Clear vision requires that eye movements keep images stationary on the retina. When the eyes need to change fixation to look at novel objects, or when the eyes must follow smoothly moving objects, the saccadic and smooth pursuit eye movement systems come into play. Saccades have high velocities and abrupt endings. These characteristics allow the eyes to get on target quickly, minimizing the amount of time that vision is interrupted. The pursuit system smoothly matches eye velocity to target velocity, keeping images from slipping on the retina. Following central or peripheral diseases or injuries these eye movements may become affected by the weakness of one or more extraocular muscles. We have shown in monkeys that retinal image slip is sufficient to elicit adaptive changes in saccadic innervation. The required changes consist of two components: an adjustment of the tonic level of innervation needed to determine the final eye position, and control of the shape (size and time constant) of the transition from the phasic burst of innervation during the saccade to the final tonic level. Smooth pursuit movements depend on retinal feedback to alter eye velocity to match target velocity. So, it would seem that adaptive control of the smooth pursuit system would not be necessary. However, there is a 130 millisecond delay before visual events can influence pursuit eye movements, which creates two contradictory goals: the pursuit gain must be high for good tracking, but low to avoid oscillations. An adaptive controller could resolve this contradiction. We have shown that human subjects can alter the gain of their pursuit systems to compensate for peripheral muscle weakness. Such gain increases occur for movements both in the direction of action of the weakened muscle, and opposite to that direction. These results show that adaptive mechanisms are sensitive to retinal image slip. Work is continuing on the study of adaptive systems to determine how neuronal networks might be constructed to make use of error signals for the control of eye movement systems.
