The Relationship Of Visual Expertise And Human Brain's Neural Response Pattern During Chinese Character Processing

In modern civilization, reading is an essential way to obtain information. As people acquire better reading skills, visual expertise is accumulated and a word-processing neural network is simultaneously formed in the human brain. A series of behavioral and electrophysiological studies have proven that visual expertise plays an important role in word processing. Additionally, neuroimaging research has also identified the word-processing neural system, which includes distributed cerebral cortices responding to various linguistic attributes of words. Up until now, the cooperating mechanism of visual expertise and the word-processing neural system during word processing has not been fully illustrated. It is essential to explore the relationship of visual expertise and the operation of the word-processing neural system in order to further study the cognitive mechanism of word processing. It is necessary to investigate the visual expertise's influences on the activation pattern of the word-processing neural system. On the other hand, it is also important to explore the intrinsic functional organization of the word-processing neural system.The Chinese character is a type of logographic writing word, which is composed of a number of strokes and requires fine-gained visual-spatial processing. Thus, it is appropriate material for the investigation of visual expertise in word processing. In the meantime, the development of functional Magnetic Resonance Imaging (fMRI) technology facilitates the investigations on the neural basis of various human cognitive functions. In this thesis, two fMRI experiments which employ Chinese characters as stimuli are performed to study the cooperating mechanism of visual expertise and the word-processing neural system. The primary work and contributions in this thesis are listed as follows:First, the influence of inversion on Chinese characters'visual processing is studied. The inversion effect is direct evidence that object processing involves visual expertise. Although the visual physical properties of the inverted stimuli are the same as those of their upright version, the behavioral performance and the neural response pattern are affected significantly by inversion. Furthermore, this influence is disproportionally larger on objects processed with visual expertise than on those processed with little visual expertise. Previous studies have observed the inversion effect in the processing of face, body and scene pictures. With respect to a Chinese character, it shares similarities with the face in several dimensions, especially the comparable visual expertise. Thus, it is interesting to know whether or not the response of the character-processing neural system will be influenced by the presented orientation. In order to answer this question, the thesis used Chinese characters, faces, houses and their inverted versions as stimuli to compare the inversion's influences on their processing. The region of interest analysis demonstrated inversion led to neural response increases for the Chinese character in the left fusiform character-preferential area (Visual Word Form Area, VWFA), bilateral fusiform object-preferential area and bilateral occipital object-preferential area. The ANOVA test further showed such inversion-caused changes in the response pattern of character processing were highly similar to those of face processing, but were different from those of house processing. Whole brain analysis revealed that the upright characters recruited several language regions responding to phonology and semantic processing; however, the inverted characters activated extensive regions related to visual information processing. These findings indicated a shift from the character-preferential processing route to the generic object processing steam within the visual cortex when the characters were inverted, and this suggested a significant modulation of visual expertise on the activated pattern of the character-processing neural system.Second, the intrinsic functional organization of the character-processing cerebral cortices was studied by using the Resting State functional connectivity MRI (rs-fcMRI) technology. The neural inversion effect of Chinese character processing suggests that visual expertise may modulate the activated pattern of the character-processing neural system. However, it is still unclear whether or not the character-processing brain regions are intrinsically organized without any explicit processing tasks. In order to explore the intrinsic functional connectivity between character-processing brain regions during the resting state, the visual word form area, which is suggested to play a key role in the word processing, was selected to be the seeding region used to calculate the functional connectivity between all the other whole brain voxels and itself. The rs-fcMRI analysis found low-frequency fluctuations between the VWFA and a number of cortical regions. They included the left angular gyrus, left supramarginal gyrus, bilateral pars opercularis, and left pars triangularis of the inferior frontal gyrus, which have been implicated in phonological and semantic processing. Additionally, the activations were also observed in the bilateral superior parietal lobule and dorsal lateral prefrontal cortex (DLPFC), which have been suggested to provide top-down monitoring on the visual-spatial processing of character. These findings demonstrated the intrinsic functional organization of the character-processing cerebral cortices, and indicated a preparatory brain networks for the highly probable character input.Third, the intrinsic intercommunicating pattern among these brain regions responding to the visual processing of Chinese characters is studied by using of multivariate Granger Causality Model (GCM). Those regions include the left parts of inferior occipital gyrus, middle fusiform gyrus, superior parietal lobule, and middle frontal gyrus. The results demonstrated that there were significantly bidirectional Granger causality connections between the left inferior occipital gyrus and superior parietal lobule, as well as between the left middle fusiform gyrus and superior parietal lobule; the left inferior occipital gyrus Granger-caused the left middle frontal gyrus; the left middle fusiform gyrus was Granger-caused by the left middle frontal gyrus. All of the findings above revealed the intercommunications among the brain regions which responded to the visual processing of Chinese characters without any explicit tasks, and this further indicated an intrinsically organized network during the resting state that likely prepares the brain to anticipate a highly probable word input for immediate and effective processing.In this thesis, we discuss the inversion effect in visual processing of Chinese characters, the intrinsic functional organization of the character-processing brain network during the resting state and the intrinsic intercommunicating pattern among the brain regions responding to the visual processing of Chinese characters. All of these findings provide new evidence for understanding the word-processing mechanism.