Dissociation of visual and haptic vertical in two patients with vestibular nuclear lesions.

The somatosensory (haptic) vertical (HV) and visual vertical (VV) were assessed in two patients with vestibular nuclear lesions. Patient 1 had paroxysmal nystagmus, and was tested "on" and "off." The HV was normal "on" and "off" but the VV was severely tilted during vestibular paroxysms. Patient 2, with a brainstem stroke, was tested at months 1 and 6. The VV was severely tilted on both occasions (>12 degrees) but the HV was marginally tilted (4 degrees) in the acute stage only. These VV-HV dissociations suggest that vestibular nuclear lesions influence gravity perception mostly via ocular torsional effects rather than by disrupting a single, internal representation of verticality.