左侧梭状回内文字的字形加工：中文正常阅读者，中文文盲、非中文母语被试的对比研究

Reading is an important human-specific skill obtained through extensive learning experience and is reliance on the ability to rapidly recognize single words. According to the behavioral studies, the most important stage of reading is the representation of “visual word form”, which is independent on surface visual features of the reading materials. The prelexical visual word form representation is characterized by the abstractive and highly effective and precise processing. Neuroimaging and neuropsychological studies have investigated the neural basis underlying the visual word form processing. On the basis of summary of the existing literature, the current thesis aimed to address three fundamental questions involving neural basis of word recognition. First, is there a dedicated neural network that is specialized for word recognition? Second, is the orthographic information represented in the putative word/character selective region (VWFA)? Third, what is the role of reading experience in the genesis of the VWFA, is experience a main driver to shape VWFA instead of evolutionary selectivity? Nineteen Chinese literate volunteers, 5 Chinese illiterates and 4 native English speakers participated in this study, and performed perceptual tasks during fMRI scanning. To address the first question, we compared the differential responses to three categories of visual objects, i.e., faces, line drawings of objects and Chinese characters, and defined the region of interesting (ROI) for the next experiment. To address the second question, Chinese character orthography was manipulated to reveal possible differential responses to real characters, false characters, radical combinations, and stroke combinations in the regions defined by the first experiment. To examine the role of reading experience in genesis of specialization for character, the responses for unfamiliar Chinese characters in Chinese illiterates and native English speakers were compared with that in the Chinese literates, and tracked the change in cortical activation after a short-term reading training in the illiterates. Data were analyzed in two dimensions. Both BOLD signal amplitude and spatial distribution pattern among multi-voxels were used to systematically investigate the responsiveness of the left fusiform gyrus to Chinese characters. Our results provide strong and clear evidence for the existence of functionally specialized regions in the human ventral occipital-temporal cortex. In the skilled readers a region specialized for written words could be consistently found in the lateral part of the left fusiform gyrus, line drawings in the median part and faces in the middle. Our results further show that spatial distribution analysis, a method that was not commonly used in neuroimaging of reading, appears to be a more effective measurement for category specialization for visual objects processing. Although we failed to provide evidence that VWFA processes orthographic information in terms of signal intensitiy, we do show that response pattern of real characters and radical collections in this area is different from that of false characters and random stroke combinations. Our last set of experiments suggests that the selective bias to reading material is clearly experience dependent. The response to unknown characters in both English speakers/readers and Chinese illiterates is fundamentally different from that of the skilled Chinese readers. The response pattern for unknown characters is more similar to that for line drawings rather as a weak version of character in skilled Chinese readers. Short-term training is not sufficient to produce VWFA bias even when tested with learned characters, rather the learned characters generated a overall upward shift of the activation of the left fusiform region. Formation of a dedicated region specialized for visual word/character might depend on long-term extensive reading experience, or there might be a critical period for reading acquisition.

阅读是人类特有的技能。视觉词形加工是词汇前水平的重要加工阶段，是关于词形信息的抽象表征，具有对字形信息进行整体加工和高效精确的特征。神经成像和神经心理学对“视觉词形”的脑结构基础进行了长期的研究，取得了丰富的实验结果。但关于视觉词形加工的神经基础研究中仍有三个基本问题需要澄清：1）文字字形相对其它视觉物体是否具有特异化的加工神经网络？2）字/词形加工区功能反应特性是什么？反应是否对正字法规则敏感？3）视觉文字视觉经验是否是促进字/词形加工区形成的主要驱动力？本文设计三组实验分别对上述三个问题进行研究。在针对字形类别特异性研究中，采用汉字和面孔、物体线条图的视觉加工对比，考察文字同其它视觉物体的加工选择性差异，同时确定字形加工感兴趣区（ROI）；11名中国大学生参加了本实验。在针对正字法层级敏感性研究中，上述11名被试参与信号强度差异的研究，而另外8名被试参与反应模式差异研究。在确定的ROI中，考察正字法规则不同层级间即真字、假字、部件组合、笔画组合的选择性加工差异。在针对视觉经验在字/词形加工区形成中的作用的研究中，4名英文母语被试及5名中文文盲参与本研究。实验考察了中文文盲和英文母语被试对汉字的脑反应特征，并观察文盲阅读训练后有关脑区对汉字反应的变化。被试统一执行与刺激材料无关的知觉判断任务。分别测量BOLD信号的强度差异、BOLD信号的空间分布及信号反应模式三个维度，对不同刺激类型、不同被试组之间的反应差异进行系统地考察。实验结果显示：第一、左侧梭状回对文字字形信息的确具有选择性反应，但与以往某些研究不同的是，这种选择性并不表现在文字同其它视觉物体的反应强度差异上，但不同类别刺激材料所引起的BOLD信号有不同的空间分布模式。尽管不同类别刺激的反应在梭状回有很大重叠，但文字刺激的反应峰值更偏向于外侧，物体线条图的反应峰值更偏向于内侧，而面孔的反应峰值更偏向于中部偏外侧，介于文字和线条图峰值之间。第二，尽管不同正字法层级的刺激在左侧梭状回外侧引起的BOLD信号强度没有显著差异，但在对不同层级刺激的反应模式考察中发现，左侧梭状回对真字和部件组合较对假字和笔划组合具有显著的敏感性，从而把对不同正字法层级刺激的反应加以区分。第三，在视觉经验对字形加工区影响的研究中发现，缺乏汉字视觉经验的英文母语被试和中文文盲被试对汉字的反应模式与熟练中文阅读者之间存在显著差异。在英文母语被试中，对汉字的反应模式与对英文的反应模式完全不同，而与物体刺激的反应模式相似；在中文文盲被试中，汉字也不能在左侧梭状回引起明显的反应信号变化。对文盲进行短期阅读训练后，文字在左侧梭状回的反应有所提高，但是没有形成同正常被试相似的反应模式。总之，本研究结果提示：左侧梭状回对字形信息具有特异性加工功能，不同类别刺激引起的反应在该脑区显示出不同的空间分布模式；字形特异性加工区对字形信息敏感，不同层级的正字法信息引起不同的皮层反应模式；短期的阅读学习不足以形成特异性的文字加工区，文字的特异性识别过程是长期视觉经验积累的结果，或者文字习得也可能存在关键期。