Editing Eye tracking (section)

== Eye-tracking vs. gaze-tracking ==
<!-- Deleted image removed: [[File:Gaze Tracking Example.jpg|thumb|left|An example of eye gaze trace overlay.]] -->Eye-trackers necessarily measure the rotation of the eye with respect to some frame of reference. This is usually tied to the measuring system. Thus, if the measuring system is head-mounted, as with EOG or a video-based system mounted to a helmet, then eye-in-head angles are measured. To deduce the line of sight in world coordinates, the head must be kept in a constant position or its movements must be tracked as well. In these cases, head direction is added to eye-in-head direction to determine gaze direction.
However, if the motion of the head is minor, the eye remains in constant position.<ref>{{cite journal|vauthors=Aharonson V, Coopoo V, Govender K, Postema M|title=Automatic pupil detection and gaze estimation using the vestibulo-ocular reflex in a low-cost eye-tracking setup|journal=SAIEE Africa Research Journal|year=2020|volume=111|issue=3|pages=120–124|doi=10.23919/SAIEE.2020.9142605|doi-access=free}}</ref>

If the measuring system is table-mounted, as with scleral search coils or table-mounted camera (remote) systems, then gaze angles are measured directly in world coordinates. Typically, in these situations head movements are prohibited. For example, the head position is fixed using a bite bar or a forehead support. Then a head-centered reference frame is identical to a world-centered reference frame. Or colloquially, the eye-in-head position directly determines the gaze direction.

Some results are available on human eye movements under natural conditions where head movements are allowed as well.<ref>{{cite journal | last1 = Einhäuser | first1 = W | last2 = Schumann | first2 = F | last3 = Bardins | first3 = S | last4 = Bartl | first4 = K | last5 = Böning | first5 = G | last6 = Schneider | first6 = E | last7 = König | first7 = P | year = 2007 | title = Human eye-head co-ordination in natural exploration | journal = Network: Computation in Neural Systems | volume = 18 | issue = 3| pages = 267–297 | doi = 10.1080/09548980701671094 | pmid = 17926195 | s2cid = 1812177 }}</ref> The relative position of eye and head, even with constant gaze direction, influences neuronal activity in higher visual areas.<ref>{{cite journal | last1 = Andersen | first1 = R. A. | last2 = Bracewell | first2 = R. M. | last3 = Barash | first3 = S. | last4 = Gnadt | first4 = J. W. | last5 = Fogassi | first5 = L. | year = 1990 | title = Eye position effects on visual, memory, and saccade-related activity in areas LIP and 7a of macaque | journal = Journal of Neuroscience | volume = 10 | issue = 4| pages = 1176–1196 | doi = 10.1523/JNEUROSCI.10-04-01176.1990 | pmid = 2329374 | pmc = 6570201 | s2cid = 18817768 }}</ref>