CONCEPT

By stereoscopic vision is understood the vision in relief or the full degree of binocular vision. It is the third degree of binocular vision according to Worth, with simultaneous perception and fusion being the first and second degrees respectively.

His study is of interest from a diagnostic, evolutionary, therapeutic and prognostic point of view.

However, the perception of relief is not the exclusive heritage of binocular vision, but is determined by multiple factors, among which it is worth highlighting, following Hugonnier:

- Extrinsic factors.

Among the extrinsic factors, all of them used in painting to achieve the third dimension, it is worth highlighting:

The color of objects: distant objects appear bluish.

The shadows of the objects: that make them linger on the background.

The overlapping of the contours: the near object partially covers the far one.

The sharpness or definition of an object: the nearby object looks sharp, with detail. The distant is more undefined.

The convergence of parallel lines: For example, the train rails converge at infinity.

The absolute and relative size of things: a large object seems close and vice versa. Two equal objects of different size create a space between them.

- Intrinsic factors

Monoculars:

Among which is described the accommodation and parallax, or apparent displacement of the objects between them when moving the head. If looking straight ahead we move our head to one side, due to parallax, the objects farthest away seem to move in the same direction and those closest to it in the opposite direction.

Binoculars:

Convergence

Stereoscopic vision: when fixing a very distant object and when both eyes are in a different situation than the fixed object, their retinal images are somewhat disparate. Stereoscopic vision results from the union of these in the cyclopean eye. It is what has been called by Bela Julesz the "binocular cyclopean retina".

MEASURE

According to D. Goddé-Jolly, stereoscopic vision (V. E.) is a phenomenon that can be quantified with the binocular presentation of images of determined disparity, measured in arc seconds. To make a regulated quantitative assessment of stereopsis, a series of pairs of images of increasing similarity will be available, until they do not produce a sensation of relief.

The fine stereoscopic vision (V.E.) rests on the foveolar retina and is considered normal stereoscopic visual acuity with 100 ”to 24” of arc according to the authors. Although sometimes 14 "in foveolar vision and 400" with an eccentricity of 90 can be achieved.

In strabismic subjects it is rarely greater than 200 ”, although there are cases described with 80” and 67 ”(M. Parks, Goddé-Jolly).

To study the V.E. In a rude way, you can go to commercial tourist stereoscopes, to coincidence tests such as the Hering drop test, the Howard-Dollman apparatus, the test of the two verticals, the test of the three bars, ... But the correct study of VE It requires quantitative tests although its results are not strictly superimposable: the TNO seems more critical than the Wirt test.

In clinical practice, three are the most used methods, for the measurement of V.E.

1. Synoptophore

The synoptophore has sights of V.E. (Worth third degree of binocular vision) that offers the sensation of relief, but does not quantify it. The most widely used test consists of two eccentric circles that, when fused, produce the image of a cube in relief.

To proceed with this examination, it is convenient to place the sights at an angle slightly more convergent than the objective angle, thereby stimulating the V.E.

The result can be falsified, given the strong stereoscopic suggestion in monocular vision.

PULFRICH stereograms require a stereoscope for their presentation, on which transparencies with disparities of 200 "to 20" arc are offered. Their use is not widespread.

2. Stereo projector

R. Pigassou-Albuy's stereoprojector. Based on other devices with the same principle, such as Burian (1941), Halden (1952) and Kretzmarck (1955), it allows projected polarized tests of known disparity on a metallized screen.

The projector is equipped with two lenses and each lens has a polarization filter oriented at 452 and 1352 respectively. The tests are mounted in pairs on a laminar rod that allows simultaneous projection. The screen is metallized so as not to destroy the polarization of the light and allow good reflection. In its upper left margin it has a horizontal and vertical graduated ruler that measures in cm. the separation of the tests.

The projector and screen will be placed parallel so that the polarized light is perpendicular to the screen. Likewise, the examined subject will wear polarized glasses corresponding to the polarized filters of the stereoprojector, will always be placed in front of the screen, will not turn his head, the polarized glasses or clips will be perfectly adjusted and will have an adaptation time of 8 to 10 ' :

Fusion tests respond to series                           100

Simultaneous Perception tests respond to series 200

Stereopsis tests respond to the                         300 series

Binocular visual acuity tests respond to the        400 series

Stereoscopic vision tests are based on parallax and projection. In the 2-eye parallax tests they see identical images, but separated by a variable distance. The stereoscopic visual acuity values ​​are usually set with the projector and the subject at 5 m., Although these values ​​can be varied by modifying the distance of the stereo projector, the screen or the subject itself. A one centimeter disparity between the 2 images equates to 400 "stereoscopic visual acuity, 0.5 cm equates to 240".

