Line bisection in a case of visual neglect: Psychophysical studies with implications for theory

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Abstract
We report a study of line bisection in a patient with left hemiplegia and a dense left homonymous hemianopia. The patient, PS, sustained a subarachnoid haemorrhage at the bifurcation of the basilar artery; right fronto-temporal craniotomy was performed with clipping of the neck of the basilar aneurysm. The only neuropsychological sequela of note was florid left neglect. In a series of six experiments, we show that the magnitude of PS's transection displacements is linearly related to the length of the stimulus line, and that rightwards displacements reliably cross over to leftwards displacements at lengths shorter than 25mm. This crossover is due neither to visuo-motor inco-ordination nor to range effects from the presence of longer lines within the experimental set. We also stress the increased standard deviations that characterise PS's performance on the line bisection task, and show that these measures of intra-subject variability are likewise strongly correlated with stimulus length. A new theory of line bisection is elaborated in which both normal and pathological performance is interpreted in terms of two constructs: the size of the Weber fraction (the “just noticeable difference” between two magnitudes) and the attentional direction of approach to a psychophysical “indifference zone”, the extent of which is determined by the Weber fraction.