EyeLink II User Manual version 2.12
(07/02/2006)
©
2002-2006 SR Research Ltd.
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analog eye trackers. These systems use multi-pole filters for noise reduction
that adds delay and smoothes the data, significantly reducing the measured
acceleration.
The saccadic motion threshold is used to delay the onset of a saccade until the
eye has moved significantly. This can add 4 to 8 msec to the reported duration
of fixations in cognitive research. 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 II 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 the 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 expensive for real-time systems using data sent via the link. Fixation
updates solve this problem by sending updates on eye position, pupil size,
