These two mechanisms work together to give humans stable gaze as the head moves. Our implementation of an ocular-motor system is an approximation of the human system. Kismet's eyes periodically saccade to new targets chosen by the attention system, tracking them smoothly if they move and the robot wishes to engage them. Vergence eye movements are more challenging, since errors in disjunctive eye movements can give the eyes a disturbing appearance of moving independently. Errors in conjunctive movements have a much smaller impact on an observer, since the eyes clearly move in lock-step.
Humans exhibit four characteristic types of eye motion.
Saccadic movements are high-speed ballistic motions that center a target in the field of view. Smooth pursuit movements are used to track a moving object at low velocities. Vergence movements serve to maintain an object in the center of the field of view of both eyes as the object moves in depth.