Depth perception is an important aspect in our every day life, and therefore the need to evaluate and consider before multifocal IOL implantation. Before we talk about depth perception, it is important to clear, what perception is. Perception is about giving a meaning to our sensations. Everyday we receive a lot of sensations from different sensory organs of our body. The duty of meaningfully interpreting them is however rested on the brain. The organization process in the brain of these sensations leads to a perception of meaning, or the brain gives a meaning to these sensations. Consider a rose. When we see it we only see a colour and its shape. But our brain interprets these sensory feedback to give
a definite meaning – the rose.
Our perception of the outer world has three aspects – height, width and depth, or what we call as the three dimensional outer world. It has been a puzzle for scientists over the centuries as to how we can interpret a three dimensional world, when our retina is two dimensional. We see the outer world when rays of light are refracted and falls on the retina, and more particularly on the fovea. However, interpretation of the three dimensional surrounding, as it is, on a two dimensional film (retina) is truly fascinating. Herein comes the importance of the depth perception through monocular and binocular cues. Part of our depth perception is caused by visual monocular cues or cues that are generated from a single eye, and part of the depth perception is caused by visual binocular cues, or cues originating from both eyes.
Let us look into some of the Monocular and Binocular cues.
Monocular cues of vision are the kind of cues that are sometimes used by painters to give a three dimensional effect on vision on a two dimensional drawing board. The first monocular cue is the cue of size. The larger the image of an object on the retina, the larger we perceive it as. Hence in the outer world, the larger an image is on the retina, the nearer the brain perceives it as. Consider a picture of two men standing – one image bigger and the other smaller. Which of the images of men will you perceive as standing closer to you.
If your answer is that the image of the man that is bigger in size appears closer to you, then this is because it is visual monocular cue of size that helps the brain to interpret as standing closer to you.
The second monocular cue is the cue of linear perspective. The linear perspective says that as the distance of objects or image increases from the retina, parallel lines appear to converge, that is the distance of the two parallel lines tend to converge as they are of an increasing distance from the eye. Think of you standing on the railway tracks. How do you see the railway lines appear as you look at them ? Don’t you feel the two lines of the railway track appear to merge together as you look at the distance ? The more the merger the more further you feel the railway tracks are. Painters do utilize this important monocular cue to show things at a distance, or at a depth.
Another important monocular cue of vision that gives us depth perception is the monocular cue of clarity or the atmospheric perspective. Think of your last visit to the hill stationthat you visited. When the sky was cloudy and the air foggy, you felt the next hill that you saw from the balcony of your hotel room was quite far. But on the next day, when the sun was shinning bright, you felt the hill was not as further away as it seemed to be yesterday under fog. Clearer an object is, closer they appear to us. Diffractive multifocal IOLs work on the principal of splitting light, and hence less the light utilized, or more the light lost, the monocular cue of clarity for depth perception is lost.
The next monocular cue is the cue of interposition or overlap. If one object appears to block the view of another object, our brain would consider the object whose image is blocked is further away from us. This cue is often used by painters to make us perceive things that are at a distance in the painting.
The monocular cue of shadows is also important for the brain to perceive depth. The darker the shadows, the further the objects are perceived to us, and the lighter the shadows the closer they seem to be.
The monocular cue of movement or motion parallax is important for depth perception. If you move your head from one point to another, the closer objects seem to move in the opposite direction and further objects appear to move in the same direction. This is also seen in trains, when the trees and eletric poles near by tend to move in the opposite direction and the trees and hills further away seem to move in the same direction of the train.
There are other monocular cues as well, but one monocular cue that is very important is the cue of accommodation. Depending on the distance an object is from the eye, the muscles of the eye and the lens has to adjust itself so that the image falls on the retina. Accommodation is a complex process that involves pupillary constriction, orbital inward movement and lens shape change. The closer the object is to us, the more the two eyes converge. A resultant pressure on the muscles of the eye gives an important feedback of depth perception to the brain.
There are some important binocular cues as well, for the brain to interpret depth perception. The two eye distance from each other or the pupillary distance is an important factor. The image of the left eye would be slightly different than the image on the right eye. This difference in the two images is interpreted by the brain as the distance the object is away from our eye. That is the image of the single object in both eyes on the fovea is fused together by the brain thereby helping us to see as a single image. In the process the interpretation of the images from the two eyes gives a perception of depth. The more the brain has to work to fuse objects together, the more closer the object is to the eye. This is also referred to as stereopsis.
To maintain depth stereopsis, the two eye should point to the same object accurately. The position of the eye in its orbit is determined by extraocular muscles, notably six of them. Slight differences in position of the two eye may cause two eyes to drift away from each other.
The good thing about assessing the binocularity of vision is that most of the tests are simple , cost effective, and may not be hugely time consuming. One such test is the Cover Test. It reveals the amount of deviation of the eye movement and can determine esophoria or exophoria, the different terms to express eye alignment. Another test is the Fly test, a very simple test to help determine streopsis of the eye.
Still another is the Worth Four Dot Test that is often used for assessing a patient's degree of binocular vision. You may have already seen the four dot shaped lights just under the Snellen Chart. It uses a set of lights -two green, one red and one white. The patient is asked to wear a goggles that has red and green filters. The red filter goes to the patient's right eye, while the green on the left eye. A patient with good binocular vision with no suppresion of vision in either of the eye will see the dots as it is. Because the red filter blocks the green light and the green filter in the goggles block red light, it is therefore possible to determine if the patient is using both eyes simultaneously and in a coordinated manner. Thus the Worth 4 Dot test is used to understand binocular function and if the two eyes are working in coordination.
The success of Multifocal IOL outcome demands a binocularity of vision and therefore the need to test the binoculariy is important to judge the candidacy of the patient. Along with dry eye testing and pupil evaluation, depth steropsis and binocularity of vision should remain an important area to focus while determining the candidacy of the patient for diffractive IOLs.