Stereopsis and Tests for Stereopsis
Introduction Since the time we know, we have two eyes. Not only humans, cows tigers cats dogs frogs, all have two eyes. Couldn’t we have lived well enough with just one? NO. The benefits of two eyes are many. The major ones I would like to elaborate with comparisons to the animal kingdom. Long before humans, the jungle had two types of animals. The prey, and the predator. Prey as in cows, deers, horses etc and predator as in tiger, lion, frog etc. If you notice, the prey group has their eyes on the sides of their heads and the predator group has in front of their heads. As wise men say, the prey needs to be aware of his surroundings, looking out for the predator. It needs to cover more ground at any given time. The side-mounted eyes give the maximum field of view. In terms of clock hours, the two eyes of the cow miss only about two clock hours of field. He is happy grazing in open grasslands since he can see nearly all around. Then what about the predator? What does the predator do with two eyes? Here the wolf needs to know the distance of the prey before he pounces. That is made possible by the use of two eyes. The two eyes give something called as depth perception. (1–3) Depth perception: definition What is depth perception? Let us try explaining with sound. If somebody just yells your name from your right side, ho do you figure out the location? Because, your right ear will hear it louder than your left. So your brain tells you that somebody on your right is calling you. There is dissimilarity in the sound heard from your two ears. Somewhat similar process happens with our eyes. They see dissimilar. When we look at the same point with both of our eyes, both the eyes see the object at the same time and the eye uses its accommodation to focus at that particular object. However, there is a slight similarity in the images perceived by each eye. The dissimilarity is in the light, shadows, size, orientation etc. This dissimilarity is processed by our brain to give a three dimensional image in the occipital cortex (2,4). Let us introduce some new words at this juncture. When we look at an object, it is known as fixation of gaze on the target object or just ‘fixation’. The eye is fixed at a given distance. The arc formed at this distance without changing the accommodation of the eye is called horopter. So everything that lies on the horopter is in focus for the eye without changing the accommodation. Imagine reading a book. You are at a line focused on it. The line you are reading is on the horopter. Notice that the line above and the line below is in focus as well. You can see few lines above and below clearly. So there is a range. A zone on either side of the horopter remains in focus. This zone or area is known as Pannum’s fusional area. Anything outside this area will appear two. Try it while reading a book. It works. In short, the two eyes have slightly different images at any given time. The ones on the horopter, that is, the object in focus is the clearest single image. Objects within pannum’s fusional area also appear single but they are not as clear as the ones exactly on the horopter, however, your brain manages to fuse these images and you see them single. Beyond the Pannum’s fusional area, the brain gives up. The dissimilarity in the images is beyond what the brain can fuse leading to double images (1,2).
Basis of stereopsis Now lets see the basis of stereopsis. Firstly, both the eyes should be able to perceive images simultaneously at any given time. That means the brain should process the image from your right eye as well as left eye. There are some situations where the brain suppresses the images from one of the eye. We will come to it later. So both the eyes should be able to work reasonably well independently when separately checked. This is known as simultaneous perception (Figure 1). Once the two eyes are able to see the images separately, the brain then learns to fuse these images. If the right eye is seeing a person waist above and the left is seeing waist below, the brain should be capable of fusing these images to see a complete human being. This is known as the development of fusion. First we made both eyes see one object simultaneously, and then the image got fused. Now comes the analysis. The brain analyses the object in image of one of the eyes and compares it to the other. By looking at the finer differences in the images generated by the fact that both our eyes are spaced away from each other the brain generates a three dimensional perception in our visual cortex. This is known as stereopsis. These stages are gradually acquired by visual behaviour. A newborn human being spends about 8 years of its initial life span developing the capability of depth perception through these stages. Defective stereopsis and strabismus Any interference to these stages within these formative years of life will adversely affect the capability of stereopsis. How do these interferences happen? In case a child has a congenital cataract, which has not been corrected immediately, the child will not have simultaneous perception, the first step. The same is true in all cases where the vision is deprived since birth, for that matter even a small period of lack of vision during these formative 8 years will hinder in the complete development of simultaneous perception (5,6). What if the newborn has a unilateral refractive error? That is anisometropia. In this case, the child creates images in his brain from both his eyes. In high refractive errors, the image quality is affected. If the right eye sees a sharp image of a tree, the left eye will see a blurred tree. With maintained simultaneous perception, the child will see a tree with blurred borders. The brain will then refuse to fuse the images since the brain is unable to get similar points in the images good enough to act as anchor points for fusion. This hinders the development of stereopsis. Further, imagine the child has strabismus. Even with a normal visual acuity, the eyes would be looking at different objects with both eyes at any given time. This also causes a cortical confusion (6,7). In such cases, the brain has to choose between the capability of a good visual acuity and stereopsis. The human being ends up preferring good