Editing Eye tracking (section)

== Tracker types ==
Eye-trackers measure rotations of the eye in one of several ways, but principally they fall into one of three categories: 
# measurement of the movement of an object (normally, a special contact lens) attached to the eye
# optical tracking without direct contact to the eye
# measurement of electric potentials using electrodes placed around the eyes.

=== Eye-attached tracking ===
The first type uses an attachment to the eye, such as a special contact lens with an embedded mirror or magnetic field sensor, and the movement of the attachment is measured with the assumption that it does not slip significantly as the eye rotates. Measurements with tight-fitting contact lenses have provided extremely sensitive recordings of eye movement, and magnetic search coils are the method of choice for researchers studying the dynamics and underlying physiology of eye movement. This method allows the measurement of eye movement in horizontal, vertical and torsion directions.<ref>{{cite journal | first=David A. | last=Robinson | title=A Method of Measuring Eye Movemnent Using a Scieral Search Coil in a Magnetic Field | journal=IEEE Transactions on Bio-medical Electronics | publisher=Institute of Electrical and Electronics Engineers | volume=10 | issue=4 | date=October 1963 | issn=0096-0616 | doi=10.1109/tbmel.1963.4322822 | pmid=14121113 | pages=137–145 }}</ref>

=== Optical tracking ===
{{See also|Video-oculography}}
[[File:EYE-SYNC eye-tracking analyzer.JPG|thumb|An eye-tracking [[head-mounted display]]. Each eye has an LED light source (gold-color metal) on the side of the display lens, and a camera under the display lens.]]

The second broad category uses some non-contact, optical method for measuring eye motion. Light, typically infrared, is reflected from the eye and sensed by a video camera or some other specially designed optical sensor. The information is then analyzed to extract eye rotation from changes in reflections. Video-based eye trackers typically use the corneal reflection (the first [[Purkinje images|Purkinje image]]) and the center of the pupil as features to track over time. A more sensitive type of eye-tracker, the dual-Purkinje eye tracker,<ref>{{cite journal|doi=10.1364/AO.24.000527|pmid=18216982|last=Crane|first=H.D.|author2=Steele, C.M.|s2cid=10595433|title=Generation-V dual-Purkinje-image eyetracker|journal=Applied Optics|volume=24|issue=4|pages=527–537|year=1985|bibcode=1985ApOpt..24..527C}}</ref> uses reflections from the front of the cornea (first Purkinje image) and the back of the lens (fourth Purkinje image) as features to track. A still more sensitive method of tracking is to image features from inside the eye, such as the retinal blood vessels, and follow these features as the eye rotates. Optical methods, particularly those based on video recording, are widely used for gaze-tracking and are favored for being non-invasive and inexpensive.

=== Electric potential measurement ===
The third category uses electric potentials measured with electrodes placed around the eyes. The eyes are the origin of a steady electric potential field which can also be detected in total darkness and if the eyes are closed. It can be modelled to be generated by a dipole with its positive pole at the cornea and its negative pole at the retina. The electric signal that can be derived using two pairs of contact electrodes placed on the skin around one eye is called [[EOG|Electrooculogram (EOG)]]. If the eyes move from the centre position towards the periphery, the retina approaches one electrode while the cornea approaches the opposing one. This change in the orientation of the dipole and consequently the electric potential field results in a change in the measured EOG signal. Inversely, by analysing these changes in eye movement can be tracked. Due to the discretisation given by the common electrode setup, two separate movement components – a horizontal and a vertical – can be identified. A third EOG component is the radial EOG channel,<ref>Elbert, T., Lutzenberger, W., Rockstroh, B., Birbaumer, N., 1985. Removal of ocular artifacts from the EEG. A biophysical approach to the EOG. Electroencephalogr Clin Neurophysiol 60, 455-463.</ref> which is the average of the EOG channels referenced to some posterior scalp electrode. This radial EOG channel is sensitive to the saccadic spike potentials stemming from the extra-ocular muscles at the onset of saccades, and allows reliable detection of even miniature saccades.<ref>{{cite journal | last1 = Keren | first1 = A.S. | last2 = Yuval-Greenberg | first2 = S. | last3 = Deouell | first3 = L.Y. | year = 2010 | title = Saccadic spike potentials in gamma-band EEG: Characterization, detection and suppression | journal = NeuroImage | volume = 49 | issue = 3| pages = 2248–2263 | doi=10.1016/j.neuroimage.2009.10.057 | pmid=19874901| s2cid = 7106696 }}</ref>

Due to potential drifts and variable relations between the EOG signal amplitudes and the saccade sizes, it is challenging to use EOG for measuring slow eye movement and detecting gaze direction. EOG is, however, a very robust technique for measuring [[saccade|saccadic eye movement]] associated with gaze shifts and detecting [[blink]]s. 
Contrary to video-based eye-trackers, EOG allows recording of eye movements even with eyes closed, and can thus be used in sleep research. It is a very light-weight approach that, in contrast to current video-based eye-trackers, requires low computational power, works under different lighting conditions and can be implemented as an embedded, self-contained [[wearable technology|wearable]] system.<ref>{{cite journal|last=Bulling|first=A. |author2=Roggen, D. |author3=Tröster, G.|year=2009|title=Wearable EOG goggles: Seamless sensing and context-awareness in everyday environments|journal= Journal of Ambient Intelligence and Smart Environments|volume=1|pages=157–171|issue=2|doi=10.3233/AIS-2009-0020|hdl=20.500.11850/352886 |s2cid=18423163 |hdl-access=free}}</ref><ref>Sopic, D., Aminifar, A., & Atienza, D. (2018). e-glass: A wearable system for real-time detection of epileptic seizures. In IEEE International Symposium on Circuits and Systems (ISCAS).</ref> It is thus the method of choice for measuring eye movement in mobile daily-life situations and [[rapid eye movement sleep|REM]] phases during sleep.  The major disadvantage of EOG is its relatively poor gaze-direction accuracy compared to a video tracker.  That is, it is difficult to determine with good accuracy exactly where a subject is looking, though the time of eye movements can be determined.