When right goes left: an investigation of line bisection in a case of visual neglect.

We report a case of severe "left" visual neglect consequent upon lesion of the right temporo-parietal region. The patient's performance on line bisection is studied in three conditions: (a) standard line bisection; (b) evaluating and correcting the patient's previous erroneous bisections; and (c) evaluating and "correcting" correctly bisected lines. In all conditions, the magnitude of absolute displacement increases as a function of stimulus length; in all conditions, the direction of displacement changes from rightwards to leftwards at smaller lengths. We propose a new theory of line bisection in which both normal and pathological performance is interpreted in terms of two constructs: the magnitude of the Weber fraction, and the direction of approach to an "indifference zone" whose size is determined by the Weber fraction.