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The saccadic motion threshold is used to delay the onset of a saccade until the
eye has moved significantly. A threshold of 0.1° to 0.2° is sufficient for
shortening saccades. Larger values may be used with caution to eliminate short
saccades: for example, a threshold of 0.4° will always merge fixations separated
by 0.5° or less, but may eliminate some 1° saccades as well. The threshold
should be set to zero for non-cognitive research, or where statistics such as
saccadic duration, amplitude and average velocity are required.
Examples of the commands to set these thresholds are:
saccade_velocity_threshold = 30
saccade_acceleration_threshold = 8000
saccade_motion_threshold = 0.15
4.3.6 Pursuit Thresholds
During smooth pursuit and nystagmus, saccades must be detected against a
background of smooth eye motion as fast as 70°/sec. While acceleration can be
used to detect these saccades, velocity data must also be used for reliable
detection of all saccades. The EyeLink parser raises the saccadic velocity
threshold during pursuit by the average velocity over the last 40 milliseconds.
This is reliable, and does not degrade parser performance during non-pursuit
eye movements.
During long saccades such as a return sweep in reading, this fix-up causes the
saccadic velocity threshold to be raised. This is not a problem as long as the
adjustment is limited, as it helps to prevent prolongation of these saccades by
overshoots and glissades. The pursuit threshold limits the amount that the
saccadic threshold can be raised. A limit of 60°/sec works well for most pursuit
and other research, but may have to be raised if very rapid pursuit or
nystagmus is being recorded.
The limit is set in degrees per second. An example of this command is:
saccade_pursuit_fixup = 60
4.3.7 Fixation Updates
Monitoring eye position or pupil size during fixations usually requires
processing all samples produced by the tracker. This is acceptable for file data,
but is computationally expensive for real-time systems using data sent via the
link. Fixation updates solve this problem by sending updates on eye position,
pupil size, velocity etc. at regular intervals during a fixation. The first update is
sent one update interval after the start of the fixation, and the last is sent at the
end of the fixation. Data is aggregated over a preset period, which lowers data
noise. The interval between updates and the data accumulation period can both
be set.
Data Files
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