Linguistic competence and related functions in the right cerebral hemisphere of man following commissurotomy an hemisphererectomy

Abstract

A simple new contact lens technique has been developed to permit the presentation of continuously lateralized visual information to one visual half field at a time. Free unilateral scanning of the information and monitoring of performance in the subjects' lap makes it possible to administer a variety of standard perceptual and cognitive tasks to either hemisphere in order to assess hemispheric specialization under natural conditions. Two representative commissurotomy patients have been fitted with the new device and have undergone an extensive series of language and related studies focusing on the right hemisphere. All tests were administered unilaterally to each hemisphere and subsequently in free vision. The results were correlated with data from a group of three hemispherectomy patients in two of whom the right (non dominant) and in one of whom the left (dominant) cerebral hemisphere has been surgically removed for the treatment of post infantile tumor. The case of dominant hemispherectomy is particularly rare and permits the study of language competence and performance in a girl whose language lateralization for speech and hearing was well under way (perhaps completed) when the tumor set in. Results of extensive clinical aphasia tests reveal a distinct hierarchy of language functions from a relatively good auditory comprehension through a more severe speech deficit, to almost complete alexia, agraphia and acalculia. Theoretical aphasiological analysis of the pattern of impairment in language functions here shows that in spite of characteristic nonfluency and anomia in speech, the syndrome is unique and does not correspond to either a Broca's or anomic aphasia. The agraphia and especially the alexia in this patient are more severe than in the separated right hemisphere of the two commissurotomy patients which can read a wide range of individual words and even short sentences. This is in contrast to her superior expressive speech relative to the right hemisphere of the same two commissurotomy patients. In a series of studies comparing the psycholinguistic abilities of the two hemispheres in the two commissurotomy and three hemispherectomy patients it was shown that the right cerebral hemisphere had extensive ability to elicit meaning from pictures and to recognize semantic associations and form concepts. In particular it was able to ignore perceptual for semantic similarity. Lateralized tests of visual closure reveal the conditions under which right hemisphere visual feature extraction mechanisms fail. Previous results on superior right hemisphere competence in completing patterns from fragmented information must now be qualified by the provision that when the gestalt of the visual ground is strong and in competition with the figure, the right hemisphere is unable to complete partial patterns. Neither can it recognize complete embedded figures in the face of distracting gestalt in the ground. Right hemisphere competence in various aspects of auditory language comprehension has been investigated with the aid of an experimental paradigm involving matching an auditory message to one of unilaterally presented alternative line drawings. Lower limit age estimates for right hemisphere comprehension of vocabulary were obtained and it was shown that the right hemisphere can comprehend not only abstract words but also a variety of syntactic structures including verbs, and sentential transformations and to a lesser degree long nonredundant and semantically abstract references. Right hemisphere pattern of syntactic competence has weak correlation with order of acquisition in children and somewhat stronger with aphasics. In contrast, aural vocabulary in the right hemisphere, although consistently inferior to the left, follows the same function of frequency as the left just as do children and aphasics.