Spatial and Dynamic Aspects of Visual Fixation

The physiological processes concerned with the difficult dynamic task of fixating the retinal image during normal body and head movement are examined with a control engineering perspective. Spatial relationships between the two main system inputs (visual and vestibular) and three main outputs (operating on the eye-in-skull, skull-on-body, and body-in-space platforms) are examined in the context of the geometry of the environment with the aid of an information flow diagram. From dynamic considerations a picture emerges in which the visual tracking system has adequate accuracy and dynamic range for following most naturally moving objects when the head is still. But with head free, the added perturbations of natural movement exceed these tracking capabilities. However, over the frequency range 0.1-5.0 c/s the semicircular canal subsystem then provides angular velocity information for powerful complementary servostabilization.