Patients with hemispatial neglect fail to cancel lines distributed on one side of a piece of paper. This defect is thought to be induced by a deficit in the neuronal systems that mediate attention, intention, and exploration toward and in the hemispace contralateral to the lesion. However, an alternate (but not mutually exclusive) interpretation is that the patients are either strongly attracted to or impaired in disengaging from the stimuli occupying the other, non-neglected hemispace. We tested ten patients with neglect on two versions of a cancellation test. In the control test they cancelled lines by drawing over them, and in the experimental test they erased lines. There were significantly more omissions in the drawing-over task than in the erasing task. The improved performance when lines were cancelled by removal instead of by marking them suggests that hemispatial neglect is influenced by the presence of stimuli in the non-neglected hemispace.

The defect occurring when a patient fails to report a visual stimulus presented in a visual half-field may be attributed to hemianopia (deafferentation) caused by a geniculocalcarine lesion. However, failure to report a stimulus presented in a visual field may also be caused by hemispatial visual inattention. We report a patient with right thalamic and temporo-occipital lesions who had a left visual field defect when her eyes were directed either straight ahead (midsagittal plane) or toward left hemispace. However, this visual field defect abated when her eyes were directed to right hemispace, suggesting that the patient had hemispatial visual inattention rather than hemianopia.

Animal experiments suggest that neurochemical and anatomic asymmetries exist within the basal ganglia, particularly the globus pallidus, and that these asymmetries correlate with both preferred direction of rotation and limb preference in lever pressing. Human neurotransmitter studies have also revealed a significant asymmetry within the globus pallidus, the left containing greater amounts of dopamine and choline acetyltransferase than the right. Recent anatomic studies of human subcortical regions, limited to the striatum, have failed to show a size asymmetry. We examined 18 normal brains in subjects ranging in age from 26 weeks gestation to 86 years for volumetric asymmetry of the globus pallidus. We found a significant asymmetry, the left side measuring larger in 16 of 18 brains. Considering the role of the basal ganglia in motor control, this data may reflect a structural basis of either axial or limb motor dominance in humans.

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