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FAQ: How are fixations / saccades defined?
#1
The EyeLink online parser uses a "saccade picking" approach, in which instantaneous velocity and acceleration thresholds to determine the onset and offset of saccades; samples above threshold are determined to be in saccade, and samples below threshold are determined to be in fixation. The standard parser setting uses velocity and acceleration thresholds of 30°/s and 8000°/s^2, respectively. So the onset of a fixation is determined by the offset of the previous saccade, and the offset of a fixation is determined by the onset of the subsequent saccade. 

For more information about the EyeLink parser, please see Section 4.3: The EyeLink On-Line Parser of the relevant User Manual: