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The role of light scatter in the residual visual sensitivity of patients with complete cerebral hemispherectomy

Published online by Cambridge University Press:  02 June 2009

Sheila M. King ,
Paul Azzopardi ,
Alan Cowey ,
John Oxbury  and
Susan Oxbury
Sheila M. King
Affiliation:
Department of Experimental Psychology, University of Oxford, UK
Paul Azzopardi
Affiliation:
Department of Experimental Psychology, University of Oxford, UK
Alan Cowey
Affiliation:
Department of Experimental Psychology, University of Oxford, UK
John Oxbury
Affiliation:
Radcliffe Infirmary, Oxford, UK
Susan Oxbury
Affiliation:
Radcliffe Infirmary, Oxford, UK
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Abstract

Various residual visual capacities have been reported for the phenomenally blind field of hemispherectomized patients, providing evidence for the relative roles of cortical and subcortical pathways in vision. We attempted to characterize these functions by examining the ability of five patients to detect, localize, and discriminate high-contrast flashed, flickering and moving targets. Dependent measures were verbal, manual, and oculomotor responses. As a control for light scatter, intensity thresholds for monocular detection of targets in the hemianopic field were compared with thresholds obtained when using an additional half eyepatch to occlude the blind hemiretina of the tested eye. One unilaterally destriate patient was tested on the same tasks. In photopic conditions, none of the hemispherectomized patients could respond to visual cues in their impaired fields, whereas the destriate patient could detect, discriminate, and point to targets, and appreciate the apparent motion of stimuli across his midline. Under reduced lighting, the threshold luminance required by hemispherectomized patients to detect stimuli presented monocularly was similar to that required for their detection when all visual information was occluded in the blind field, and only available to the visual system indirectly via light scatter. In contrast, the destriate patient's monocular threshold in his blind field was substantially lower than that for stimuli directly occluded in the blind field. As we found no range of stimuli which the hemispherectomized patients could detect or discriminate that was not also associated with discriminable scattered light, we conclude that the subcortical pathways which survive hemispherectomy cannot mediate voluntary behavioural responses to visual information in the hemianopic field.

Keywords

Residual visionSubcortical visual pathwaysCortical visual pathwaysLuminance intensity thresholdsHemianopic field
Type
Research Articles
Information
Visual Neuroscience , Volume 13 , Issue 1 , January 1996 , pp. 1 - 13
Copyright
Copyright © Cambridge University Press 1996

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