Responses of the Saccadic System to Sudden Changes in Target Direction

Abstract

Little is known on how the brain derives a motor-command signal for initiating and directing a saccade from the visual information on the retina. Often two subsystems are distinguished. One system, called the WHERE system in this study, determines the metrics of saccades. Another system, denoted here as the WHEN system, initiates the saccade. An interesting property of the WHERE system was discovered by Becker and Jürgens (1979). They showed that saccades are directed at a delayed and filtered (or time-averaged) version of the stimulus trajectory. Several groups have observed rapid eye movements which abruptly changed course when the stimulus reversed direction. These responses have been interpreted as the sum of two separate saccades (Becker and Jürgens, 1979) or as a single saccade modified in midflight (Robinson, 1975). Recently Georgopoulos et al. (1981) found that hand-movement trajectories were curved while saccade trajectories were straight when monkeys tracked a stimulus which suddenly changed direction. Using essentially the same type of stimulus we did find curved saccade trajectories. Saccades to single step stimuli had approximately straight trajectories. These results are discussed in terms of a two-dimensional version of Robinson’s model.