Spatial Orientation and Vestibular Function

Spatial orientation is achieved through central integration of various sensory inputs and prior knowledge in a statistically optimal way based on the reliability of the different signals. When upright, the subjective visual vertical (SVV) is accurate and precise in healthy human subjects. However, when roll-tilted, both systematic physiologic, roll-angle dependent errors (termed A- und E-effect) and a decrease in precision of SVV estimates have been described. In case of a sudden unilateral vestibular deficit (UVD) a significant imbalance between the two vestibular organs occurs at the level of the vestibular nuclei, disrupting the percept of vertical. The most frequent cause for such a unilateral vestibular deficit is an inflammation of the vestibular nerve by viral infection, termed vestibular neuritis (VN). While in the acute stage these patients are usually immobilized due to the severity of symptoms, recovery is overall good and most patients return to their daily activities within a few weeks. Central compensation is considered the most important contributor to recovery in these patients, while recovery of the damaged vestibular nerve occurs only in a minority of cases. While acute VN presents with sudden UVD, bilateral vestibular deficits (BVD) typically evolve more slowly and re-sult in distinct complaints. The percept of vertical can be quantified by assessing the subjective visual vertical or SVV, which is usually done by letting subjects adjust a luminous line along perceived direction of gravity. Modifications of this paradigm which are independent from retinal input are e.g. adjustments of a rod along perceived vertical in complete darkness (termed subjective haptic vertical or SHV) and self-alignments along perceived vertical (subjective postural vertical or SPV) and perceived horizontal (subjective postural horizontal or SPH) in complete darkness. Previous research has proposed no unified percept of vertical as errors assessed in different domains (v