Effects of lateral head inclination on multimodal spatial orientation judgments in neglect: evidence for impaired spatial orientation constancy.

Recent research revealed that patients with spatial hemineglect show deficits in the judgment of the subjective vertical and horizontal. Systematic deviations in the subjective axes have been demonstrated in the visual and tactile modality, indicating a supramodal spatial orientation deficit. Further, the magnitude of the bias was shown to be modulated by head- and body-position. The present study investigated the effect of passive lateral head inclination on the subjective visual and tactile vertical and horizontal in neglect patients, control patients with left- or right-sided brain damage without neglect and healthy controls. Subjects performed visual- and tactile-spatial judgments of axis orientations in an upright head orientation and with lateral head inclination 25 degrees in clockwise (CW) or counterclockwise (CCW) direction. Neglect patients displayed a marked variability as well as a systematic tilt in their spatial judgments. In line with a multisensory spatial orientation deficit their subjective vertical and horizontal was tilted CCW in the visual and in the tactile modality, while such a tilt was not evident in any other subject group. Furthermore, lateral head inclination had a differential effect in neglect patients, but not in control subjects. Neglect patients' judgments were modulated in the direction of the head tilt ('A-effect'). That is, a CCW inclination further increased the CCW spatial bias whereas a CW inclination decreased the spatial bias and thus led to approximately normal performance. The increased A-effect might be caused by a pathologically strong attraction of the subjective vertical by an idiotropic vector relying on the actual head orientation, as a consequence of impaired processing of gravitational information in neglect patients.