Cyclovergence

Cyclovergence is the simultaneous occurring cyclorotation (torsional movement) of both eyes which is performed in opposite directions to obtain or maintain single binocular vision.

Normal cyclovergence and cycloversion

Conjugate cyclorotations of the eye (that is, cyclorotations in the same direction) are called cycloversion.[1] They mainly occur due to Listing's law, which, under normal circumstances, constrains the cyclorotation in dependence on the vertical and horizontal movements of the eye.

Visually evoked cyclovergence

Listing's law, however, does not account for all cyclorotations. In particular, in the presence of cyclodisparity (that is, when two images are presented which would need to be rotated in relation to each other in order to allow visual fusion to take place), the eyes perform cyclovergence, rotating around their gaze directions in opposite directions, as a motor response to cyclodisparity.

Such additional, visually evoked cyclovergence appears to superimpose linearly onto the cycloversion due to Listing's law.[2]

Visually induced cyclovergence of up to 8 degrees has been observed in normal subjects. Together with the 8 degrees that can usually be compensated by sensory means, this means that the normal human observer can achieve binocular image fusion in presence of cyclodisparity (also called orientation disparity in the case of a line image) of up to approximately 16 degrees. Larger cyclodisparity normally results in double vision.[3] It has been shown that the tolerance of human stereopsis to cyclodisparity of lines (orientation disparity) is greater for vertical lines than for horizontal lines.[4]

The visually evoked cyclovergence relaxes once the cyclodisparity is reduced to zero. The effect also relaxes when the eyes are presented with darkness; however, experiments show that in the latter case the cyclovergence does not disappear completely straight away.[5]

Cyclovergence can also be evoked by cyclodisparity of the visual field; the cyclodisparity can be introduced by dove prisms.[6] Here, use is made of the fact that a pair of dove prisms rotate an image optically if they are arranged one after the other and with an angular displacement relative to each other. Conversely, the range of cyclovergence-based cyclofusion can be trained using dove prisms that actively rotate the field of view: "The patient fixates a vertical line target, and the dove prism is rotated in the direction to increase the action of the insufficient muscle while fusion is maintained."[7]

The cyclorotation of the eyes can normally not be performed under voluntary control; nonetheless it is possible to do so after extended practice.[8] Voluntary cyclorotation after extended practice was first demonstrated in 1978.[9][10]

Measurement

It has long been known that the human visual system compensates for cyclical mismatch in such a way that cyclofusion and thereby stereo vision is achieved. There has been agreement on this point since the question was raised[11] in 1891. However, for a long time the mechanism of the compensation was unclear: many thought that cyclofusion was due exclusively to high-level processing of the visual images, while others suggested a motor cyclovergence response. In 1975, motor cyclovergence was demonstrated for the first time with photographic methods.[12]

Cyclovergence, and more generally torsional eye positions, can be measured using scleral coils or using video-oculography. Torsional eye positions can also be measured using fundus cyclometry, which is based on infrared scanning laser ophthalmoscopy.[13]

There have been contradictory statements on whether cyclovergence can be measured subjectively, that is, by an evaluation of the subjects' own statements on whether lines in a scene appear at an angle in the two eyes. Recent evidence based on an analysis of the empirical horopter suggests that subjective estimates of cyclovergence are accurate if they are performed using horizontal lines to the left and to the right of the fixation, not vertical lines above and below it which would be affected by shear of retinal correspondence points.[14]

