Nonstationary properties of the saccadic system: new constraints on models of saccadic control.

1. We tested the predictions of two models of the saccadic burst generator by electrically stimulating sites in primate superior colliculus (SC) immediately following visually guided movements. 2. The amplitude and direction of stimulated saccades depend systematically on the amplitude and direction of preceding visually guided saccades, and that effect decays exponentially with a time constant of approximately 45 ms. The saccadic system, then, displays an amplitude-dependent non-stationarity that follows an exponential time course during the intersaccadic interval (ISI). 3. These results are consistent with a variant of the eye displacement model proposed by Jurgens et al. but not with Robinson's classic model of the burst generator. Moreover, since all models of saccadic control must predict either stationary or nonstationary behavior during the ISI, these results provide a powerful new constraint on those models. 4. Finally, the success of the displacement model in accounting for our data suggests a new explanation for the results of colliding saccade experiments.