Research On The Mechanisms Of Brain Cross-modal Plasticity After Early Deafness Using Functional Magnetic Resonance Imaging

Although the auditory cortex has been traditionally considered as a unimodal system involved in processing auditory singals,recent studies have found that the auditory cortex responds to other sensory stimuli,especially in individuals with early auditory deprivation.However,whether and how auditory regions engage into non-auditory cognitive processing remains unclear after early auditory deprivation.It was an example of early deaf(ED)subjects for investigating the neural substrates of the cross-modal processing of the visual working memory(WM)tasks in the deprived auditory cortex at the root of my Ph.D.thesis.First,it is hypothesized that auditory regions may be involved in visual WM processing and activation of auditory regions may contribute to the superior behavioural performance of early deaf subjects.In this study,41 ED subjects and 40 age-and gender-matched hearing controls underwent functional magnetic resonance imaging(fMRI)during a visuo-spatial delayed recognition task that consisted of encoding,maintenance and recognition stages.The early deaf subjects exhibited faster reaction times on the spatial WM task than did the hearing controls.Compared with hearing controls,deaf subjects exhibited increased activation in the superior temporal gyrus(STG)bilaterally during the recognition stage.This increased activation amplitude predicted faster and more accurate working memory performance in deaf subjects.Deaf subjects also had increased activation in the STG bilaterally during the maintenance stage and in the right STG during the encoding stage.These increased activation amplitude also predicted faster reaction times on the spatial WM task in deaf subjects.These findings suggest that cross-modal plasticity occurs in auditory association areas in ED subjects.These areas are involved in visuospatial WM.Furthermore,amplitudes of cross-modal activation during the maintenance stage were positively correlated with the age of onset of hearing aid use and were negatively correlated with the percentage of lifetime hearing aid use in deaf subjects.These findings suggest that earlier and longer hearing aid use may inhibit cross-modal reorganization in ED subjects.Granger causality analysis revealed that,compared to the hearing controls,the deaf subjects had an enhanced net causal flow from the prefrontal cortex to the STG.These findings indicate that a top–down mechanism may better account for the cross-modal activation of auditory regions in ED subjects.Second,to further indicate the changes in the organization of these regions related spontaneous functional network and the cognitive relevance are still unknown following the early deafness.To clarify this issue,the technology of resting-state fMRI was applied on 42 ED subjects and 39 hearing controls.Then,these subjects performed two visual tasks for acquiring their visual spatial and numerical N-back WM information.Compared with hearing subjects,the ED subjects exhibited superior visual WM performance in both spatial and numerical domains as revealed by inverse efficiency scores.Furthermore,ED subjects also exhibited significantly increased functional connectivity(FC)between right STG and bilateral anterior insula,right dorsal anterior cingulated cortex and right dorsolateral prefrontal cortex.Finally,the FC of right STG could predict visual spatial WM performance,even after controlling for numerical WM performance.Our findings suggest that early auditory deprivation can strengthen the spontaneous FC of STG,which may contribute to the cross-modal involvement of this region in visual WM tasks.Third,to explore whether the cross-modal reorganization of the auditory cortex in the ED subjects cause the changes of large-scale functional and anatomical networks in their whole brains.In this study,37 ED subjects and 33 hearing controls were recruited for detecting the topological characteristic changes of functional networks,anatomical networks and couplings between both networks.Compared with hearing controls,ED subjects only exhibited increased functional properties in Local connections but no differences in the Rich-club connections and the Feeder connections.Additionally,ED subjects also showed an enhanced trend towards couplings between the whole brain functional and anatomical network.Furthermore,the whole-brain couplings were positively correlated with the age of onset of hearing aid use and were negatively correlated with the percentage of lifetime hearing aid use in the ED subjects.These findings suggest that normal developments of large-scale brain networks occur in the ED subjects while enhanced Local connections and whole-brain couplings contributes to effective application of residual sensory information in the ED subjects.In summary,it is an example of ED subjects for first exploring neural mechanisms of cross-modal activation in their auditory cortex during visual WM in this research.From the new perspective,these results provide basic evidences and preliminary answers about whether ED promoted the deprived auditory cortex processing visuospatial WM,advanced the changes in the organization of deprived regions related spontaneous functional activities and evoked the changes in the couplings between functional and anatomical networks.Neuroplasticity is a lifelong capability in human brain by learning and training.This article will contribute to further understanding the mechanisms of the disease or trauma induced changes of the brain structure and function for increasing the rehabilitation of brain injury and the development of cognitive neuroscience.