Sm. King et al., THE ROLE OF LIGHT SCATTER IN THE RESIDUAL VISUAL SENSITIVITY OF PATIENTS WITH COMPLETE CEREBRAL HEMISPHERECTOMY, Visual neuroscience, 13(1), 1996, pp. 1-13
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.