See also

References
  1. Laurence Harris; Michael Jenkin (1993). Spatial Vision in Humans and Robots: The Proceedings of the 1991 York Conference on Spatial Vision in Humans and Robots. Cambridge University Press. p. 349. ISBN 978-0-521-43071-5. Retrieved 8 July 2013.
  2. Hooge, IT.; van den Berg, AV. (May 2000). "Visually evoked cyclovergence and extended listing's law". J Neurophysiol. 83 (5): 2757–75. doi:10.1152/jn.2000.83.5.2757. PMID 10805674.
  3. Arthur Lewis Rosenbaum; Alvina Pauline Santiago (1999). Clinical Strabismus Management: Principles and Surgical Techniques. David Hunter. p. 63. ISBN 978-0-7216-7673-9. Retrieved 8 July 2013.
  4. Philip M Grove; Hiroshi Ono (2012). "Horizontal/vertical differences in range and upper/lower visual field differences in the midpoints of sensory fusion limits of oriented lines". Perception. 41 (8): 939–949. doi:10.1068/p7091. PMID 23362671. S2CID 22083757.
  5. Matthew J. Taylor; Dale C. Roberts; David S. Zee (April 2000). "Effect of Sustained Cyclovergence on Eye Alignment: Rapid Torsional Phoria Adaptation". Investigative Ophthalmology & Visual Science. Vol. 41, no. 5. pp. 1076–1083. Archived from the original on 2014-05-12. Retrieved 2013-07-08.
  6. J.S. Maxwell; C.M. Schor (1999). "Adaptation of torsional eye alignment in relation to head roll". Vision Res. 39 (25): 4192–4199. doi:10.1016/s0042-6989(99)00139-x. PMID 10755157. S2CID 11043880.
  7. Mitchell Scheiman; Bruce Wick (2008). Clinical Management of Binocular Vision: Heterophoric, Accommodative, and Eye Movement Disorders. Lippincott Williams & Wilkins. p. 432. ISBN 978-0-7817-7784-1. Retrieved 22 July 2013.
  8. Ian P. Howard Center for Vision Research York University; Brian J. Rogers Department of Experimental Psychology Oxford University (30 November 1995). Binocular Vision and Stereopsis. Oxford University Press. p. 417. ISBN 978-0-19-802461-3. Retrieved 29 July 2013.
  9. Balliet, R.; Nakayama, K. (1978). "Trained Human Voluntary Torsion". Augenbewegungsstörungen / Disorders of Ocular Motility. Symposien der Deutschen Ophthalmologischen Gesellschaft. pp. 221–227. doi:10.1007/978-3-642-48446-9_33. ISBN 978-3-8070-0303-0.
  10. Training of voluntary torsion Archived 2014-09-17 at archive.today, Invest. Ophthalmol. Vis. Sci. April 1978 vol. 17 no. 4 303–314 (full text)
  11. Nagel, 1891, cited after: Hooten, Kenneth; Myers, Earl; Worrall, Russell; Stark, Lawrence (1979). "Cyclovergence: The motor response to cyclodisparity". Albrecht von Graefes Archiv für Klinische und Experimentelle Ophthalmologie. 210 (1): 65–68. doi:10.1007/BF00414790. PMID 312031. S2CID 1789329.
  12. Crone, R. A.; Everhard-Halm, Y. (1975). "Optically induced eye torsion". Albrecht von Graefes Archiv für Klinische und Experimentelle Ophthalmologie. 195 (4): 231–239. doi:10.1007/BF00414936. PMID 1080373.; also as cited in: Hooten, Kenneth; Myers, Earl; Worrall, Russell; Stark, Lawrence (1979). "Cyclovergence: The motor response to cyclodisparity". Albrecht von Graefes Archiv für Klinische und Experimentelle Ophthalmologie. 210 (1): 65–68. doi:10.1007/BF00414790. PMID 312031. S2CID 1789329.
  13. Ehrt, Oliver; Boergen, Klaus-Peter (2001). "Scanning laser ophthalmoscope fundus cyclometry in near-natural viewing conditions". Graefe's Archive for Clinical and Experimental Ophthalmology. 239 (9): 678–682. doi:10.1007/s004170100347. PMID 11688667. S2CID 30275189.
  14. Emily A. Cooper; Johannes Burge; Martin S. Banks (28 March 2011). "The vertical horopter is not adaptible, but it may be adaptive". Journal of Vision. 11 (3, article 20): 20. doi:10.1167/11.3.20. PMC 3804431. PMID 21447644. Archived from the original on 19 July 2013. Retrieved 19 July 2013. See page 16.

Further reading
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