Identification of voluntary and reflex stabilization strategies of head control

Control of the head involves the interaction of multiple control systems. These systems may influence head motion in varying ways and degrees. Because of this complexity, it is difficult to quantify their individual characteristics of head control. Previous studies have provided conflicting views of individual control system characteristics, which may be due to differences in experimental paradigm and similarities in control system characteristics. The goal of these studies was to test the hypothesis that head control systems are redundant, and can compensate for deficits in control due to changes in physiology that may occur with disease. The focus of this work was on vestibular deficit. A physiologically based model was used to quantify the individual contributions of head control mechanisms in healthy subjects and subjects with bilateral vestibular deficit during reflex and voluntary head stabilization paradigms in the horizontal plane. Optimization methods were used to fit model parameters to experimental data from individual subjects. Results show that individuals are able to modulate the influence of each control mechanism according to situational demand and individual preference. Optimization results and model simulations show that head stabilization may be achieved through redundant control mechanisms. Tuning of viscoelastic properties of the neck and neural weighting of residual vestibular signals were important for compensation of vestibular deficit. The models and data fitting techniques developed in this work may be used to further investigate head control in other situations and planes of movement.