Psychophysical properties of line bisection and body midline perception inunilateral neglect

Past research associated unilateral neglect with a systematic ipsilesional shift of the perceived position of the body midline; however, this was not confirmed by recent experiments. We used the constant stimuli method to con trol for potential artifacts intrinsic to the techniques used in previous s tudies. Body midline perception was measured in the visual and propriocepti ve modalities in ten patients with left unilateral neglect, ten control pat ients and ten normal subjects and compared with a visual line bisection tas k, also using the constant stimuli method. Neglect patients showed a significant rightward bias in the line bisection task, but no consistent directional bias either in the proprioceptive or in the visual body midline task. These results clearly counter the associatio n between neglect and an ipsilesional shift of the body midline. However, in the body midline tasks neglect patients made more errors in jud gement on both sides of their subjective midline, both with respect to the control groups and with respect to the line bisection task. This may imply that a specific impairment of body-centered representations is indeed prese nt and manifests as a non directional increase in response variability, rat her than as a systematic shift. It is suggested that body- and object-related tasks (such as line bisection ) may be processed by independent cognitive computations. This interpretati on is discussed with reference to a recent neuroimaging study investigating the same kinds of tasks.