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Title: Neural mechanisms of spatial coding : a multimodal imaging study
Authors: Derbie, Abiot Yenealem
Degree: Ph.D.
Issue Date: 2021
Abstract: Allocentric and egocentric spatial coding has been revealed involving similar processes of the parieto-frontal attention network. Because of the difference in nature and conceptual underpinnings of experimental paradigm used, the neural mechanisms subserving allocentric (aSC) and egocentric spatial coding (eSC) has been controversial. In this thesis, using multimodal neuroimaging methods, series of studies aimed at enhancing and integrating the model of spatial representations in human, were carried out. The aim of the first study was to give a synopsis of converging evidence about the present state of these two types of spatial coding. By using activation likelihood estimation (ALE) (N = 28; Subjects = 447, voxel height p < 0.001, cluster p < 0.05 FWE-corrected)), both common and differentiated clusters of convergence for eSC and aSC were revealed. The common clusters were the right precuneus and the right superior frontal gyrus indicating attention selection/maintenance and response mapping are required in both types. The differences were the clusters in the superior occipital gyrus for aSC and in the middle occipital gyrus for eSC. They suggested visualizing and maintaining spatial relationships of objects in space were prevailed in aSC. Task-specific designs, i.e. spatial judgment and virtual environment were found to bias the convergent results of aSC but not eSC. The findings enable better understanding of the construct of spatial coding as well as the design of experimental tasks for assessing their neural underpinnings. The second study was aimed to use fine-grained cue-to-target paradigm, required to allocate top-down attention control, for characterizing the neural processes associated with aSC and eSC. Twenty-two participants completed a custom visuospatial task, and changes in the concentration of oxygenated haemoglobin (O2-Hb) were recorded using functional near-infrared spectroscopy (fNIRS). The least absolute shrinkage and selection operator-regularized principal component (LASSO-RPC) algorithm was used to identify cortical sites that predicted the aSC and eSC conditions' reaction times. Significant changes in the O2-Hb concentration in the right superior frontal gyrus (SFG) and in the post-central gyrus (PoG) were common in both conditions. In contrast, the O2-Hb concentration changes unique to aSC were in the left precentral gyrus (PG) and intraparietal sulcus (IPS); those changes unique to eSC were in the right posterior inferior parietal lobule (IPL). The fNIRS results suggest that top-down attention, encoding visual representation, and response-mapping processes were common to both spatial coding types. When compared with egocentric coding, allocentric spatial coding demands more orienting attention and updating of spatial information. A future study will use other visuospatial tasks to further inform the task-specificity in spatial coding processes.
The aim of the third study was, using diffusion tensor imaging (DTI) and functional magnetic resonance imaging (fMRI), to investigate how age-related changes in white-mater integrity, as indexed by the fractional anisotropy (FA), could be accounted for age-related changes of neural processes associated to aSC and eSC. In this study, older (n = 24) and younger (n = 27) participants completed the DTI and fMRI scans during which they engaged in a cue-to-target task to elicit aSC or eSC processes. To define white matter ROI's, the FA seed regions were correlated with the task-related blood-oxygen-level dependent (BOLD) signal changes. The relationships between the white matter and functional ROI's were tested by with hierarchical multiple regression. The significant structural ROIs were then used to construct functional connectivity models using generalized psychophysiological interaction analyses (gPPI). The results revealed that white-matter tract of the posterior corona radiata (PCR) facilitated aSCĀ­and eSC-modulated connectivity of the frontal eye fields (FEF) with other neural substrates in the parieto-occipital circuits. White-matter tract of the superior longitudinal fasciculus (SLF) exerted positive influences on the aSC-modulated connectivity of the FEF with neural substrates in the parieto-frontal circuits (involving precuneus, anterior supramarginal gyrus, and somatosensory association cortex) and with the neural substrates in the dorsal and ventral streams (inferior and superior temporal gyrus). It is noteworthy that the connectivity with the middle frontal gyrus (MFG), which plays a gate control role for signals projected from the dorsal attention network (DAN) and the ventral attention network (VAN) during aSC and from the DAN during eSC. The white-matter tract of the superior corona radiata (SCR) facilitated the aSC and eSC-modulated connectivity of the posterior parietal cortex (PPC) with neural substrates in its sub-regions. Disregarding of ageing, effect modulation of spatial coding in FEF requires structure-function interaction and connected to dorsal and ventral attention systems equally. FEF were a critical node for feedforward and feedbackward loops for the resource demanding aSC (involving iLOC, ITG, MFG). The aSC functional efficiency were associated to the function-structure interaction mediated by WM tract in PCR for parieto-occipital gray matter (GM) functions, SLF for near and far neural areas along the fronto-parietal attention network, and SCR for PPC GM functions.
Subjects: Brain -- Imaging
Brain -- Magnetic resonance imaging
Diffusion tensor imaging
Hong Kong Polytechnic University -- Dissertations
Pages: xv, 168 pages : color illustrations
Appears in Collections:Thesis

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