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A saccade is a quick, simultaneous movement of both eyes between two or more phases of fixation in the same direction. In contrast, in smooth pursuit movements, the eyes move smoothly instead of in jumps. The phenomenon can be associated with a shift in frequency of an emitted signal or a movement of a body part or device. Controlled cortically by the frontal eye fields (FEF), or subcortically by the superior colliculus, saccades serve as a mechanism for fixation, rapid eye movement, and the fast phase of optokinetic nystagmus. The word appears to have been coined in the 1880s by French ophthalmologist Émile Javal, who used a mirror on one side of a page to observe eye movement in silent reading, and found that it involves a succession of discontinuous individual movements.
In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Binocular vision does not typically refer to vision where an animal has eyes on opposite sides of its head and shares no field of view between them, like in some animals.
Depth perception is the ability to perceive distance to objects in the world using the visual system and visual perception. It is a major factor in perceiving the world in three dimensions. Depth perception happens primarily due to stereopsis and accommodation of the eye.
Strabismus is a vision disorder in which the eyes do not properly align with each other when looking at an object. The eye that is focused on an object can alternate. The condition may be present occasionally or constantly. If present during a large part of childhood, it may result in amblyopia, or lazy eyes, and loss of depth perception. If onset is during adulthood, it is more likely to result in double vision.
Binocular rivalry is a phenomenon of visual perception in which perception alternates between different images presented to each eye.
Stereopsis is the component of depth perception retrieved through binocular vision. Stereopsis is not the only contributor to depth perception, but it is a major one. Binocular vision happens because each eye receives a different image because they are in slightly different positions on one’s head. These positional differences are referred to as "horizontal disparities" or, more generally, "binocular disparities". Disparities are processed in the visual cortex of the brain to yield depth perception. While binocular disparities are naturally present when viewing a real three-dimensional scene with two eyes, they can also be simulated by artificially presenting two different images separately to each eye using a method called stereoscopy. The perception of depth in such cases is also referred to as "stereoscopic depth".
A vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision.
Congenital fourth nerve palsy is a condition present at birth characterized by a vertical misalignment of the eyes due to a weakness or paralysis of the superior oblique muscle.
Fixation disparity is a tendency of the eyes to drift in the direction of the heterophoria. While the heterophoria refers to a fusion-free vergence state, the fixation disparity refers to a small misalignment of the visual axes when both eyes are open in an observer with normal fusion and binocular vision. The misalignment may be vertical, horizontal or both. The misalignment is much smaller than that of strabismus. While strabismus prevents binocular vision, fixation disparity keeps binocular vision, however it may reduce a patient's level of stereopsis. A patient may or may not have fixation disparity and a patient may have a different fixation disparity at distance than near. Observers with a fixation disparity are more likely to report eye strain in demanding visual tasks; therefore, tests of fixation disparity belong to the diagnostic tools used by eye care professionals: remediation includes vision therapy, prism eye glasses, or visual ergonomics at the workplace.
Strabismus surgery is surgery on the extraocular muscles to correct strabismus, the misalignment of the eyes. Strabismus surgery is a one-day procedure that is usually performed under general anesthesia most commonly by either a neuro- or pediatric ophthalmologist. The patient spends only a few hours in the hospital with minimal preoperative preparation. After surgery, the patient should expect soreness and redness but is generally free to return home.
Binocular disparity refers to the difference in image location of an object seen by the left and right eyes, resulting from the eyes’ horizontal separation (parallax). The brain uses binocular disparity to extract depth information from the two-dimensional retinal images in stereopsis. In computer vision, binocular disparity refers to the difference in coordinates of similar features within two stereo images.
Heterophoria is an eye condition in which the directions that the eyes are pointing at rest position, when not performing binocular fusion, are not the same as each other, or, "not straight". This condition can be esophoria, where the eyes tend to cross inward in the absence of fusion; exophoria, in which they diverge; or hyperphoria, in which one eye points up or down relative to the other. Phorias are known as 'latent squint' because the tendency of the eyes to deviate is kept latent by fusion. A person with two normal eyes has single vision (usually) because of the combined use of the sensory and motor systems. The motor system acts to point both eyes at the target of interest; any offset is detected visually. Heterophoria only occurs during dissociation of the left eye and right eye, when fusion of the eyes is absent. If you cover one eye you remove the sensory information about the eye's position in the orbit. Without this, there is no stimulus to binocular fusion, and the eye will move to a position of "rest". The difference between this position, and where it would be were the eye uncovered, is the heterophoria. The opposite of heterophoria, where the eyes are straight when relaxed and not fusing, is called orthophoria.
Listing's law, named after German mathematician Johann Benedict Listing (1808–1882), describes the three-dimensional orientation of the eye and its axes of rotation. Listing's law has been shown to hold when the head is stationary and upright and gaze is directed toward far targets, i.e., when the eyes are either fixating, making saccades, or pursuing moving visual targets.