visual acuity without the confusion. In an anisometropic patient where there is a difference of visual acuity more than what the brain can work with, the brain has an option of suppressing the bad eye. This is known as amblyopia. Amblyopia is a protective mechanism to prevent diplopia and confusion at the cost of stereopsis (8,9,2). Why only anisometropia. In case a child, or even an adult has any ocular unilateral condition effecting the visual acuity, the brain can even cause a strabismus which result in separation of fields. In case a child has strabismus, both the eyes look at different targets. These images do not make sense when the brain tries to fuse them. The centres of both images are different since each eye is looking at different objects. This causes the brain to turn off one of the eye to prevent this mismatch and resulting confusion. This is known as strabismic amblyopia. In the cases the eye may be emmetropic. These adaptive changes occur till the completion of development of stereopsis, that is upto about 8 years of age. In case these errors in vision occurs after the formative years of development of stereopsis, there is no loosing of stereopsis, but the adaptive changes occur. More towards strabismus, and rarely towards amblyopia. Since there is no amblyopia, the patient continue to see with both eyes equally well but due to acquired adult onset strabismus, the eyes see different objects at the fovea. Even with the dying desire of the brain to suppress one image, it is unable to do so after crossing the formative years of stereopsis. The brain is left with only one option, deviate the eye so that the visual fields don't overlap or else the patient has to live with a highly disturbing diplopia. This forms the basis of adult onset sensory deviations. Those patients with adult onset deviations continue to complain about their diplopia. Grades of binocular vision There are grades and methods of assessing binocular vision. The grades are the different steps in the development of stereopsis during the visual maturation. Testing of the grades is done by a synaptophore and graded as - no binocular single vision grade zero, simultaneous perception grade 1, fusion grade 2 and stereopsis grade 3. Limited form of testing can be done with worth four-dot test or Bagolini’s glasses. The grade 3, which is the stereopsis, is further quantified using different charts. The charts presently available are titumus fly chart, random chart etc. These charts are discussed in detail subsequently. Monocular cues Depth perception and binocular vision will be incomplete without the mention of simulated stereopsis. This is the process, by which we achieve the sense of depth perception, albeit incomplete, using a single eye (Figure 2). In such situation, the brain utilises something known as monocular visual cues. These cues are simple interpretations from the image, which enable the cortex to judge the distance and depth. It’s like a painting. If tree covers a part of the house, then the tree is towards you and the house should be behind it. Such cues are derived from size, shadows, position etc. these cues help in depth perception in a mixed and familiar image. Not in a blank background or new surroundings with new objects. Monocular cues are more susceptible to optical illusion. Heard about trick photography? Quantification of stereopsis Now lets see the unit of measurement of stereopsis. Stereopsis is quantified in a unit called as seconds of arc. Any given point has 360 degrees around it. One degree is divided into 60 minutes of arc, and one minute into 60 seconds of arc. And now we talk about seconds of arc. It can be easily understood that we are talking about measurement of an angle. Now we need to know which is this angle that we are talking about. Imagine you are sitting at the back of a bus. Look at the first seat and the seat just before you. Both are at different distance from you. Each seat connects with both of your fovea with separate line of sight for each eye. The right and left eye has a different line of sight for each seat. The line for the right eye meets the line for the left eye at the point of fixation. At the meeting point, they subtend an angle. This angle is known as visual angle. The visual angle for objects closer to you is more open than the angle for the objects away from you. The difference of these two angles gives the relative distance between the two objects. This angle expressed in seconds of arc is the unit for stereopsis. It states how much minute difference in depth you can perceive. When the angle is very small, it means that the two objects in question is very close together. The stereopsis of an individual is defined quantitatively as the least seconds of arc difference in depth that the individual can perceive binocularly. And yes, this value will differ as per the distance of the object system from the eye. Closer to your eyes, you would have a better stereopsis than at distance. In unitary terms, the value of stereopsis in seconds of arc for near will be lesser than for distance. Repeat, lesser the value for stereopsis, better is the depth perception when denoted in milliseconds of arc (see Figure 4). The ideal way to test stereopsis would be to present two point targets separated in depth placed in a uniformly lit, shadow less room without any form of monocular visual cues. However, we are far from ideal in our clinical setting. This has led to the development of various teats for stereopsis. Tests for stereopsis As a basic principle, all tests for stereopsis incorporate a way of delivering dissimilar images to each eye. This is known as methodology of dissociation. The tests differ in the methodology of dissociation used. Haploscope Lets go to the era of haploscope first. Technically, haploscope is an optical device used to deliver different image to each eye at the same time. In earlier days, a pair of mirrors placed at an angle to each other was used. Imagine two mirrors placed at 45-degree angle, shiny surface away and the joint at your nose. The right mirror will show you the images of your absolute right side and the left one from the left. Your brain will be forced to fuse these two images. If these images are very very different, then it creates confusion. However, if the