus during sound localization for both late- and early-onset blind groups. Between-group dissociations, however, were found in the left superior frontal gyrus (BA 6) and right middle occipital gyrus (BA 17), whose neural activities were significantly correlated with sound localization performance in the late- (P=0.024) and early-onset (P=0.042) blind groups, respectively. Neural activities of the left superior frontal gyrus were also correlated with visuospatial working memory abilities among the late-onset blind participants (r=0.585 and P=0.022 for 2D matrix; r=0.562 and P=0.029 for 3D matrix). These findings support the notion that late- and early-onset blind individuals rely more on the prefrontal and occipital regions for auditory-spatial processing, respectively, which serve visuospatial working memory and cross-modal plasticity.|
In Study 2, the late-onset blind group showed greater improvement than the early-onset blind group in the sound localization performance after training. Three neural correlates: the left precuneus (BA 7) (F=6.089, P=0.023), right precuneus (BA 7) (F=5.151, P=0.034), and left inferior frontal gyrus (BA 9) (F=14.442, P=0.001) demonstrated significant interaction effects between the two groups before and after training. Neural activities in the three neural correlates were increased in the late-onset blind participants, which can be attributed to more involvement of visuo-spatial processing and auditory attention. It is postulated that the late-onset blind individuals could achieve higher level cross-modal learning, which relies on synthesization of auditory-spatial stimuli. In contrast, the early-onset blind individuals demonstrated decreased neural activities in the same three neural correlates, which can be attributed to less involvement of visuo-spatial processing and auditory attention. It is postulated that the early-onset blind participants, because of no visual experience, may rely on rote memory of the sound-location associations. As a consequence, the early-onset blind participants could only achieve lower level cross-modal learning, which relies on association of stimuli or stimulus-response pairs.It seems that prior visual experience in late-onset blind individuals enables visuospatial working memory during cross auditory-spatial processing and higher level cross-modal learning via meaningful encoding and stnthesization of the spatial sounds. Without prior visual experience, however, early-onset blind individuals rely on cross-modal plasticity during cross auditory-spatial processing and possibly rely on rote memory during cross auditory-spatial learning. We demonstrate a higher and lower level cross-modal learning within the blind population due to great versus little visual experience. Visual experience can modulate neuroplasticity at different developmental stages. This modulation is not only limited to a blind model, but also generalizes to other disabled populations. Greater knowledge of visual experience modulation on cross-modal processing and cross-modal learning promises rehabilitation of sensory disabled population through the development of more effective cross-modal training strategies and sensory substitution devices.
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