Translational vestibulo-ocular reflex evoked by a "head heave" stimulus

The gain and symmetry of vestibulo-ocular reflexes for high-frequency, high-acceleration movements of the head are altered following unilateral vestibular lesions. These changes have been well characterized for rotational head movements (thrusts), and provide reliable markers of dysfunction in individual semicircular canals. Alterations in the vestibulo-ocular reflex (VOR) evoked by lateral, whole-body translations have also been observed. In an approach directed at the development of a bedside test of otolith function, we have recorded (scleral search coil) the VOR evoked by brief, high-acceleration lateral translations (heaves). We delivered these stimuli manually and also developed a "head sled" device that minimizes any rotational contaminating component of the stimulus. Our geometrical analysis of the stimuli enables us to take into account the translational and rotational components of the movement, and to calculate an ideal response required for stabilization of images on the fovea at different fixation distances. We observed a tracking response (visually assisted VOR) that was close to ideal for image stabilization when these methods were used to analyze responses to slow, low-amplitude lateral translations of the head. When applied to rapid, high-acceleration (0.5 g) translations, the VOR was found to be less than compensatory in subjects with normal vestibular function. In a patient with unilateral vestibular hypofunction following intratympanic gentamicin injections, both the rotational and the translational VOR were asymmetric. Responses for translations toward the treated side had lower gain than those for translations toward the normal side. These findings provide a basis for further development of this technique as a clinical test and as a method for quantitative evaluation of otolith function.