Importing Data from Eyetracking devices

Eyetracking devices record a persons point of gaze, usually in relation to a screen. Typically, gaze position (also referred to as eye or pupil position) and pupil size are recorded as separate channels. This section describes how to read data from supported eyetracking manufacturers.

MNE-Python provides functions for reading eyetracking data. When possible, MNE-Python will internally convert and store eyetracking data according to an SI unit (for example radians for position data, and meters for pupil size).

Note

If you have eye tracking data in a format that MNE does not support yet, you can try reading it using other tools and create an MNE object from a numpy array. Then you can use mne.preprocessing.eyetracking.set_channel_types_eyetrack() to assign the correct eyetrack channel types.

See also

Some MNE functions may not be available to eyetracking and other physiological data, because MNE does not consider them to be data channels. See the Glossary of Common Terms and API Elements for more information.

SR Research (Eyelink) (.asc)

Note

MNE-Python currently only supports reading Eyelink eyetracking data stored in the ASCII (.asc) format.

Eyelink recordings are stored in the Eyelink Data Format (EDF; .edf), which are binary files and thus relatively complex to support. To make the data in EDF files accessible, Eyelink provides the application EDF2ASC, which converts EDF files to a plain text ASCII format (.asc). These files can be imported into MNE using mne.io.read_raw_eyelink().

Note

The Eyelink Data Format (EDF), should not be confused with the European Data Format, the common EEG data format that also uses the .edf extension.

Supported measurement types from Eyelink files include eye position, pupil size, saccadic velocity, resolution, and head position (for recordings collected in remote mode). Eyelink files often report ocular events (blinks, saccades, and fixations), MNE will store these events as mne.Annotations. Blink annotation descriptions will be 'BAD_blink'. For more information on the various measurement types that can be present in Eyelink files. read below.

Eye Position Data

Eyelink samples can report eye position data in pixels, units of visual degrees, or as raw pupil coordinates. Samples are written as (x, y) coordinate pairs (or two pairs for binocular data). The type of position data present in an ASCII file will be detected automatically by MNE. The three types of position data are explained below.

Gaze

Gaze position data report the estimated (x, y) pixel coordinates of the participants’s gaze on the stimulus screen, compensating for head position changes and distance from the screen. This datatype may be preferable if you are interested in knowing where the participant was looking at on the stimulus screen. The default (0, 0) location for Eyelink systems is at the top left of the screen.

This may be best demonstrated with an example. In the file plotted below, eyetracking data was recorded while the participant read text on a display. In this file, as the participant read the each line from left to right, the x-coordinate increased. When the participant moved their gaze down to read a new line, the y-coordinate increased, which is why the ypos_right channel in the plot below increases over time (for example, at about 4-seconds, and at about 8-seconds).

See also

Working with eye tracker data in MNE-Python

import mne

fpath = mne.datasets.misc.data_path() / "eyetracking" / "eyelink"
fname = fpath / "px_textpage_ws.asc"
raw = mne.io.read_raw_eyelink(fname, create_annotations=["blinks"])
cal = mne.preprocessing.eyetracking.read_eyelink_calibration(
    fname,
    screen_distance=0.7,
    screen_size=(0.53, 0.3),
    screen_resolution=(1920, 1080),
)[0]
mne.preprocessing.eyetracking.convert_units(raw, calibration=cal, to="radians")

Visualizing the data

cal.plot()

custom_scalings = dict(pupil=1e3)
raw.pick(picks="eyetrack").plot(scalings=custom_scalings)

Note that we passed a custom dict to the 'scalings' argument of mne.io.Raw.plot. This is because MNE expects the data to be in SI units (radians for eyegaze data, and meters for pupil size data), but we did not convert the pupil size data in this example.

Head-Referenced Eye Angle (HREF)

HREF position data 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 stimulus screen. This datatype might be preferable for analyses that are interested in eye movement velocities and amplitudes, or for simultaneous and EEG/MEG eyetracking recordings where eye position data are used to identify EOG artifacts.

HREF coordinates are stored in the ASCII file as integer values, with 260 or more units per visual degree, however MNE will convert and store these coordinates in radians. The (0, 0) point of HREF data is arbitrary, as the relationship between the screen position and the coordinates changes as the subject’s head moves.

Below is the same text reading recording that we plotted above, except a new ASCII file was generated, this time using HREF eye position data.

fpath = mne.datasets.misc.data_path() / "eyetracking" / "eyelink"
fname_href = fpath / "HREF_textpage_ws.asc"
raw = mne.io.read_raw_eyelink(fname_href, create_annotations=["blinks"])
custom_scalings = dict(pupil=1e3)
raw.pick(picks="eyetrack").plot(scalings=custom_scalings)

Pupil Position

Pupil position data contains (x, y) coordinate pairs from the eye camera. It has not been converted to pixels (gaze) or eye angles (HREF). Most use cases do not require this data type, and caution should be taken when analyzing raw pupil positions. Note that when plotting data from a Raw object containing raw pupil position data, the plot scalings will likely be incorrect. You can pass custom scalings into the scalings parameter of mne.io.Raw.plot so that the signals are legible when plotting.

Warning

If a calibration was not performed prior to data collection, the EyeLink system cannot convert raw pupil position data to pixels (gaze) or eye angle (HREF).

Pupil Size Data

Pupil size is measured by the EyeLink system at up to 500 samples per second. It may be reported as pupil area, or pupil diameter (i.e. the diameter of a circle/ellipse model fit to the pupil area). Which of these datatypes you get is specified by your recording- and/or your EDF2ASC settings. The pupil size data is not calibrated and reported in arbitrary units. Typical pupil area data range between 800 to 2000 units, with a precision of 1 unit, while pupil diameter data range between 1800-3000 units.

Velocity, resolution, and head position data

Eyelink files can produce data on saccadic velocity, resolution, and head position for each sample in the file. MNE will read in these data if they are present in the file, but will label their channel types as 'misc'.

Warning

Eyelink’s EDF2ASC API allows for modification of the data and format that is converted to ASCII. However, MNE-Python assumes a specific structure, which the default parameters of EDF2ASC follow. ASCII files should be tab-deliminted, and both Samples and Events should be output. If the data were recorded at 2000Hz, timestamps should be floating point numbers. Manual modification of ASCII conversion via EDF2ASC is not recommended.

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