Hierarchical Processing of Visual Word Forms
Date Issued
2011
Date
2011
Author(s)
Kao, Chien-Hui
Abstract
The present study is to investigate the processing of Chinese character in the visual system and its neural network. In study 1, we used a dual task paradigm with both detection and discrimination tasks in spatial summation to measure the spatial constraints for visual word form perception. Five types of stimuli (real, pseudo-, non-, Jiagu and scrambled characters) were used in the experiments. The detection thresholds for the same stimulus size and the same eccentricity were the same for all types of stimuli. When the target size was small, the detection threshold of a character decreased with the increase in its size, with a slope of -1/2 on log-log coordinates, up to a critical size at all eccentricities and for all stimulus types. Beyond the critical size, there was little, if any, improvement of visual word form detectability as target size further increased. That is, the detectability is based on local feature analysis regardless of character types. The contrast thresholds for discriminating real and pseudo- or non-characters were higher than for discriminating real characters and Jiagu characters or scrambled characters. The results suggested that the discrimination threshold is mediated by well-learned forms of components in a character. In study 2, we used a 2 (“rotating wedge of characters” vs. “expanding ring of characters”) by 2 (“with a blank background” vs. “with a scrambled background”) design with fMRI to define the visual cortical regions produce unique representations for Chinese characters. The ventral occipital region showed a retinotopic representation for character stimuli as we found the fusiform gyrus is selective for the contralateral visual fields and has a centre-to-periphery map with a preference for characters presented in fovea. This implies that the fusiform gyrus is selectively to the spatial location information of stimuli. In study 3 and 4, we examined the spatial configuration processing in the visual system for a character by comparing the visual performance for upright character with that for their inverted versions with psychophysics and fMRI. We showed a robust inversion effect for real character regardless of eccentricities. Such an inversion effect was absent for non-characters. The inversion effect was also found in radicals or components in a character. These results suggested that a visual analysis of characters and their components involved a spatial configuration processing. Our neuroimaging evidence showed that the left fusiform gyrus analyzes the spatial configuration of components in a character while the right occipitotemporal regions analyze the spatial configuration of strokes in a component. In addition, the right occipitoparietal region analyzes local features of a character. Together, we demonstrated a hierarchical processing of visual word forms in the visual cortex.
Subjects
Chinese character
spatial configuration
inversion effect
visual cortex
psychophysics
fMRI
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