User manual

90 Data Files

4.4.2.1 PUPIL

4.4.2.2 HREF

Each sample may contain several data field, including:

• Time of the sample (timestamp) in milliseconds

• eye position data in gaze, HREF, or RAW data, monocular or binocular

• Pupil size, monocular or binocular

• 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 1000 tracker are

recorded. Other types of sample data are discussed in greater detail below.

4.4.2 Position Data

Eye position data is produced by the EyeLink 1000 tracker every 0.5, 1, 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.

Pupil position data is raw (x, y) coordinate pairs from the camera. 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.

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: