Individual variation in line bisection: a study of four patients with right hemisphere damage and normal controls

In a previous study of line bisection, [Manning et al., Neuropsychologia 28, 647-655, 1990] we have shown that normal young adults show significant linear regressions of both transection displacement (TD) and standard deviation (SD) on line length. Furthermore, there was considerable individual variation in the magnitude of SDs and the direction and magnitude of TDs. These two sources of variance were uncorrelated. In the current study we provide a constructive replication of these findings on an older population that constitutes a more appropriate control sample for patients with stroke. The performance of four patients with right hemisphere stroke is then interpreted in terms of the constraints observed in normal subjects. All the patients showed qualitatively intact line bisection, but with quantitative impairments of varied severity. When analysed in two pairs, the patients also showed a double-dissociation between the magnitude of their mean transection displacements and the SDs of those means. This result is in conformity with the prediction based upon a lack of correlation between these two measures in normal subjects.