Brain, Vol. 112, No. 6, 1485-1502, 1989
© 1989 Guarantors of Brain
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VISUOSPATIAL PROCESSES OF LINE BISECTION AND THE MECHANISMS UNDERLYING UNILATERAL SPATIAL NEGLECT
Department of Neurology, Faculty of Medicine, Tokyo Medical and Dental University Tokyo, Japan
Correspondence to:
Correspondence to Dr Sumio Ishiai, Department of Rehabilitation, Tokyo Metropolitan Institute for Neurosciences, 2–6 Musashidai, Fuchu City, Tokyo 183, Japan
Patients with unilateral spatial neglect fail to report or respond to stimuli contralateral to the lesion which usually involves the right parietal lobe. When asked to mark the centre of a horizontal line, these patients place the mark to the right of the true midpoint. It has been considered that they neglect the left part of the line and bisect the perceived line segment We investigated the eye-fixation patterns of hemianopic patients with or without unilateral spatial neglect during the bisection of lines, using an eye camera
Hemianopic patients without unilateral spatial neglect saw the whole line, searching to the endpoint on the hemianopic side, and bisected it correctly. In contrast, left hemianopics with unilateral spatial neglect never searched to the left hemianopic side. Once they fixated a certain point on the right part of the line, they persisted with this point and marked the subjective midpoint there. Taking left homonymous hemianopia into account, the subjective midpoint appeared to be marked, not at the centre of the line segment perceived in the seeing right visual field, but at the leftmost point of it However, they could appreciate the deviation of the subjective midpoint in the right visual field when forced to fixate the left endpoint of the line. These findings suggest that the left hemisphere has the ability to estimate the midpoint of the line through the right visual field and that visuospatial disorder in the line bisection test is attributable to the pathological change in the right hemisphere.
The results are interpreted to mean that left hemianopic patients with unilateral spiral neglect see a totalized image of a line extending equally to either side of the point where they are going to mark the subjective midpoint We considered that the right hemisphere completes the line, using the visual input relating to the right part of the line perceived by the left hemisphere.
Received July 19, 1988. Revised January 4, 1989. Accepted February 8, 1989.
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