閱讀障礙之巨細胞系統功能異常假說

THE MAGNO DEFICIT HYPOTHESIS OF DYSLEXIA: A CRITICAL REVIEW

近年來關於閱讀障礙（dyslexia）生理成因的探討受到學界相當程度的關注。Livington等人根據視覺誘發電位（visual evoked potential）以及遺體解剖證據，提出閱障者的巨細胞（magno）系統具有結構及功能上的缺陷之假說（Livingstone, Rosen, Orislane, & Galaburda, 1991）。由於這個假說深深影響過去近十年來閱讀障礙研究的方向，本文之目的即在於以批判性的角度回顧：（1）導致巨細胞功能異常假說之提出的相關背景文獻，尤其是過去各種關於閱障者視知覺功能異常之研究；（2）支持以及反對該假說的證據，包括最近在Wernicke語言區、角狀回（angular gyrus）以及主要視覺皮質區（striate cortex）的大腦功能造影所獲得的不同意義的發現；（3）有無形式簡潔的理論框架可以整合大多數表面上互相衝突的證據；（4）這樣的理論框架對於中文閱障研究的啟示。本文提出（a）將巨細胞功能異常與語音失常型（而非圖像失常型）的閱讀障礙聯在一起，可統合兩個不同理論陣營的主要證據；（b）以語音覺識的觀點來解釋語音失常型的閱讀障礙，則可解釋大部分來自不同語系的國家的資料。這兩個假設也預測中文閱障者的書寫以及閱讀困難源自不同的功能障礙，前者可能與英文閱障者相同，源自語音覺識能力的不足，後者的原因則仍然未明。

There has been a growing interest in searching for the malfunctioning brain sites associated with dyslexia. Based on the evidence from both YEP and postmortem anatomical studies, Livingstone and her colleagues proposed that dyslexic readers might have deficits in their magna system (Livingstone, Rosen, Drislane, & Galaburda, 1991). As many studies in recent years were framed around this hypothesis, a critical review of relevant backgrounds and the interpretations of data is in order. Themes covered in this article include: (1) Thinking about the pathology of dyslexia that leads to the magna deficit hypothesis. Researches on the visual aspects of poor readers are stressed. (2) Positive and negative evidence for this hypothesis. Recent brain imaging reports on weak activations of the Wernicke area, angular gyrus, and striate cortex of dyslexics are also discussed. (3) Possible theoretical framework that might reconcile most of the conflicting observations. (4) Conjectures on the language-specific aspects of Chinese dyslexia. It is argued that: (a) magna deficits are only associated with dysphonetic, but not with dyseidetic, dyslexia; (b) poor phonological awareness is the critical determinant of dysphonetic dyslexia. Given these two assumptions, one could account for most discrepancies found in the literature. It is also predicted from this view that writing and reading difficulties in Chinese dyslexics are of different origins. While the former might be due to inadequate phonological awareness, the cause for the latter remains undetermined.