Negative feedback control model of proximal convergence and accommodation.

A comprehensive model has been developed to illustrate the interactions between the observer and the surrounding environment in the control of oculomotor responses to distance or 3-D space. Accommodation and vergence respond to both spatiotopic (body reference) proximal percepts and retinotopic (eye referenced) physical stimuli of blur and disparity. Both spatiotopic and retinotopic stimuli are derived respectively from perceptual and physical correlates of negative feedback for eye position. The spatiotopic and retinotopic stimulus errors are combined in the feedforward path and drive a common oculomotor controller which has a phasic-tonic organization. Spatiotopic and retinotopic stimuli are shown to be effective over complementary operating ranges. Perceptual spatiotopic errors of gaze provide optimal stimuli for near responses to large depth intervals whereas physical-retinotopic cues of blur and disparity provide quantitative information about small binocular fixation errors. Small dynamic variations of target distance are sensed both spatiotopically and retinotopically. Coarse and fine spatiotopic errors of gaze are processed differently. Large spatiotopic errors are sampled intermittently at the beginning of the near response, whereas small retinotopic position errors and spatiotopic velocity errors are sampled continuously throughout the near response. Former reports of empirically observed higher velocity of vergence responses to very large depth intervals is explained in terms of stimulus sampling modes rather than in terms of separate oculomotor control mechanisms. The model demonstrates a complementary function of top-down spatiotopic cues, which are used to initiate the near response, and bottom-up retinotopic cues, which are used to refine and complete the near response. Cross-couplings by vergence-accommodation and accommodative-vergence serve to coordinate the components of the near response when feedback from sensed response of one motor system (i.e. vergence) is more accurate than that of the other motor system (i.e. accommodation). The model presented here is concerned primarily with the near response mediated by accommodation and disjunctive eye movements and not by the independent vergence mediated by non-conjugate or yoked saccades of unequal amplitude.