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Directional vision
Physiological process of seeing

Directional vision refers to how stimulation of a point on the retina results not only in a light sensation but also a directional sensation, forming a combined image centered egocentrically between both eyes. The 11th-century Arab scholar Alhazen first proposed that vision occurs because light reflects off objects into the eyes, with perception involving the observer’s eye movements. In the 19th century, Ewald Hering expanded this by introducing the concept of a “cyclopean eye,” a single point between the two eyes from which direction is perceived. Research on the horopter helped explain how single and double images arise, and how sensory fusion merges two images into one with a new direction, based on simultaneous nerve cell activation in the brain. Depth perception through disparity is explored separately in Stereopsis.

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Visual direction

Each eye has a small area, the fovea (f), with which it can see most sharply. When seeing, this area is automatically focused on the point that has the attention, the fixation point. The direction in which the eye then looks is called the principal oculocentric direction by Hering.

If a point on the retina is stimulated by light, this not only gives a light sensation, but also a directional sensation. This direction is called visual direction and is expressed in the angle with the principal direction.

Single image and double image

Double image

If two eyes simultaneously see a point P in space, the brain receives information about two visual directions that, due to the positioning of the eyes in the horizontal plane, lie next to each other. The brain interprets this as seeing two objects next to each other. These are called double images.

Singel image

In case the visual directions in teh two eyes are equal, the double images seem to coincide with each other. This is called a single image.

Because both eyes look at space from different positions, the visual directions in the two eyes are unequal for most points in space. If you pay attention, you can see these two different directions. Normally double images are invisible because we are not aware of them, unless there is a deviation such as diplopia.

Egocentric direction

Hering described in 1861 that we seem to perceive the world from a point, midway between both eyes, instead of from each eye separately.3 He called this point a cyclopean eye, after the cyplopes in Greek mythology. Hering also described a method to show how the two retinal images in this cyclopean eye are apparently merged into a combined image. He called this method cyclopean projection. He illustrated this method with a pencil held so that it points away from the observer, see the figure. The method basically involves rotating the images of the two eyes around the fixation point towards each other until they coincide in the cyclopean eye.

Horopter

Hering's Cyclopean projection is intended as a description of what he observed and not as an explanation of how the egocentric image is formed in the brain. How the images from the two eyes are merged has been extensively investigated using a theoretical model, the geometric horopter. The geometric horopter is a circle through the fixation point and both eyes. Points on this circle have been shown to project (approximately) onto corresponding points in both eyes. These are points that look in the same visual direction in both eyes.4 In empirical research it has been established that it is possible to measure and compare the visual directions in both eyes and thus determine the observed horopter (empirical horopter). It turns out that this horopter is not a line but a vertical (frontoparallel) plane through the fixation point and that the shape of this plane differs per person. At short fixation distances the plane as seen from the observer is concave (hollow), at slightly greater distances flatter and at greater distances flat or convex. Furthermore, the plane curves towards the observer at the top and away from the observer at the bottom. It is generally assumed that these are adaptations to the environment, including how the eyes move in the recesses in the skull.56

Horopter as a screen

In early theories, the geometric horopter is presented as a screen on which the visual world is depicted. Points that are closer (C) or further away (P) hit the screen at two different points.

Although the above-mentioned screen does not actually exist, it does provide a correct description of how double images are seen. In the case where the directions of both eyes cross before the horopter (point C), the left double image comes from the right eye and vice versa. This situation is known as "crossed disparity". At a point that is further away than the horopter (P), the image of the left eye is seen to the left of the image of the right eye. Researchers call this "parallel disparity". And at a point on the horopter, the two directions coincide and so only 1 image is seen.

Disparate points

Disparate points are retinal points with a different directional characteristic in both eyes, which therefore look in different visual directions. The size of the difference is expressed as an angle difference, or in a distance on the retina or on the horopter. The angle difference of the two visual directions is called disparity. Disparity is a measure of the distance between two double images and is also a measure of depth (stereopsis).

Horopter with cyclopean eye

Hering's view can be combined with the results of the empirical research on the horopter by adding a cyclopean eye. This also allows the egocentric direction to be drawn.

The procedure for this is as follows: draw the principal direction of the cyclopean eye by drawing a line from the cyclopean eye to the fixation point. Then place the left eye on the cyclopean eye and rotate it so that the gaze direction of the left eye coincides with the principal direction of the cyclopean eye. Repeat this for the right eye.

Points on (the distance of) the horopter now end in two coincident egocentric directions and all other points in two different directions. Hering's cyclopean projection thus provides an explanation for the occurrence of single vision versus double vision.

