Transaural linear vestibulo-ocular reflexes from a single utricle

Probing the human vestibular system with galvanic stimulation

Galvanic vestibular stimulation (GVS) is a simple, safe, and specific way to elicit vestibular reflexes. Yet, despite a long history, it has only recently found popularity as a research tool and is rarely used clinically. The obstacle to advancing and exploiting GVS is that we cannot interpret the evoked responses with certainty because we do not understand how the stimulus acts as an input to the system. This paper examines the electrophysiology and anatomy of the vestibular organs and the effects of GVS on human balance control and develops a model that explains the observed balance responses. These responses are large and highly organized over all body segments and adapt to postural and balance requirements. To achieve this, neurons in the vestibular nuclei receive convergent signals from all vestibular receptors and somatosensory and cortical inputs. GVS sway responses are affected by other sources of information about balance but can appear as the sum of otolithic and semicircular canal responses. Electrophysiological studies showing similar activation of primary afferents from the otolith organs and canals and their convergence in the vestibular nuclei support this. On the basis of the morphology of the cristae and the alignment of the semicircular canals in the skull, rotational vectors calculated for every mode of GVS agree with the observed sway. However, vector summation of signals from all utricular afferents does not explain the observed sway. Thus we propose the hypothesis that the otolithic component of the balance response originates from only the pars medialis of the utricular macula.

Vertical eye-movement oscillation with a frequency double that of lateral linear acceleration in patients with long-standing unilateral vestibular loss

OBJECTIVE

To examine our hypothesis that an unusual floating or tilting sensation claimed by patients with long-standing unilateral vestibular loss might be attributed to incomplete central compensation in the otolith system.

METHODS

Seven patients who were with or without symptoms for 6-101 months after intratympanic gentamicin therapy for unilateral endolymphatic hydrops were sinusoidally exposed to lateral linear acceleration, and their compensatory eye movements were compared with those of 18 normal controls, using electro-oculography (EOG). The subjects, secured firmly in the chair of a linear accelerator (sled), were oscillated at three different G-loadings of 0.1 (0.11 Hz), 0.2 (0.16 Hz) and 0.3 G (0.19 Hz), respectively. During displacement, they gazed at a real (visible) or imaginary target which was located on the wall (earth-fixed), or moved on the wall in sync with the sled (body-fixed).

RESULTS

Vertical EOG (V-EOG) with a frequency two-fold that of the stimulus frequency was characteristically evoked in all patients, but in none of the normal controls. One of the doubled V-EOG was larger in amplitude than the other, when the stimulus acceleration was directed from the affected ear to the intact ear in those patients whose symptoms still remained. Such a directional difference tended to be greater with the imaginary target than with the visible target. In the horizontal EOG, there was no marked difference between the patients and normal controls. Nobody reported tilt perception but only horizontal translation during the sled displacement, except one patient who was examined twice; a roll tilt was perceived in the first examination when she still had symptoms, but no tilt sensation in the second one when she no longer had symptoms.

CONCLUSION

It was suggested that incomplete compensation in the otolith system could cause unusual sensations even after long-standing unilateral vestibular loss, and that V-EOG induced by a sled might be helpful in evaluating functional recovery.

Assessing otolith function by the subjective visual vertical

The effects of peripheral vestibular diseases on the subjective visual vertical (SVV) are resumed and provide the basis for some insights into the otolith pathophysiology. With a normal range of 0 +/- 2 deg (when measured in an upright body position), the SVV was shifted by 11 +/- 6 deg toward the ipsilateral ear in 40 patients following an acute unilateral vestibular deafferentiation (UVD), but in the opposite direction in 9 of 52 patients after stapes surgery. These opposite effects suggest a push-pull mechanism of the pairs of otolith organs with respect to the SVV. The dissociation between the SVV and the perception of body position indicates influences by unconscious reflexive mechanisms such as ocular cyclotorsion on the SVV. In chronic UVD patients, lateral shifts of the subjects during constant angular velocity rotation into various eccentric positions (+/- 16 cm) revealed a shift of the "center of graviception" close to the remaining intact contralateral inner ear. To date, this seems to be the most consistent test for clinical identification of a chronic compensated unilateral loss of otolith function. The findings regarding asymmetries in otolithic sensitivity to medially and laterally directed roll-tilts remain controversial, probably mainly because of influences of extravestibular cues.

Horizontal otolith-ocular responses to lateral translation in benign paroxysmal positional vertigo

Benign paroxysmal positional vertigo (BPPV) is assumed to result from utricular damage, but it is controversial if patients have manifest utricular dysfunction. Therefore, we investigated linear vestibulo-ocular reflexes (LVORs) during lateral whole-body translation in 14 patients with unilateral BPPV. Patients were subjected to linear acceleration steps of 0.24 g along the interaural axis, which were applied randomly to the left and right, both in the dark and in the light with a visual target at a distance of 60 cm. The LVOR was measured by EOG from the slow phase velocity of the averaged and desaccaded compensatory eye movement. In normal cases, maximum asymmetry of LVOR velocity was 13% in the dark and 10% in the light. In patients, LVOR velocities were normal in the dark but mildly reduced in the light (p < 0.05). Five patients had mild LVOR asymmetries in the dark (range 18-38%) and two in the light (11 and 13%), but there was no consistent relationship to the affected side. The absence of gross changes of the LVOR may be explained either by minor utricular damage that is functionally irrelevant or by central compensation of a chronic unilateral deficit.