THE ROLE OF LIGHT SCATTER IN THE RESIDUAL VISUAL SENSITIVITY OF PATIENTS WITH COMPLETE CEREBRAL HEMISPHERECTOMY

Various residual visual capacities have been reported for the phenomen ally blind field of hemispherectomized patients, providing evidence fo r the relative roles of cortical and subcortical pathways in vision. W e attempted to characterize these functions by examining the ability o f five patients to detect, localize, and discriminate high-contrast fl ashed, flickering and moving targets. Dependent measures were verbal, manual, and oculomotor responses. As a control for light scatter, inte nsity 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 u nilaterally destriate patient was tested on the same tasks. In photopi c conditions, none of the hemispherectomized patients could respond to visual cues in their impaired fields, whereas the destriate patient c ould detect, discriminate, and point to targets, and appreciate the ap parent motion of stimuli across his midline. Under reduced lighting, t he threshold luminance required by hemispherectomized patients to dete ct stimuli presented monocularly was similar to that required for thei r detection when all visual information was occluded in the blind fiel d, and only available to the visual system indirectly via light scatte r. In contrast, the destriate patient's monocular threshold in his bli nd field was substantially lower than that for stimuli directly occlud ed in the blind field. As we found no range of stimuli which the hemis pherectomized patients could detect or discriminate that was not also associated with discriminable scattered light, we conclude that the su bcortical pathways which survive hemispherectomy cannot mediate volunt ary behavioural responses to visual information in the hemianopic fiel d.