Binocular summation refers to the improved visual performance of binocular vision compared to that of monocular vision. The most vital benefit of binocular vision is stereopsis or depth perception, however binocular summation does afford some subtle advantages as well. By combining the information received in each eye, binocular summation can improve visual acuity, contrast sensitivity, flicker perception, and brightness perception. Though binocular summation generally enhances binocular vision, it can worsen binocular vision relative to monocular vision under certain conditions. Binocular summation decreases with age and when large interocular differences are present.
Cyclodisparity refers to the difference in the rotation angle of an object or scene viewed by the left and right eyes. Cyclodisparity can result from the eyes' torsional rotation (cyclorotation) or can be created artificially by presenting to the eyes two images that need to be rotated relative to each other for binocular fusion to take place.
Cyclotropia is a form of strabismus in which, compared to the correct positioning of the eyes, there is a torsion of one eye about the eye's visual axis. Consequently, the visual fields of the two eyes appear tilted relative to each other. The corresponding latent condition – a condition in which torsion occurs only in the absence of appropriate visual stimuli – is called cyclophoria.
The Maddox rod test can be used to subjectively detect and measure a latent, manifest, horizontal or vertical strabismus for near and distance. The test is based on the principle of diplopic projection. Dissociation of the deviation is brought about by presenting a red line image to one eye and a white light to the other, while prisms are used to superimpose these and effectively measure the angle of deviation. The strength of the prism is increased until the streak of the light passes through the centre of the prism, as the strength of the prism indicates the amount of deviation present. The Maddox rod is a handheld instrument composed of red parallel plano convex cylinder lens, which refracts light rays so that a point source of light is seen as a line or streak of light. Due to the optical properties, the streak of light is seen perpendicular to the axis of the cylinder.
The prism fusion range (PFR) or fusional vergence amplitude is a clinical eye test performed by orthoptists, optometrists, and ophthalmologists to assess motor fusion, specifically the extent to which a patient can maintain binocular single vision (BSV) in the presence of increasing vergence demands. Motor fusion is largely accounted to amplitudes of fusional vergences and relative fusional vergences. Fusional vergence is the maximum vergence movement enabling BSV and the limit is at the point of diplopia. Relative fusional vergence is the maximum vergence movement enabling a patient to see a comfortable clear image and the limit is represented by the first point of blur. These motor fusion functions should fall within average values so that BSV can be comfortably achieved. Excessive stress on the vergence system or inability to converge or diverge adequately can lead to asthenopic symptoms, which generally result from decompensation of latent deviations (heterophoria) or loss of control of ocular misalignments. Motor anomalies can be managed in various ways, however, in order to commence treatment, motor fusion testing such as the PFR is required.
The FourPrism Dioptre Reflex Test is an objective, non-dissociative test used to prove the alignment of both eyes by assessing motor fusion. Through the use of a 4 dioptre base out prism, diplopia is induced which is the driving force for the eyes to change fixation and therefore re-gain bifoveal fixation meaning, they overcome that amount of power.
The Hering–Hillebrand deviation describes the mismatch between the theoretical and empirical horopter. The horopter is the set of points that projects at the same location in the two retinae. Geometrically the horopter is a circle passing through the nodal point of the two eyes and through the fixation point. This is known as the horizontal geometrical horopter, or as the Vieth–Müller circle. This is the set of points that correspond geometrically to the intersection between visual lines at identical eccentricities. There is also a vertical horopter which the a straight line on the sagittal plane and passing through the intersection between the sagittal plane and the Vieth–Müller circle.
References
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ J.S. Maxwell; C.M. Schor (1999). "Adaptation of torsional eye alignment in relation to head roll". Vision Res. Vol. 39, no. 25. pp. 4192–4199. PMID 10755157.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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)
- ↑ Nagel, 1891, cited after: Kenneth Hooten; Earl Myers; Russell Worrall; Lawrence Stark (1979). Springer (ed.). "Cyclovergence: the motor response to disparity". Albrecht V. Graefes Archiv für Klinische und Experimentelle Ophthalmologie. No. 210. pp. 65–68.
- ↑ R.A. Crone; Y. Everhard-Halm (1975). Albrecht von Graefes Archiv für klinische und experimentelle Ophthalmologie, 4. VII. (ed.). "Optically induced eye torsion". Vol. 195, no. 4. pp. 231–239.; also as cited in: Kenneth Hooten; Earl Myers; Russell Worrall; Lawrence Stark (1979). Springer (ed.). "Cyclovergence: the motor response to disparity". Albrecht V. Graefes Archiv für Klinische und Experimentelle Ophthalmologie. No. 210. pp. 65–68.
- ↑ Oliver Ehrt; Klaus-Peter Boergen (September 2001). "Scanning laser ophthalmoscope fundus cyclometry in near-natural viewing conditions". Graefe's Archive for Clinical and Experimental Ophthalmology. Vol. 239, no. 9. pp. 678–682.
- ↑ 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 . PMID 21447644. Archived from the original on 19 July 2013. Retrieved 19 July 2013. See page 16.
