Brain, Vol 120, Issue 9 1647-1657, Copyright © 1997 by Oxford University Press
D Gaffan and J Hornak
Macaque monkeys were trained preoperatively in a visual search task.
Neglect after a unilateral lesion was measured as the tendency to make
errors (choices of non-target stimuli) ipsilateral to the lesion. Neglect
was observed after optic tract section combined with forebrain
commissurotomy (n = 6) and after parietal leucotomy, i.e. unilateral
section of the white matter between the fundus of the intraparietal sulcus
and the lateral ventricle (n = 3). Neglect was not observed after optic
tract section alone, or forebrain commissurotomy alone, or posterior
parietal cortical ablation, or posterior parietal ablation plus frontal
eyefield ablation, or after frontal lobectomy plus forebrain commissurotomy
(n = 3 in each of these groups). We propose that the cortex of each
hemisphere maintains a retinotopically organized representation of the
visible halfworld that is contralateral to the animal's current point of
fixation, and that this representation is based not only on analysis of the
current retinal input but also on memory. Visual neglect reflects an
impairment in this representation. According to this proposal, the fact
that neglect is not caused by optic tract section alone is explained by the
ability of the blind hemisphere to build a memory-based representation of
what is contralateral to the current point of fixation, using memories of
visual information which arrived from the ipsilateral visual field in
previous fixations that were directed contralateral to the current fixation
point. However, neglect does follow when unilateral optic tract section is
combined with forebrain commissurotomy, even though the cortex is intact,
because then the blind hemisphere is not only deprived of information
arriving from the contralateral field but it is also cut off from
information arriving from the ipsilateral visual field, and therefore
cannot build a memory-based representation of the currently contralateral
visible world.
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Visual neglect in the monkey. Representation and disconnection
Department of Experimental Psychology, Oxford University, UK.
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