THE PERCEPTION OF BODY VERTICALITY (SUBJECTIVE POSTURAL VERTICAL) IN PERIPHERAL AND CENTRAL VESTIBULAR DISORDERS

The perception of body verticality (subjective postural vertical, SPV) was assessed in normal subjects and in patients with peripheral and c entral vestibular lesions and the data were compared with conventional neuro-otological assessments. Subjects were seated with eyes closed i n a motorized gimbal which executed cycles of tilt at low constant spe ed (1.5 degrees s(-1)), both in the frontal (roll) and sagittal (pitch ) planes. Subjects indicated with a joystick when they entered and lef t verticality, thus defining a sector of subjective uprightness in eac h plane. The mean angle of tilt (identifying a bias of the SPV) and th e width of the sector (defining sensitivity of the SPV) were then dete rmined In normal subjects, the angle of the 'verticality' sector was 5 .9 degrees for pitch and roll. Patients with bilateral absence of vest ibular function, patients with vertigo, i.e. acute unilateral lesions, benign paroxysmal positional vertigo (BPPV) and Meniere's disease, an d patients with positionally modulated up-/downbeat nystagmus all had enlarged sectors (i.e. loss in sensitivity). Mean sector angle in thes e groups ranged from 7.8 to 11 degrees and the abnormality was present both in pitch and roll, regardless of the direction of nystagmus or b ody sway. Patients with chronic unilateral peripheral vestibular lesio ns and those with position-independent vertical nystagmus had normal s ensitivities. No significant bias of the SPV was found in arty patient group, not evert those with acute unilateral vestibular lesions who h ad marked tilts of the subjective visual vertical (SW). Complementary experiments in normal subjects tested under galvanic vestibular or Pol l-plane optokinetic stimulation also failed to show biases of the SPV. In contrast, a significant bias in the SPV could be induced in normal subjects by asymmetric cycles of gimbals tilt, presumably by proprioc eptive adaptation. The following conclusions can be drawn. (i) The per ception of body verticality whilst seated is mainly dependent on propr ioceptive/contact cues but these are susceptible to tilt-mediated adap tation. (ii) Vestibular input improves the sensitivity of the SPV even in vestibular disorders, as long as the abnormality is stable. (iii) There can be marked dissociation between vestibule-motor (ocular and p ostural) phenomena and the perception of body verticality, and between the SPV and SVV. (iv) The postural sway asymmetries in patients with peripheral and central vestibular lesions, like those induced by galva nic or optokinetic stimulation in normal subjects, are not consequence s of changes of the SPV.