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Data Files

4.5.3.4 Saccades

The EyeLink 1000 tracker's parser detects saccades by the velocity and

acceleration of the eye movements. Because of variations in acceleration

profiles, the onset and offset point of saccades can vary by one or two samples

from "ideal" segmentation done by hand. Nonetheless, the saccadic data

compiled by the parser is sufficient for most neuro-psychophysical research,

including smooth pursuit. Most cognitive research will ignore the saccadic data,

using the fixation data produced by the EyeLink 1000 parser. The saccadic data

produced for saccades includes:

• The time of the first and last sample in the saccade

• The eye that generated the event

• Start and end HREF or gaze position data

• Peak eye-movement velocity

• Start and end gaze-data angle.

• Gaze-data angular resolution

All of these data may appear in the ENDSACC event that terminates the

fixation. Only the starting data can appear in the STARTSACC event that

initiates the saccade.

In a sorted EDF file or a text ASC file (produced by EDF2ASC) that contains

both samples and events, the STARTSACC event will precede the first sample in

the file that is part of the saccade, and the ENDSACC event will follow the last

sample in the saccade. This allows the sample data in the files to be processed

by saccade or fixation in a single pass. The data contained in STARTSACC and

ENDSACC events may be configured by modifying the DATA.INI file for the

EyeLink tracker. Saccadic events may be eliminated entirely, if only fixation

data is required. STARTSACC events may also be configured to contain only the

start time of the saccade.

The peak and average velocity data for saccades is especially valuable for neuro-

psychophysical work. These are the absolute velocities measured as the

Euclidean sum of x and y components. The EyeLink 1000 parser computes

velocity by use of a 9-sample moving filter. This is optimal for detection of small

saccades, minimizes extension of saccade durations, and preserves saccadic

peak velocities.

Other data in the ENDSACC event may be useful for some types of analysis. The

start and end position, and start and end resolution, may be used to compute

saccadic amplitude. This is more easily done by multiplying average velocity by

the saccadic duration:

dist = 1000.0 * (end_time - start_time + 1.0) * avg_velocity;