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Data Files
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2013-2017 SR Research Ltd.
This resolution data may be used to estimate distances between gaze positions,
and to compute velocities of eye movements. To compute the angular distance
of two points, compute the x and y angular distances of the points separately by
dividing the distance in pixels by the average of the resolutions at the two
points, then compute the Euclidean distance from the x and y distances. For
instantaneous velocity in degrees per second, compute the x and y velocities,
then divide each by the x or y resolution, square and add the x and y velocities,
and take the square root.
Resolution is computed at the point of gaze on the display, and can vary up to
15% over the display. The resolution data in an EDF file is recorded using a
prescaler for extra precision, and noted in the gaze-position section.
4.4.3 Pupil Size Data
Pupil size is also measured by the EyeLink 1000 Plus system, at up to 2000
samples per second depending on your tracker version. It may be reported as
pupil area, or pupil diameter. The pupil size data is not calibrated, and the
units of pupil measurement will vary with participant setup. Pupil size is an
integer number, in arbitrary units. Typical pupil area is 100 to 10000 units,
with a precision of 1 unit, while pupil diameter is in the range of 400-16000
units. Both measurements are noise-limited, with noise levels of better than
0.2% of the diameter. This corresponds to a resolution of 0.01 mm for a 5 mm
pupil.
Pupil size measurements are affected by up to 10% by pupil position, due to the
optical distortion of the cornea that accompanies rotations of the eye to view the
peripheral parts of the display, and camera-related factors. If you intend to
measure pupil size, the participant should not move their eyes during the trials.
They can be presented with a fixation point with aural stimulus presentation, or
a single stimulus position at display center may be used. It is also possible to
counterbalance stimulus position during the experiment. As well, since pupil
size is largely affected by luminance, cognitive load, and emotional responses,
those factors not being explicitly manipulated must be equated across
conditions to achieve a valid measure of pupil size. Lastly, since pupil size is
recorded in arbitrary units that is not calibrated across participants, measures
of pupil size are best recorded as percent change relative to a baseline period.
4.4.4 Button Data
The state of up to 8 buttons or input port bits may be recorded in each sample.
Button ports, bits, and polarity may be set in the EyeLink 1000 Plus tracker
configuration file BUTTONS.INI.
The button data consists of two 8-bit fields, recorded as a 16-bit number. The
lower 8 bits contain the current status of the 8 buttons (bit = 0 if off, 1 if
pressed). Each of the upper 8 bits will be set to 1 if its button has changed