Modulation of hemispatial neglect by directional and numerical cues in the line bisection task

We investigated the effects of arrows, eye gaze, and digits presented as irrelevant flankers in a line bisection task that was administered to 17 right brain damaged patients with or without left neglect. The rightward bias of neglect patients was selectively modulated by the direction of eye gaze and by the magnitude of two identical digits. The bisection error was shifted contralesionally by leftward-gazing eyes and "small" digits, whereas it was shifted ipsilesionally by rightward-gazing eyes and "large" digits. Therefore, the performance of neglect patients was influenced by task-irrelevant cues whose directional meaning was either explicitly represented (eye gaze) or related to the activation of a spatially oriented mental representation (digits). Regression analyses of the overall performance revealed that size of the rightward bias and error variability were predicted by neglect assessment scores across the entire sample of right brain damaged patients. The increased variability in line bisection performance is consistent with the "indifference zone" theory and it appears to be a subtle but stable marker of neglect.