Hemispheric lateralization in visual word processing : evidence from ERP

• Main Author: Lau, Fun
• Other Authors: Alice Hiu Dan Chan
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: Humanities::Linguistics
• Online Access: https://hdl.handle.net/10356/88881; http://hdl.handle.net/10220/50462
• Institution: Nanyang Technological University

Hemispheric lateralization differences between alphabetic and logographic scripts in the occipitotemporal region are well-documented in the literature. The N170 ERP component, which has been source-localized to the occipitotemporal region, is typically left-lateralized for alphabetic scripts, and bilateral or even right-lateralized for logographic scripts. Such cross-linguistic N170 lateralization differences have been explained in terms of visuoperceptual and phonological processing demands, with the former citing distinctions in spatial configuration and visual complexity, and the latter quoting differences in orthographic depth and phonological mapping. At present, the functional significance of N170 lateralization in visual word processing remains an open question and continues to receive substantial research interest. The two studies in this thesis together investigated if N170 laterality is modulated by visuoperceptual and/or phonological mapping demands. Study One featured an artificial language training paradigm that aimed to uncover how spatial configuration and orthographic transparency shape N170 laterality independently and interactively. Findings demonstrated strong spatial configuration effects on N170 laterality at both pre- and post-training, such that the N170 was left-lateralized for linear configurations and bilateral for square configurations. The effects of orthographic transparency, on the other hand, were not significant. Study Two primarily investigated the coarse (orthographic stimuli vs. visual control) and fine (further distinctions within orthographic stimuli) N170 (commonly referred to as N1 in the print tuning literature) print tuning effects documented in the literature. Coarse N1 print tuning effects were observed, where orthographic stimuli elicited more left-lateralized responses than non-orthographic stimuli. Although fine N1 print tuning effects did not emerge to be significant in the primary analysis, retrospective analysis within each individual hemisphere found greater negativity for words and pseudowords compared to nonwords only in the left hemisphere, suggesting that the left hemisphere is sensitive to orthographic regularity. Effects of spatial configuration were replicated in Study Two with the inclusion of familiar orthographic stimuli and alphanumeric symbols, reflecting the robustness of spatial configuration effects on N170 lateralization. Study Two further revealed that the spatial configuration effects mainly stemmed from a left hemisphere preference for linear over square stimuli. In light of the robust visuoperceptual effects and relatively weak phonological mapping effects observed, the findings in this thesis favor the notion that the N170 lateralization in visual word processing is mainly driven by hemispheric visuoperceptual processing capacities, particularly in the left hemisphere. The left hemisphere exhibited a preference for linear stimuli, as well as a sensitivity to well-formed orthographic strings, which may have arisen from general statistical learning mechanisms. In contrast, the right hemisphere demonstrated heightened activity for symbols. Considering the close relationship between phonology and orthography especially in alphabetic scripts, this thesis puts forth an alternative view that phonological mapping processes exert influence over N170 lateralization indirectly, through the shaping of orthographic regularity.