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Clarifying the roles of spatial configuration and orthographic depth in shaping brain laterality in visual word processing
Poster Session D, Saturday, October 26, 10:30 am - 12:00 pm, Great Hall 3 and 4
Alice HD Chan1, Fun Lau2; 1Nanyang Technological University
Introduction: Neuropsychological evidence has consistently shown that logographic and alphabetic scripts are represented differently in the occipitotemporal (OT) regions (bilateral for logographic scripts, left-lateralized for alphabetic scripts). Such distinction has been attributed to differences in spatial configuration and orthographic depth. However , how these factors independently influence neural representation of visual words has not been comprehensively investigated. Adopting an artificial orthography training paradigm, this study aims to clarify the effects of spatial configuration and orthographic depth on the laterality of the N170 ERP component, which is source-localized to the occipitotemporal region. Methods: Sixty-two subjects took part in a five-day artificial language training study, with pre- and post-training EEG sessions. Subjects were randomly assigned to learn 1 of 4 artificial orthographies. The artificial orthographies were composed using the same set of artificial letters, and differed only in spatial configuration and orthographic depth such that there were Linear Shallow (n = 15), Linear Deep (n = 16), Square Shallow (n = 17) and Square Deep (n = 14) conditions. All subjects learnt a total of 18 artificial words over the five days, and their word learning performance was assessed at the end of every session. In the EEG sessions, subjects performed a visual one-back task of the artificial words they were assigned to learn. A series of mixed-design ANOVAs were performed on the N170 amplitudes at the left OT and right OT to examine N170 laterality differences among the 4 artificial orthography conditions at pre-training, post-training, as well as the changes in N170 laterality from pre-training to post-training. Results: Subjects attained high levels of learning accuracy at the end of the training (M = 85.8%), indicating that subjects have successfully acquired the word-to-sound correspondences in their assigned artificial orthography. At pre-training, there was a sole significant interaction effect of hemisphere x spatial configuration [F(1, 58) = 6.203, p = .016], driven by left-lateralized N170 for the linear orthographies and bilateralized N170 for the square orthographies. Importantly, this effect of spatial configuration on N170 laterality is sustained at post-training [F(1, 49) = 4.139, p = .047], and remains to be the sole significant effect observed. Effects of orthographic depth, while not salient when pre- and post-training N170 amplitudes are analyzed in isolation, could potentially be captured by the change in N170 laterality from pre- to post-training, as indicated by a near-significant interaction effect of session x hemisphere x orthographic depth. Conclusion: Findings from this study indicate that the laterality of the N170 ERP component in visual word processing is primarily driven by spatial configuration, in that the processing of linear orthographies is left-lateralized and the processing of square orthographies is bilteralized. This difference in lateralization patterns was observed regardless of whether the subjects were naïve to (pre-training) or familiar with (post-training) the orthographies. While orthographic graph could possibly contribute to change in laterality patterns from pre-training to post-training, ultimately its effects on N170 laterality appear to be overshadowed by the effects of spatial configuration.
Topic Areas: Reading, Language Development/Acquisition