Dissociation of body-centered and stimulus-centered representations in unilateral neglect

Background: Previous studies on unilateral neglect have shown that there are at least two types of neglect-i.e., body-centered and stimulus-centered neglect. These symptoms suggest that the human brain has at least two different reference frames for processing external space. It is unknown, however, whether these two frames are represented independently in the brain and if so, which areas (or networks) of the brain are responsible for each frame of reference. Objective: To determine whether body-centered neglect and stimulus-centered neglect can be dissociated in patients with brain injury. Methods: New figure discriminative cancellation tasks were designed to simultaneously assess body-centered neglect and stimulus-centered neglect. Two neglect patients with lesions located in different anatomic regions were required to circle every complete figure and to cross out every figure with a missing portion on a sheet of white (29.7 × 42 cm) paper. Results: Patient 1 omitted leftward stimuli on the paper, but the stimuli he found were correctly circled or crossed out. On the other hand, Patient 2 marked stimuli across the paper although he mistakenly circled stimuli that were missing a portion of their left side. Neither patient manifested interaction between the two types of neglect. Conclusion: The results of this study clearly showed double dissociation between the two types of neglect. Furthermore, it not only provides evidence that there are two distinct systems of reference frame for external space in the human brain, but also adds new knowledge indicating that these two systems function independently, at least in part.