images are only slightly dissimilar, then we will able to process it. Imagine looking at a man standing in front of a tree (Figure 1). Your right eye sees the image differently than your left. Take a picture of the man about from the right and the left. In the first image, the man will be seen standing on the left side of the frame and in the second, the tree stands in the left of the frame. The objects are the same, while the perspective changes. Place these two images in the haploscope. Right sided in right and left sided in left. With the haploscope, we can see both images simultaneously. Since there is a similarity in the images, the brain can fuse it, process it and give stereopsis. We can perceive the distance of the man from the tree. (1,4,5) Haploscopes were difficult to construct. Over time, prisms replaced the mirrors. In order to reduce the size of the device, eye piece lenses made an entry. Just like the ones David Brewster built. In his times, haploscope was a recreational device. These stereoscopes, as they were called, went into mass production. Anaglyph Optically speaking, these devices altered the line of sight of the user. Hence it was less physiological to be used for diagnostic purposes. People started looking for newer techniques to deliver different images to each eye while maintaining the line of sight. This give birth to anaglyph. Anaglyph uses different colours to present different images. Lets bring back our photographs of the man with the tree. Take the right-sided photograph; make it into a red coloured image. Take the left one and make it cyan or simply blue coloured (Figure 5). Take a pair of glasses. Put red filter in right and cyan on the left. With these glasses, look into the print. The right eye will see only the red coloured image and the left will see only the cyan image, although both images are printed one over the other. The brain gets a pair of dissimilar images and viola, stereopsis. Historically, in the year 1922, the first 3D movie by the name ‘the power of love’ was made 3D by the use of anaglyph technique. The audiences had to wear anaglyph red-blue glasses for the 3D experience. The TNO test for stereopsis is based on anaglyph principle (1). However, anaglyph glasses with the images gave a differential colour and contrast perception even though they managed to overcome the visual axis problem to a large extent. We needed better. Vectograph Vectograph was born from the realisation that light waves can be polarised. What do you mean by polarisation? A beam of light travelling in space has many waves. These waves generally socialite in all directions perpendicular to the central axis. There will be few horizontal waves, few vertical waves; few oblique waves etc. imagine these waves stopped by a wall with a thin vertical slit opening. Only the vertical oscillating waves can go through then and all other waves are stopped. In a vectograph, there is a pair of dissimilar images, which are covered by polarising film (Figure 5). The viewer wears a set of polarising glasses in which the axis of polarisation are in right angles to each other. If right eye is vertically polarising, then the left is horizontal. The right eye will be able to see only the vertically oscillating light waves and the left will see the horizontal ones. This makes it possible to project different images to each eye while maintaining the illumination and line of sight. The birth of vectograph was followed by the development of titmus fly test. The test image is a fly with different depth printed in different parts of the image of a large housefly. The fly can be replaced with shapes or other familiar images for the convenience of testing esp children. (2,9–11) These tests are used clinically to put a value to the stereopsis of an individual. However, the clinician should be aware of the fact that the stereo acuity gets better as the object comes nearer to the near point. Hence the value of stereopsis in seconds of arc should always have a testing distance mentioned along with it. Stereopsis in daily life Enough of clinical aspects. Lets explore something else in stereopsis. Ever wondered how the current 3D movies work? Presently 3D viewing devices have two different techniques to choose from. One is the vectograph, which we have already discussed where polarisation of light is used to present dissimilar images. Second method is the oscillation technique or the active shutter 3D display technique. The screen doesn’t simultaneously project both images but alternate between the two dissimilar images at a very high frequency in the tune of 60 hertz or more. The viewer in this case wears bulkier 3D glasses, which has a built-in battery. These glasses can turn opaque at the same frequency. So when the right-sided image is on the screen, the left eye glass turns opaque and vice versa. This oscillation between images happen at a speed of 60 hertz or more. That means each eye gets to see 30 images in a second. This rate is more than the threshold of temporal summation for the human eye and we are able to appreciate the movie. Most of the 3D televisions work this way (Figure 3). This technique preserves the brightness and contrast of the movie which otherwise would have been lost by polarisation technique since the polarised glasses block more than half of the light. But this technique is costlier and requires costly viewing glasses. (12) Stereo-Sue Another story you would be interested to know is about Prof Susan R. Barry from Massachusetts. She didn’t have stereovision till adulthood and suddenly and miraculously gained it then. She herself being a prof of neurobiology could comprehend the change she underwent. Her story is available all over the Internet and also in her book titled ‘Fixing My Gaze: A Scientist's Journey into Seeing in Three Dimensions’ (13). A wonderful journey into the relm of stereopsis. Conclusion The concept of stereopsis makes us realise the importance of two eyes in our lives and the aim in treating the bad eye with almost care even if the patient has another good eye. It’s the quality of life we are talking about. It is a very less understood concept among ophthalmologists even (14). Foundation of these concepts in early training days will go very long way ahead in better clinical practice.