User manual

Data Files
© 2005-2008 SR Research Ltd.
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4.5.3.1
4.5.3.2 Fixations
Eye-movement events are always labeled by which eye generated the event. If
binocular data is recorded, a separate start and end event is generated for each
eye. The time differences between eyes are very important for neurological
analysis, for example. The main classes of data events are summarized below.
Start events contain the time of the start of the eye-movement condition. They
may also contain the state of the eye at the onset of the condition: for example,
the position and pupil size at the start of a fixation.
End events contain both the start and end time of the condition. The end time
is actually the time of the last sample in the condition, so length of a condition
must be computed as the difference between the end and start times plus the
time between samples (1, 2 or 4 milliseconds). End events also contain
summary data on the condition as well: average gaze position of a fixation, for
example.
Record Blocks
Each block of recorded data in an EDF file begins with one or both of a
STARTSAMPLES or STARTEVENTS event. These contain the time of the
recording start, and specify what data can be expected to follow. This allows for
flexible adaptation to almost any file-data configuration. Information included
in the start events include:
• Which eye recorded from
• Sample data rate
• Sample data contents
• Event data contents
• Event types included
• Gaze-position and velocity prescalers
Each block of recorded data ends with one or both of an ENDSAMPLES or
ENDEVENTS event. This simply terminates the data block, and specifies the
time that recording ended.
The text files generated from EDF files by the EDF2ASC translator utility create
a simplified form of START and END events. A single START or END line is
produced for both sample and event blocks, which specifies which eye was
recorded from, and whether samples, events, or both, are present in the
following data block. Other data is given on following SAMPLES, EVENTS,
PRESCALER, etc. lines.
The on-line EyeLink 1000 tracker parser processes eye-position data,
identifying saccades and fixations and compiling data on these conditions. For
fixations, these data include: