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detection of saccades as small as 0.3°, ideal for smooth pursuit and

psychophysical research. A conservative threshold of 30°/sec is better for

reading and cognitive research, shortening saccades and lengthening fixation

durations. The larger threshold also reduces the number of microsaccades

detected, decreasing the number of short fixations (less than 100 msec in

duration) in the data. Some short fixations (2% to 3% of total fixations) can be

expected, and most researchers simply discard these.

Use of eye-movement acceleration is important for detection of small saccades,

especially in smooth pursuit. Acceleration data has much more noise than

velocity data, and thresholds of 4000°/sec

for small saccade detection and

8000°/sec

for reading and cognitive research are recommended. Lower

acceleration thresholds will produce false saccade reports. Acceleration data

and thresholds for the EyeLink 1000 system may be larger than those reported

for 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. 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 1000 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