The projection tests comprise three pairs each of which represents a circle and an ellipse, varying only the dimension of the horizontal axis of the ellipse.

Disparities range from 120 to 11 "of arc.

3. Stereograms

These include 2 types:

- Wirt test.

Designed by S.E. Wirt in 1947.

It consists of a series of polarized stereoscopic sheets, which the Titmus firm presents in a folder with 3 parts: 9 groups of 4 concentric circles, 3 series of animals and a large fly. As far as I know it is also known as the fly test.

The patient with polarized glasses and at a working distance of approximately 33cm. You must say if you see the flies, an animal or a circle in relief. The raised vision of the fly is equivalent to a visual acuity of approximately 1000 ", and it is of doubtful significance since it can give the impression of relief in monocular vision. It may be the steropsis of a strabismus with small angle CRAA: child operated on Strabismus at 2 years of age with minimal residual objective angle. This test is very rude, and only affirms the existence of peripheral stereoscopic vision. The disparity of the animals is 400, 200 and 100 "from the top row to the bottom row; and that of the circles ranges between 400 and 40 ".

Reindeer test or Reindeer test. It is similar to the Titmus test. It features a rude stereoscopic test (reindeer antlers) and 6 circles with disparity between 600 and 30 ".

- Anaglyphs

Anaglyph means "vase or carved work of rough relief". And in stereoscopic vision the anaglyphs are based on Aschenbrenner's studies and subsequent independent rediscovery by Bela Julesz, using the principle of the "Random dots test" or surface of almost uniform and random points. If they are compared monocularly, they do not offer clear differences, but observed binocularly, a specific structure appears.

These tests seem more critical in the measurement of stereoscopic visual acuity, which is why they are the ones chosen in the United States. for screening tests in children with possible stereoscopic vision disturbances: anisotropy, amblyopia with visual anisoacuteness greater than 2/10, oculomotor imbalance ...

Currently there are basically 3 tests with this principle:

TNO by Julesz

Test designed by the TNO Perception Institute and carried out in the Larmeris Ophthalmology department. It is based on the works of Julesz adapted by Walraven. The plates are red and green dots, some strictly corresponding, and others with increasing separation. The separation between these points is measurable, and therefore the psychological differences obtained.

It consists of 7 plates where the 2 stereograms (one red and one green) are superimposed and appear in relief thanks to red-green glasses (principle of anaglyphs). 3 of these plates are scaled and show monocularly visible motifs, and other similar ones that only appear in binocular vision (butterflies, circles and geometric figures). Slide IV, with 3 discs, is included as a suppression control. Slides V, VI, and VII are quantitative and evoke a circular cake lacking a sector, only visible in binocular vision. The corresponding retinal disparities are between 480 "and 15", for the examination distance of 40cm. If you vary this distance, these values ​​will be multiplied by the factor 40 / d, where "d" is the examination distance in cm.

The lack of monocular suggestion confirms it as the stereoscopic test of choice.

RDE by Reinecke

This is a polarized test and the relief is only appreciated with polarized glasses.

It consists of 2 sheets of random scattered points, which do not evoke any figure in monocular vision. 1 of them acts as a control for simulators, and the other represents an "E", only visible with polarized glasses. There is another sheet with an embossed "E", which is used for demonstration.

The stereoscopic acuity is 900 "at 28cm. And decreases with the distance of the test until 50" at 5m. In practice and according to its author, the subject who perceives relief at 1 meter is considered to have normal stereopsis, although it is advisable to present the test at 50cm. so that the little one is not distracted.

Frisby

It offers the advantage of not using any dissociating device (red-green glasses, polarized filters ...)

It consists of three sheets of plastic of different thickness, with a circular area visible only in binocular vision on a background of similar structure. On each sheet, the relief area is printed on one side and the uniform background on the other. The sheet is divided into 4 squares, but only one contains the circle. The examinee must specify which of them.

The disparity is given by the thickness of the plates, and ranges from 875 "to 20" depending on the examination distance.

 

NOTATION

In the record of the measurement of stereoscopic visual acuity it will be stated:

The method followed
The technique practiced
The measure obtained
And other circumstances of interest in the assessment of the test (lighting, attention, explorer ...)

For example:

TNO, 40cm, good collaboration, 30 ".

This example indicates that the TNO method was used to examine stereoscopic vision at a distance of 40cm, that the examinee exhibited good collaboration and that the stereoscopic visual acuity measured was 30 "of arc, that is, he responded correctly up to the slide. VII top.