Spatial orientation of the human linear and angular vestibulo-ocular reflexes during centrifugation

Six independent co-ordinates comprising three rotational and three translational degrees of freedom are required to specify the orientation and motion of a body in inertial space without ambiguity. The vestibular sensors of the inner ear are designed to provide the organism with precisely this information: three approximately orthogonal semicircular canals and two otoliths (utricle and saccule) on either side of the head transduce the three angular and three linear degrees of freedom, respectively. Activation of either angular or linear vestibular sensors results in compensatory reflexes including an angular VOR (AVOR), various linear VOR's (LVOR's) and the vestibulo-spinal reflexes which mediate static and dynamic postural control. The combined input from the vestibular labyrinth thus serves to orient the body and stabilise the eyes during head movement in any spatial plane. The vestibular sensors are not tied to any particular muscle group; they drive a number of motor systems with disparate physical requirements and functions. How this brainstem system accurately and reflexively partials out the relevant information in the source signal to drive a particular compensatory reflex is the subject of several different sensory-motor modelling approaches. Recent evidence suggests that, analogous to a modern inertial guidance system, the vestibular system uses frequency-selective filtering, realised in the physical properties of the endorgan, and the hair cell receptors, to effect the appropriate channelling. Considerable interspecies variation in the frequency response of the VCR reflects the range of natural movements of different animals. Analogously, variation in the working ranges of different body movements within a species should be reflected in differential frequency-tuning of the various vestibule-ocular and vestibule-spinal reflexes. Evidence for frequency-selective filtering of all vestibulo-ocular reflexes is presented in the following chapters, and a case developed for a peripheral filtering system which generates appropriate canal-ocular and otolith-ocular reflexes during combined, often ambiguous linear and angular acceleration stimulation