MULTIDIMENSIONAL DYNAMICS OF VESIBULAR-OCULAR REFLEX

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Compensatory eye movements generated by the vestibular system are the result of a coordinated "integration" of information from all semicircular canals and the otoliths. A study of continuous horizontal nystagmus generated during off vertical axis rotation and pitching the head while rotating has shown how the otoliths and the vertical semicircular canals can generate horizontal nystagmus by coupling to the velocity storage integrator. A particularly striking phenomena discovered during the last grant period is that there is a coupling between storage associated with nystagmus in the horizontal and vertical planes and that it is significantly altered by changes in the gravitational field. This suggests that velocity storage would be represented by a higher dimensional dynamical system with the states of that system establishing a gravity dependent coordinate basis for interpreting head movement and movements of the visual surround. The aim of this research is to extend the model of the vestibulo-ocular reflex for visual-vestibular interaction in one plane to a three dimensional representation of the vestibulo-ocular reflex that can be used to explain the role of velocity storage in complex rotations of the head. Specifically, we will determine the peak velocity and rising and falling time constants of vertical optokinetic after nystagmus following pure horizontal optokinetic stimulation when the animal is in different orientations with regard to gravity. This should enable us to obtain quantitative estimates of the coupling between the horizontal and vertical modes of excitation of the velocity storage integrator. Activity will be recorded extracellularly from second order vertical as well as horizontal units in the vestibular nuclei. The second order vertical unit activity will then be compared to the vertical eye velocity during off-vertical axis rotation to determine what effect cross coupling has on vertical eye position and nystagmus. This research should give a mathematical framework in which to view the dynamic transformations that occur for head and surround movement in all planes. We should also have a clearer idea of the parameters that are important for cross coupling in the vestibulo-ocular reflex and have a better understanding of the functional significance of velocity storage in three dimensions.