Sensory fusion

Observational research shows that points with unequal visual directions in both eyes are sometimes not seen double, but single, in a direction that lies between the two visual directions. This phenomenon is called sensory fusion. With a strong dominant eye, the perceived direction of the fused image is closer to the direction seen by the dominant eye.7

The occurrence of fusion is due to Panum89 and measured by Ogle10 by moving a vertical rod in space and determining when the rod is perceived as single or double. The result is not a line, like the geometric horopter, but an area extending on either side of the geometric horopter. This area is called Panum's fusion area. Ogle also found that double images with a large disparity are seen on the horopter and double images with smaller disparities are seen (slightly) further away or closer than the horopter; the depth sensation is less vivid and less great than with fused images.11

Hering's experiment

Hering described an experiment to test Panum's theory. In the experiment, a thin rod is held straight ahead in the direction of view in the midsagittal plane.12 If the center of the rod is fixed, then the center of the rod should appear single and the ends double and receding in depth, as in the figure. Hering did not find this and thus rejected the theory.

Hering's experiment is a special case of the midsagittal-strip illusion. This illusion explains that and why the fused image is seen transversely to the direction of view and not as Hering assumed.

Vergence horopter

As soon as a point that has the attention threatens to move outside Panum's fusion area, an automatic eye movement (vergence movement) ensures that the point comes to lie in the middle of Panum's fusion area again. In research on this reflex, the vergence horopter is defined as the set of points within Panum's fusion range that do not elicit a reflex. The measured horopter falls centrally within Panum's fusion range. In the vertical plane, the vergence horopter is less inclined than the corresponding fusion range.13

Explanation

The existence of fusion is not explained by classical theory. Theories that make use of the fact that neurons have been found in the visual cortex that look in a specific visual direction via each eye seem to be able to explain this phenomenon better.14

References

  1. Adamson, Peter (2016). Philosophy in the Islamic World: A History of Philosophy Without Gaps. Oxford University Press. p. 77. ISBN 978-0-19-957749-1. Archived from the original on February 5, 2023. Retrieved October 3, 2016. 978-0-19-957749-1

  2. David B. (2012),“The Oxford Handbook of the History of Psychology: Global Perspectives”, Oxford University Press,isbn 978-0-19-536655-6

  3. Zum Lehre von Ortsinne der Netzhaut. E. Hering (1861), Leipzig: Engelmann, p.37.

  4. Howarth PA (2011). "The geometric horopter". Vision Research. 51 (4): 397–9. doi:10.1016/j.visres.2010.12.018. PMID 21256858. https://doi.org/10.1016%2Fj.visres.2010.12.018

  5. Sprague; et al. (2015). "Stereopsis is adaptive for the natural environment". Science Advances. 1 (4): e1400254. Bibcode:2015SciA....1E0254S. doi:10.1126/sciadv.1400254. PMC 4507831. PMID 26207262. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4507831

  6. Gibaldi; et al. (2017). "The Active Side of Stereopsis: Fixation Strategy and Adaptation to Natural Environments". Scientific Reports. 7: 44800. Bibcode:2017NatSR...744800G. doi:10.1038/srep44800. PMC 5357847. PMID 28317909. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5357847

  7. Hariharan-Vilupurua S.L.;Bedella H. (2008), "The perceived visual direction of monocular objects in random-dot stereograms is influenced by perceived depth and allelotropia", Vision Research 49 (2009) 190–201

  8. Physiologische Untersuchingen über das Sehen mit zwei Augen. P.L. Panum(1858), Kiel.

  9. Unter welchem ​​umstände erscheinen doppelbilder in ungleichen Abständen vom Beobachter? Albrecht vom Graefes Arch. Klin. Exp. Ophthalm. 41, 134 157.

  10. Ogle K. N. (1950), “Researches in binocular vision,” Philadelphia: Saunders.

  11. Ogle K. N. (1950), “Researches in binocular vision,” Philadelphia: Saunders.

  12. Zum Lehre von Ortsinne der Netzhaut. E. Hering (1861), Leipzig: Engelmann, p.37.

  13. Harrold A.L.; Grove P.M. (2021),,"The vergence horopter", Vision Research Volume 180, March 2021, Pages 63-79 [1] https://doi.org/10.1016/j.visres.2020.12.003

  14. Krol J.D.(1982), "Perceptual ghosts in stereopsis, a horrible problem in binocular vision", PhD thesis ISBN 90-9000382-7.