ABSTRACT
BACKGROUND AND OBJECTIVES: The vestibulo-ocular reflexes (VOR) act a short latency to optimize vision during locomotion. The angular VOR (aVOR) has been widely studied in human subjects and preserves clear, stable vision during rotational head perturbations by generating eye movements that hold the line of sight on the target of interest. Less is known about the properties of the linear or translational VOR (tVOR), mainly due to technical difficulties posed by testing head or body translations. Geometric considerations indicate that different properties should be expected of tVOR, which can only provide stable vision of objects lying in one depth plane. MATERIALS AND METHODS: We studied the human tVOR using a moving platform to translate normal human subjects vertically at frequencies similar to those occurring during locomotion. We found that, whereas aVOR is concerned with holding retinal images fairly stable to optimize clear vision, tVOR seems best suited to minimize relative motion of retinal images belonging to objects lying in different depth planes-and thereby to optimize motion parallax information. We also investigated whether the tVOR functioned abnormally in patients with neurological disorders causing falls-progressive supranuclear palsy (PSP) and cerebellar ataxia. RESULTS: Both groups of patients showed impaired ability to modulate their tVOR during viewing of near targets; in PSP this might be attributed to failure of convergence, but cerebellar patients failed to modulate tVOR at near despite intact convergence. CONCLUSION: In both disorders, an impaired ability to adjust tVOR for viewing distance points to central disturbance of otolithic vestibular reflexes, which may also contribute to postural instability.