Vestibulo-cerebellar disease impairs the central representation of self-orientation

Abstract

Transformation of head-fixed otolith signals into a space-fixed frame of reference is essential for perception of self-orientation and ocular motor control. In monkeys the nodulus and ventral uvula of the vestibulo-cerebellum facilitate this transformation by computing an internal estimate of direction of gravity. These experimental findings motivated the hypothesis that degeneration of the vestibulo-cerebellum in humans alter perceptual and ocular motor functions that rely on accurate estimates of gravity, such as subjective visual vertical (SVV), static ocular counterroll (OCR), and gravity-dependent modulation of vertical ocular drifts. We assessed the SVV, OCR, and spontaneous vertical ocular drifts in 12 patients with chronic vestibulo-cerebellar disease and in 10 controls. Substantially increased variability in estimated SVV was noted in the patients. Furthermore, gravity-dependent modulation of spontaneous vertical ocular drifts along the pitch plane was significantly (p < 0.05) larger in the patients. However, the gain and variability of static OCR and errors in SVV were not significantly different. In conclusion, in chronic vestibulo-cerebellar disease SVV and OCR remain intact except for an abnormal variability in the perception of verticality and impaired stabilization of gaze mediated by the otoliths. These findings suggest that OCR and perceived vertical are relatively independent from the cerebellum unless there is a cerebellar imbalance like an acute unilateral cerebellar stroke. The increased trial-to-trial SVV variability may be a general feature of cerebellar disease since a function of the cerebellum may be to compensate for such. SVV variability might be useful to monitor disease progression and treatment response in patients.

Keywords: cerebellar degeneration; ocular counterroll; otoliths; spontaneous vertical deviation; subjective visual vertical.

Figure 1

Average errors (±1 SD) in perceived vertical (A) and average intra-individual SVV variability (±1 SD) (B) are plotted against whole-body roll position both for the cerebellar patients (squares) and the age-matched healthy controls (circles). The direction (indicated by the curved arrows) of arrow adjustments (straight arrows: clockwise vs. counter-clockwise), as illustrated below the x-axis for all roll positions (upright, 75° left-ear-down or LED, and 75° right-ear-down or RED) tested was pseudo-randomized.

Figure 2

(A) Means of static torsional eye position (±1 SD) in 75°, 45°, 30°, 20°, and 10° whole-body roll tilt positions to either side (positive values corresponding to right-ear-down positions, i.e., clockwise roll tilts, as seen from the subject) did not differ significantly (p > 0.05) between patients (squares) and age-matched controls (circles). (B) Variability of static torsional eye position over time (10 s) in upright and 75° right- (positive values) and left-ear-down (negative values) positions to either side was not significantly different (p > 0.05) between patients (squares) and age-matched controls (circles).