A Quantitative Theory of Control Sharing Between Accommodative and Vergence Controllers

Static responses of human accommodation (focusing) and vergence (eye-turn) are controlled by two separate feedback control systems. The two systems also interact so that each is capable of producing an isolated motor response in the other. When both systems operate together, as in normal binocular movements, they must share control in some manner. A recently proposed static oculomotor control theory predicts that a given system´s control responsibility will be determined by the strength (i.e., gain) of its interactive or cross-link pathway in addition to its own feedforward gain. To quantify this control sharing, a theoretically-based sensitivity function was developed which predicted each controller´s contributions to the overall motor response. Evaluating these sensitivity functions using data obtained from four normal subjects indicated that a range of control sharing conditions may be used by the nervous system to successfully meet the control objectives of normal binocular vision. This suggests that the theory of vergence and accommodative control is a generalization of earlier theories developed by Maddox and Fincham-Walton which represented two extremes of control sharing seen in normals.