|
Pupil Size Data
|
Pupil size is measured by the EyeLink system, at up to 2000 samples per second. It may be reported as pupil area, or pupil diameter. The pupil size data are 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 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 ideally 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. Since pupil size is also affected by luminance, cognitive load, and emotional responses, those factors not being explicitly manipulated should also be equated across conditions to achieve a valid measure of pupil size. Lastly, since pupil size is recorded in arbitrary units that are not calibrated across participants, measures of pupil size are best recorded as percent change relative to a baseline period.
For EyeLink 3, pupil ellipse data is also available when using ellipse-fitting pupil tracking algorithm. For both monocular and binocular recordings, EyeLink 3 can provide the X, Y position of the pupil ellipses for the tracked eyes in camera coordinates and the major and minor axis (in camera pixels) of the pupil ellipses. The latter two can be used to derive pupil size in physical units (e.g., in millimeters) by applying a conversion factor. EyeLink 3 also reports the angle (in degrees) between the major-axis and x-axis for the eyes; however, please note that the angle data can be unreliable when the fitted ellipse is very close to a circle. To output pupil ellipse data, add -pe switch when converting the EDF file from the command line prompt, or enable the EDF2ASC converter option "Preferences -> EyeLink 3 -> Output Pupil Ellipse".