The Programming Strategy of the Saccadic Eye Movement Control System

The purpose of this work is to learn more about the properties of and relationships between the stages of information processing in programming a saccadic eye movement. The transient and the steady-state behavior of the saccadic programming system were studied using a double-step target movement and a long sequence of random-walk target movements, respectively. Then, the performance of the saccadic system in response to a more complex stimulus pattern which requires acuity discrimination was studied. The results of the double target-step experiment suggested the following hypotheses. The information processing in the programming of a saccade consists of detection, direction computation and magnitude computation. If the new target-step arrives before the direction computation of the previous saccade is complete, this saccade will be cancelled. The partial program concerning the direction of the saccade is kept in a buffer memory; if the direction of the new saccade is not in a direction similar to the old one, this partial program will have to be erased, which takes an extra 40-80 msec of processing time. There is a stage in which the direction of the saccade cannot be reprogrammed but the magnitude can still be shortened. In other words, the magnitude computation seems to finish after the direction computation. Although the magnitude computation requires the results of the direction computation, it seems to start before the direction computation finishes. If the new target-step arrives when the computation of both direction and magnitude are complete, two full saccades will be observed. The results suggested that the second saccade can start to be programmed before the first saccade is executed, but there is an approximately 100 msec refractory period following the programming of the first saccade. When the saccadic system tracks a long sequence of random walk target movements, the percentage of target-steps that elicit saccadic responses is lower than the corresponding percentage in the double target-step situation. A third order effect was identified, which might be responsible for there being fewer saccades observed in the steady-state situation when the target rate is not too high. At a high target rate, such as 10 to 20 target-steps per second, a perceptual smearing phenomenon was observed. The size of this smearing region may define a threshold for a strong and consistent stimulus for the saccadic system at the high rate. The results from the acuity discrimination experiment suggested that the same discrimination system is used by the saccadic programming system and the psychophysical acuity system. Like the single-spot stimulus, the new acuity information which arrives early enough can be used to reprogram the saccade. When a saccade is too late to be reprogrammed, the second saccade in response to the new target information can be processed in parallel with the first one.