Listing's law
In this coordinate system, Listing's law simply states that the torsional component of eye orientation is held at zero.
(Note that this is not the same description of ocular torsion as rotation around the line of sight: whereas movements that start or end at the primary position can indeed be performed without any rotation about the line of sight, this is not the case for arbitrary movements.)
The line of sight is typically horizontal, but does not necessarily point straight ahead (perpendicular to the coronal plane).
Also, within each subject, the primary position tilts towards the temples when viewing distant objects due to vergence.
This generally results in slow movements that drive the eye torsionally out of Listing's plane.
However, when the head translates without rotating, gaze direction remains stable but Listing's law is still maintained.
Listing's law persists if a torsional bias is added, when the head is held at a tilted posture and the eyes counter-roll, and when the head is held steady upward or downward Listing's plane tilts slightly in the opposite direction.
[clarification needed] Perfect VOR would stabilize retinal image but cause violation to Listing's law, As a compromise, eye motion follows the half-Listing's law strategy, where instead of following the Listing's half-angle rule (a geometric consequence of Listing's rule), eyes react to head motion in VOR by rotating around a modified velocity plane.
The data can be explained by assuming that the eyes take the fastest possible path to their final orientation, with no constraints on torsion, except that it stays less than 15 degrees.
[1] Listing's law can be violated in neurological conditions, such as acute unilateral fourth nerve palsy.
The adaptation fails under central fascicular palsy, as even chronic patients suffer from deviation from Listing's law.
[1] While Listing's law holds only for eyes that fixate a distant point (at optical infinity), it has been extended to include also vergence.
From this binocular extension of Listing's law, it follows that vergence can lead to a change of cyclotorsion.
For example, since Listing's law defines torsion as zero about a head-fixed axis, this results in "false torsional" tilts about the line of sight when the eye is at tertiary (oblique) positions, which the brain must compensate for when interpreting the visual image.
The binocular version of Listing's law is thought to be a best compromise to simplify this problem, although it does not completely rid the visual system of the need to know current eye orientation.
However, the accumulated evidence suggests that both factors play a role in the implementation of different aspects of Listing's law.
The horizontal recti muscles of the eyes only contribute to horizontal eye rotation and position, but the vertical recti and oblique muscles each have approximately equal vertical and torsional actions (in Listing's plane coordinates).
Thus, to hold eye position in Listing's plane, there needs to be a balance of activation between these muscles so that torsion cancels to zero.
The eye muscles may also contribute to Listing's law by having position-dependent pulling directions during motion, i.e., this might be the mechanism that implements the "half-angle rule" described above.
However, the brainstem reticular formation centers that control vertical eye position (the interstitial nucleus of Cajal; INC) and saccade velocity (the rostral interstitial nucleus of the medial longitudinal fasciculus; riMLF) are equally involved in torsional control, each being divided into populations of neurons that control directions similar to those of the vertical and torsional pulling eye muscles.
Damage to any of the physiology described above can disrupt Listing's law and thus have negative impacts for vision.
The influence of strabismus surgery on the Listing's planes of the two eyes is not fully understood.
[10] The orientation of Listing's plane (equivalently, the location of the primary position) of an individual can be measured using scleral coils.
Listing's law was first measured directly, with the use of 3D eye coils in the 1980s by Ferman, Collewijn and colleagues.
Demer and Miller have championed the role of eye muscles, whereas Crawford and colleagues worked out several of the neural mechanisms described above over the past two decades.
