Visual neglect in the monkey. Representation and disconnection.

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.