Summary: The human otolith system, situated in the inner ear, plays a key role in maintaining balance, keeping a stable gaze and orienting ourselves. Assessing its function is crucial in diagnosing vestibular pathologies, but due to the complexity of the system, it remains a challenging task. This doctoral dissertation focuses on two different techniques to assess the utricular function, the unilateral centrifugation (UC) test and the ocular vestibular evoked myogenic potential (oVEMP) test. During the UC test, the patient is rotated about an Earth vertical axis and translated along the interaural axis to consecutively align both utricles with the axis of rotation, allowing a unilateral evaluation. The combined rotation and translation induces ocular counterrolling (OCR) which is measured using 3D video oculography. In this dissertation a new stimulation paradigm for the UC test was developed, along with a physiological model to interpret the test outcome. That model also includes a contribution of the semicircular canals, and it has been shown to be reliable and robust model.

The oVEMP test uses bone conducted vibration to stimulate both utricles simultaneously. A biphasic wave could be recorded from the extra ocular muscles. The methodology of that test was improved by introducing a calibration procedure. In addition, the influence of different stimulation parameters on the test outcome was investigated. A test-retest study showed an excellent reliability.

For both the UC and oVEMP test a set of normative data was gathered. A comparative study between those two tests showed the complementary character of both tests. Whereas the UC test assesses the low frequency response of afferents in all zones of the utricle, the oVEMP test assesses the high frequency response of the irregular afferents in the central, striolar zone of the utricle. The complementary character of both utricular tests with the cVEMP test was also shown.

Finally, the influence of a long duration exposure to microgravity (6 months stay in the International Space Station) on the otolith system was investigated.