off the horizontal plane and translational movements of the head in any direction stimulate a distinct subset of hair cells in the saccular and utricular maculae, while simultaneously suppressing responses of other hair cells in these organs. Ultimately, variations in hair cell polarity within the otolith organs produce patterns of vestibular nerve fiber activity that, at a population level, unambiguously encode head position and the forces that influence it. Hair bundles and the otolithic membrane Studies performed by a team at the University of California, Los Angeles elucidated the movement of the active hair bundle under the otolithic membrane, as well as the coupling between the hair bundles and the membrane. The researchers concluded that when coupled and loaded by the otolithic membrane, hair bundles of the bullfrog sacculus do not oscillate spontaneously but are poised in a dormant regime. However, when stimulated by a sinusoidal pulse, the bundles in the coupled system exhibit an active biphasic response similar to the "twitch" observed in individual bundles. The active bundle motion can generate sufficient force to move the otolithic membrane. Furthermore, the almost perfect entrainment between the hair bundles and the membrane demonstrates that coupling between the two is elastic rather than viscous. A further study further demonstrated that the motion evoked in the hair cell bundles induced by the otolithic membrane, was found to be highly phase-locked which was consistent over large portions of the sensory epithelium. Clinical significance Although the pathophysiology of otolithic dysfunction is poorly understood, a disorder of otolith function, at a peripheral or central level, may be suspected when a patient describes symptoms of false sensations of linear motion or tilt or shows signs of specific derangements of ocular motor and postural, orienting and balancing responses. When disorientation is severe the patient may describe symptoms which sound bizarre, raising doubts over the organic basis of the disease. It is important to understand otolithic involvement in a wider neurological context through knowledge of the otolith physiology and the characteristics of proven otolithic syndromes. Benign paroxysmal positional vertigo (BPPV) is the most common vestibular system disorder and occurs as a result of otoconia detaching from the otolithic membrane in the utricle and collecting in one of the semicircular canals. It is generally associated with natural age-related degeneration of the otolithic membrane. When the head is still, gravity causes the otoconia to clump and settle. When the head moves,