EyeLink II User Manual version 2.12
(07/02/2006)
©
2002-2006 SR Research Ltd.
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One or more types of eye position data, monocular or binocular
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Pupil size, monocular or binocular
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Angular resolution of eye-position data
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Button or input port state bits
All samples contain a timestamp, recorded in milliseconds. The time is
measured from the time when the tracker software was started. This
timestamp makes detection of missing samples possible, as well as simplifying
processing of data. Usually all samples produced by the EyeLink II tracker are
recorded, but the tracker may be configured to record samples at a lower rate,
or only when significant changes in eye position occur. These special modes are
not used for most data, as they may prevent re-analysis of the EDF file.
Other types of sample data are discussed in greater detail below.
4.4.2 Position Data
Eye position data is produced by the EyeLink II tracker for one or both eyes
every 2 or 4 milliseconds depending on the tracking mode and speed set. It is
then processed to compute eye rotation angles and to compensate for subject
head motions. The processed data in one or all of these forms may be recorded
in the samples. Data is written as (x, y) coordinate pairs, or two pairs for
binocular data. The types of position data available are explained below.
4.4.2.1 PUPIL
Pupil position data is raw (x, y) coordinate pairs from the eye cameras. It has
not been converted to eye angles or to gaze position. There may be a non-linear
relationship between this data and true gaze position. Pupil position is reported
in integer values, with 200 to 400 units per visual degree.
When a calibration has not been performed, the EyeLink system cannot convert
pupil data to the more useful data types. Raw pupil position is useful when
auto-sequencing calibrations, or when the application wishes to perform its own
calibration. Most users will not need this data type.
4.4.2.2 HREF
The HREF (head-referenced) position data directly measures eye rotation angles
relative to the head. It does not take into account changes in subject head
position and angle, or distance from the display. However, it may be more
accurate for neuro-psychophysical research, as it reflects real eye movement
velocities and amplitudes.
The (x, y) coordinate pairs in HREF data reflect the line of sight in the geometric
model below:
