Systematic biases in the tactile perception of the subjective vertical in patients with unilateral neglect and the influence of upright vs. supine posture

Patients with right hemisphere lesions often show contralesional neglect. Recent research focused on deficits beyond the typical neglect symptoms observed in the horizontal plane. Studies investigating deficits in the frontal and sagittal plane revealed impairments in the judgment of the subjective vertical. Systematic deviations in the subjective vertical have been demonstrated in the visual and tactile modality, indicating a supramodal spatial orientation deficit. Further, the magnitude of deviations appears to be manipulable by modulations of body posture. The present study investigated the subjective tactile vertical (STV) in neglect patients in the frontal and sagittal plane and its dependence on posture. Neglect patients and healthy controls performed tactile-spatial judgments of axis orientations in supine and upright posture. Neglect patients displayed a marked variability as well as a systematic tilt in their STV judgments. The STV was tilted counterclockwise in the frontal and backward in the sagittal plane. This tilt was larger in severe compared to moderate neglect patients, while it was not evident in healthy subjects. Our results support previous evidence and indicate a multisensory spatial orientation deficit in neglect patients which is related to neglect severity. Further, we found that performance of neglect patients deteriorated in supine compared to upright posture. This finding conflicts with the suggestion of a performance benefit in supine posture due to reduced (asymmetric) gravitational input. The negative effect of supine posture on the spatial bias in neglect is discussed with respect to a presumably further reduced intrinsic alertness state in the typically hypo-aroused neglect patients.