From small to large, all saccades follow the same timeline

Abstract

Purpose: The well-known main sequence of saccadic eye movements can have a large variability; the peak velocity of saccades with the same amplitude can be dramatically different. We explored an alternative approach to describing the saccade characteristics by looking into the in-flight shift timeline of saccades. Methods: We gathered 929 natural saccades made by two subjects with their heads restrained as they watched videos. The saccade duration and amplitude ranged from 13 to 95 ms (mean ± std. = 43ms ± 14.5) and 0.54° to 28.7° (mean ± std. = 9.27° ± 4.5), respectively. For each saccade we analyzed data in two ways: (1) displacement at time points corresponding to different proportions of its overall duration vs. amplitude; and (2) the time taken to reach various proportions of its amplitude vs. total duration. Results: In all the cases, the data showed a strong linear relationship, with slopes converging toward unity and the line-fitting errors reducing as the sampling got closer to the end of saccade trajectory (R²=0.62, 0.96, and 0.99 at 20%, 60% and 90% of saccade duration, respectively; and R²=0.93, 0.98, and 0.99 at 20%, 60%, and 90% of saccade amplitude, respectively). For the same data, saccade duration vs. amplitude resulted in R²=0.65 while peak velocity vs. amplitude was R²=0.12. Conclusion: Our finding is consistent with the fact that generally, saccade duration has a linear relationship with its size. The novelty of our finding is that, if normalized by saccade duration or size, all saccades appear very similar in their time course. This consistent time line of the normalized in-flight shift might suggest a universal controlling model in some saccade-related neural circuits, for instance, the paramedian pontine reticular formation.

Meeting abstract presented at VSS 2015