Skewness of saccadic velocity profiles: a unifying parameter for normal and slow saccades.

It has become customary to make use of so-called main sequence plots to characterize the dynamic properties of saccades. However, such a description does not account for the fact that the ratio between the accelerating and the decelerating fraction of the eye movement is not constant for all saccades. In this paper we introduce a new parameter, skewness, that characterizes this much neglected aspect of the saccade velocity profile. Human saccade data in this report demonstrate a clear relation between saccade duration (D) and skewness (S). When saccadic eye movements become extremely slow, due to fatigue or diazepam, the main sequence relation breaks down, while the S-D relation still holds. Despite large differences in amplitude, saccades of a fixed duration appear to have the same shape of velocity profile. A unifying equation relating the saccade parameters amplitude, maximum velocity and skewness, which is valid for both normal and slow saccades, is proposed.