Measuring unilateral otolith function via the otolith-ocular response and the subjective visual vertical

In the present study, attention is directed at the unilateral response of the otolithic system to static and dynamic tilt, as reflected both by the objectively measured, otolith-ocular reflex, and by the subjective estimation of the visual vertical. Measurements were performed with a variable radius rotatory chair, which permits a controlled modulation of the centripetal, or radial acceleration. By limiting the radius, i.e. eccentric displacement of the head to 3.5 cm during constant velocity rotation about the earth-vertical axis, an adequate unilateral stimulation of the otolith organ is generated, without involving the semicircular canals.
The gain and phase relationships of the OOR show a lowpass characteristic over the measured range of 0.03 - 1.0 Hz, in accordance with recent models for the discrimination of tilt and translation.
The paradigm can equally be employed to determine the subjective visual vertical, notably during exclusive stimulation to the left or the right labyrinth. In contrast to the otolith-ocular response, the subjective visual vertical reflects the processing of otolithic information in the higher brain centres (thalamus, vestibular cortex). Exploitation of these complementary approaches provides useful information for both experimental and clinical scientists.
The findings also reveal that the centripetal acceleratory stimulation to the otolith organs (predominantly utricles) during constant angular velocity with the subject centred on axis, is sufficient to localise peripheral otolith dysfunction by means of SVV estimation. This represents a novel test of otolith function that can be easily integrated into routine clinical testing.