Akustische Allästhesieと聴空間認知障害

Auditory Allesthesia and Cognitive Disorders in Auditory Sphere

I．はじめに

　「Akustische Allästhesieと聴空間認知障害」に含まれる現象について，神経科領域では恐らくhemineglect syndromeにおけるneglectからallesthesiaを経てextinctionという経過，およびCritchley^12）やHeilman^32）らのように，これら3症状がhemineglect syndromeの異なった現われであるとの指摘より，半側空間無視との関係から考え，一方耳鼻科領域ではauditory allesthesiaの現象が一種の音源定位障害であるとの考えより，この論題を方向感障害として考えるかもしれない。ただしallesthcsiaの現象を単に方向感障害として片付けてしまうことには問題があり，その近縁症状であるneglectやextinctionの現象は単なる方向感障害とは異なる。以上の観点から，この論題に含まれる現象を列挙したのがTab. 1である。そしてこれらの現象の責任病巣としては，主として頭頂葉および側頭葉が重視され，一部視床等の関与も指摘されている。

　ところで，外空間の認知障害についての検討は，そのほとんどが視空間を対象としており，これらの聴空間における認知障害に関する検討は甚だ少ない。事実，Frederiks^21）はTab. 2に示す中枢性聴覚認知障害の分類の中で，d）unilateral auditory disorder of attentionおよび f）auditory disorders of localizationいずれの項目でもこれらの現象を挙げており，その位置付けさえもが問題となっている。さらに，これらの現象の中には，その名前すら神経心理学の教科書に記載されていないものもあるので，まず各現象を紹介し，その後でallesthesiaとextinctionの現象を中心に自験例を含め文献的考察を加える。

　Referred in this article are such cognitive disorders in auditory sphere as neglect, allesthesia (alloacusis), extinction, obscuration (graded extinction), displacement, synesthesia, cross modal extinction, transmodal displacement, binaural extinction and impairment of sound localization. Each phenomenon is represented schematically according to Bender's definition (Fig. l) and the reported cases concerned are reviewed.

　The author pointed out several problems in the phenomena of neglect, allesthesia, extinction and impairment of sound localization. Especially, in auditory extinction, the author indicated that there had been no mentions about the presence of auditory extinction on nonverbal stimuli such as finger snappings in reported cases with extinction on verbal stimuli, namely, on dichotic listening test. In our five patients with auditory extinction on dichotic listening tests, there were two cases who did not show extinction on nonverbal stimuli (Table 6). One of them with a tumour lesion in the posterior portion of corpus callosum showed left ear extinction only on verbal stimuli like the splitbrain patients reported by Sparks and other authors. From this evidence in our cases, it is suggested that the mechanism of occurrence in auditory extinction in the patients showing extinction only on verbal stimuli is different from that in patients showing extinction both on nonverbal and verbal stimuli.

　Finally, referring to the notions of Pötzl, Seitelberger, and Heilman, the author proposed one hypothesis concerning the clinical course of hemineglect syndrome (Fig. 5). It is as follows. In the neglect stage, the ipsilateral pathways are physiologically out of operation, and so the stimulus from the opposite side of the lesion is neglected. When the ipsilateral pathways begin to operate, that is, in the allesthesia stage, that stimulus gets through this ipsilateral pathway to the intact hemisphere which is predominant to localize the stimulus from the contralateral side. As a result, the stimulus from the opposite side of the lesion is mislocalized to the same side of the lesion. Next, the intact hemisphere becomes able to processthe stimulus from the same side correctly in localization. In this extinction stage, the stimuli from either side correctly in localization. In this extinction stage, the stimuli from either side are correctly localized respectively in case of unilateral stimulation. However, on bilateral simultaneous stimulation, the stimulus which enters into the intact hemisphere via ipsilateral pathway is suppressed by the stimulus via crossed pathway, and consequently, the stimulus from the opposite side of the lesion is extincted.