A mathematical model of line bisection behaviour in neglect.

Subjects with left hemispatial neglect frequently demonstrate an array of abnormal behaviours on line bisection tasks. They misbisect long horizontal lines to the right of true midline. They bisect short lines to the left of true midline. They exaggerate the left-sided length of lines when placing the endpoints for 'invisible' lines, and they underestimate the length of the left side of long lines that are shown to them bisected accurately. No current theory of neglect explains all these features of line bisection behaviour. A mathematical model of line bisection behaviour in neglect is presented that proposes that subjects bisect lines at the point where they perceive the 'salience' of the two line segments created by their bisection mark to be equal. Salience is determined by the brain's attentional systems which map salience amplitude to spatial position following a bell shaped distribution. Right hemisphere strokes simulated by decreasing the 'height' and "breadth' of the right hemisphere salience to position function produced all of the above features of clinical neglect subjects' line bisection behaviour. Neglect may be conceived of as damage to brain systems performing mappings between stimulus characteristics (such as spatial location) and salience.