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

In the present study, attention is directed to the unilateral response of the otolith system to static and dynamic tilt, as reflected by subjective estimation of the visual vertical (oculogravic perception). Measurements were performed with a variable radius rotary chair, which permits controlled modulation of the centripetal, or radial, acceleration. By limiting the radius, i.e. eccentric displacement of the head by 3.5 cm during constant-velocity rotation about the earth-vertical axis, adequate unilateral stimulation of the otolith organ--predominantly the utricle--is generated, without involving the semicircular canals. This paradigm has been employed to measure the unilateral utriculo-ocular response. In contrast to the otolith-ocular response (OOR), the subjective visual vertical (SVV) reflects the processing of otolithic information in the higher brain centres (thalamus, vestibular cortex). Exploitation of these two complementary approaches provides useful information for both experimental and clinical scientists. The findings also reveal that centripetal acceleratory stimulation during constant angular velocity with the subject centred on axis is sufficient to localize 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.