User manual

92 Data Files

4.4.2.4

display coordinates (usually pixels) which can be set in the EyeLink 1000

configuration file PHYSICAL.INI. The default EyeLink coordinates are those of a

1024 by 768 VGA display, with (0, 0) at the top left.

The resolution data for gaze position data changes constantly depending on

subject head position and point of gaze, and therefore is reported as a separate

data type (see below). A typical resolution is about 36 pixels per degree for the

suggested EyeLink 1000 setup, with the distance between the subject's eyes

and the display being twice the display's width, and with a default 1024 by 768

screen resolution.

The high resolution of the EyeLink 1000 data is preserved by multiplying the

position by a prescaler, recording the value as an integer in the EDF file, then

dividing by the prescaler when the file is read. The usual prescaler value is 10,

allowing gaze position to be recorded with 0.1 pixel of resolution. Actual

EyeLink 1000 resolution is limited only by measurement noise.

Gaze Resolution Data

For gaze position, unlike the HREF data, the relationship between visual angle

and gaze position is not constant. The EyeLink 1000 tracker computes and can

record the instantaneous angular resolution at the current point of gaze. This is

measured as the units (usually pixels) per degree of visual angle, computed for

a change in x and y position separately.

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 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 subject 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.