Crossing the midline: reducing attentional deficits via interhemispheric interactions.

Patients with unilateral neglect and extinction show a profound lack of awareness of stimuli presented contralateral to their lesion. However, many processes of perception are intact and contralesional stimuli seem to reach a high level of representation, perceptual and semantic. Some of these processes can work to decrease the magnitude of the attentional deficit. Here, we examine two of these intact processes, feature detection and perceptual grouping. First, we demonstrate that feature detection occurs in parallel in the contralesional visual fields of neglect and extinction patients. Second, we attempt to dissociate the influence of perceptual contours across the vertical meridian from the presence of an object or higher-level perceptual unit (or group) that may be created by these contours. We find that connections across the midline affect attentional deficits independently of the objects they may create. This suggests that several effects of grouping on neglect and extinction may be mediated by long-range cortical interactions that arise from connections across the vertical meridian.