数の音韻，アラビア数字形式間で両方向性の変換障害を呈した左頭頂後頭葉出血の1例

Two-way Deficit between Phonological and Arabic Representation of Numbers after Left Parieto-occipital Hemorrhage

はじめに

　数の体系は(1)算術的事実，(2)計算手続き，(3)規則^1）によって構成されている。また，言語との間で(4)記号形式の変換^2）が必要となる。(1)算術的事実には加減算の表（例4＋3＝7，3－2＝1）などが含まれる。(2)計算手続きには繰り上がりや繰り下がりの手順などが含まれる。算術的事実と計算手続きを合わせて演算という。(3)規則には数の大小，交換則，分配則などが含まれる。(4)記号形式変換には，例えば“682”を「ろっぴゃくはちじゅうに」と読むなど言語と数の関係づけ，すなわち数の読み書きが含まれる。

　近年これら4つの要素が脳損傷によって別々に障害されることが明らかになってきた。しかし，比較的独立して記号形式変換の障害が生じた症例の検討は少ない^3－9）。われわれは，左頭頂後頭葉出血後に，数の音韻とアラビア数字表記間で両方向性の記号形式変換障害を呈した1例を経験したので報告する。

Abstract

　A 52-year-old right-handed man developed a deficit of two-way transcoding between phonological and Arabic representations of numbers after cerebral hemorrhage in the left occipital lobe, intraparietal sulcus, and the surrounding of inferior parietal lobule.　He showed right hemianopia, mild Wernicke's aphasia, agraphia, and phonological dyslexia.　Calculation, comparison between numbers and comprehension of arithmetic signs were almost preserved.　Retrieval of the mnemonic rhymes for the multiplication table (kuku) was moderately compromised.　He showed good performance in the following tasks: selecting coins corresponding to the number orally designated by the experimenter, reporting orally the amount of arranged coins, selecting coins according to written amount, indicating the amount of arranged coins by writing numbers, and choosing the amount of arranged coins from several alternatives of written numbers.　Thus, encoding the phonological and visual representations of numbers from the meanings as well as decoding from these 2 representations into their meanings were preserved.　However, he showed grave difficulty in dictation of numbers, in choosing the verbally indicated numbers from among a set of written numbers, and in reading aloud a number.　Thus, two-way impairment of association between phonological and visual representations of numbers was confirmed.　Regarding other categories of nouns, marked deterioration was detected only in dictation and writing the names of indicated objects.　We taught him a strategy of imaging the coins when he transcoded from Arabic representation of numbers to phonological representation and vice versa.　This strategy improved his performance to some extent.　These results suggest that there is a direct, bi-directional transcoding route between phonological and visual representations of numbers, which is not mediated by semantic representation.　This route can be impaired by a lesion located in the left parieto-occipital cortex.

(Received: May 24,2010,Accepted: November 11,2010)