SR Research EyeLink Portable Duo User Manual page 94

Average HREF or gaze position data
•
Average pupil size
•
Gaze-data angular resolution
•
All of these data may appear in the ENDFIX event that terminates the fixation.
Only the starting data can appear in the STARTFIX event that initiates the
fixation.
In a sorted EDF file or a text ASC file (produced by EDF2ASC) that contains
both samples and events, the STARTFIX event will precede the first sample in
the file that is part of the fixation, and the ENDFIX event will follow the last
sample in the fixation. This allows the sample data in the files to be processed
by saccade or fixation in a single pass.
The data contained in STARTFIX and ENDFIX events may be configured by
modifying the DATA.INI file for the EyeLink tracker. For most research, only
simple fixation statistics are required, such as average position and pupil size.
By default, STARTFIX events are configured to contain only the start time of the
fixation.
Other data in the ENDFIX event may be useful for some types of analysis. The
resolution may be used to estimate angular distance between fixations. Angular
(Euclidean) distance can be calculated by subtracting the x and y position for
the fixations, and dividing by the average corresponding resolution data.
dx = (x1 - x2) / ( (rx1 + rx2)/2.0);
dy = (y1 - y2) / ( (ry1 + ry2)/2.0);
dist = sqrt(dx*dx + dy*dy);
4.5.3.3 Fixation Updates
Data within a fixation can be broken into smaller time segments, useful for
real-time analysis and control via eye movements. FIXUPDATE events may be
produced at regular intervals within a fixation, and contain data for a specified
length of time within the fixation. The data recorded in the FIXUPDATE event is
similar to that in the ENDFIX event.
FIXUPDATE events are most useful in real-time applications using the link.
Recording samples in the EDF file is more useful for most psychophysical
research.
4.5.3.4 Saccades
The EyeLink 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
84 Data Files
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