SENSORY-MOTOR ADAPTIVE MECHANISMS IN EQUILIBRIUM CONTROL

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The control of equilibrium in the elderly is an important public health concern because aging is accompanied by an increasing incidence of both dizziness and falls. The vestibular system is crucial in maintaining equilibrium through a variety of postural and orienting reflexes. One of its major functions is the vestibulo-ocular reflex (VOR), which generates compensatory rotations of the eyes during head movements in order to maintain ocular fixation on visual targets, and therefore a stable retinal image. The VOR is activated by two kinds of head motion; angular (the AVOR), driven by the semicircular canals, and linear (the LVOR), driven by the otoliths. An important feature of the VOR is its ability to adaptively modify performance in response to prolonged visual-vestibular mismatch during head movements. In addition, recent evidence has demonstrated that the VOR is also modulated by instantaneous viewing conditions (e.g. target distance and gaze), more so in the LVOR than the AVOR. This project focuses on the VOR because it is the most direct and accessible behavior from which mechanistic inferences about vestibular function can be derived. The LVOR has generally received less attention than the AVOR. Adaptive plasticity has only been demonstrated in the AVOR, and remains unexplored in the LVOR. Studies on aging have also been limited to the AVOR. In the past support period, progressive age-dependent deterioration was uncovered in the AVOR and its adaptive capabilities, especially when stressed by high head velocities and frequencies. We now turn our attention to the human LVOR, its relationship with the AVOR and with visual following, and novel adaptive mechanisms in different age groups. New techniques will be employed to study these functions in order to define their basic performance and plastic characteristics. We will also stress the LVOR and visually-driven adaptive mechanisms to uncover limitations and deteriorations with aging. We hypothesize that the LVOR will be more sensitive to aging than the AVOR, and that a comprehensive study of both VORs will allow us to infer mechanisms of fundamental interest as well as those relevant to age-dependent disequilibrium underlying dizziness and falls.