A parametric analysis of human saccades in different experimental paradigms.

In this paper we report on human saccade dynamics in three different experimental paradigms: visual target, remembered target and anti-saccade task. We found that saccades to remembered targets and anti-saccades have strongly reduced peak velocities coupled with markedly increased durations. In addition we observed a considerable degree of asymmetry in the velocity profiles of these saccades. By using gamma functions to describe the shape of the velocity profiles a parameter characterizing the degree of asymmetry (skewness) was computed: it was found that skewness increases with saccade amplitude. Due to the large variability in saccade durations for any given amplitude in our data we could confirm the recent claim, based on pharmacologically induced slow saccades, that skewness is more tightly related to duration than to amplitude. The duration/skewness relationship appeared to be nearly invariant with saccade type. We conclude that the commonly used main-sequence description of saccades is incomplete and can be extended usefully by including skewness. The possible neural basis of the task-related differences in saccade properties and their implications for models of the saccadic system are discussed. It is suggested that the marked differences in dynamic properties among different saccade types may reflect processes in the visuomotor rather than in the motor system.