All primates, including humans, have two eyes facing forward. With this binocular vision, the views through the two eyes are nearly identical. In contrast, many other animal groups, especially herbivores such as ungulates (hooved animals, including cows, sheep and deer) and lagomorphs (rabbits, for example), have eyes pointing sideways. This perspective provides largely independent views for each eye and an enormously enlarged field of view overall. Why did primates sacrifice panoramic vision? What benefit did they gain?
We know binocular vision evolved several times independently in vertebrates. For example, among birds, predatory species such as owls and hawks have forward-pointing eyes. One theory is that the feature conferred a statistical advantage—two eyes are better than one—for detecting and discriminating objects, such as prey, in low light levels. But whatever the original reason for its emergence, the evolutionary novelty afforded a huge advantage: stereoscopic (literally, solid) vision.
Shifting Views
How does it work? Even though both your eyes point forward, they are separated horizontally so that they look at the world from two slightly different vantage points. It follows that each eye receives a slightly different picture of the three-dimensional scene around you; the differences (called retinal disparities) are proportional to the relative distances of the objects from you. Try this quick experiment to see what we mean: hold two fingers up, one in front of the other. Now, while fixating on the closer finger, alternately open and close each eye. You’ll notice that the farther the far finger is from you (don’t move the near finger), the greater the lateral shift in its position as you open and close each eye. On the retinas, this difference in line-of-sight shift manifests itself as disparity between the left and right eye images.
A simplified example shows this effect clearly. When you look at the pyramid, the right eye sees more of the right side than the left eye does, and vice versa; it is a simple consequence of geometric optics. Notice that the images in the two eyes are correspondingly different; the inner square is shifted right or left. This retinal disparity is proportional to the height of the pyramid. The brain measures the difference and experiences it as stereoscopic depth.
Although this explanation seems patently obvious today, it wasn’t elucidated until the 19th century. Leonardo da Vinci attempted to explain it several hundred years earlier and correctly observed that because the eyes normally receive different views of a 3-D scene, it is impossible, even in principle, to convey a full sense of 3-D on a 2-D canvas. Leonardo puzzled over how we can see a single world of solid objects given the different eye views (now known as Leonardo’s paradox), but he failed to grasp the critical point that retinal disparity is not a problem but is the basis for stereopsis.
This fact was finally made clear in 1838 by English physicist Charles Wheatstone, who published an elegant series of experiments on binocular vision. Recognizing the difference in perspective of the left and right eyes, he began by making line drawings of each eye’s view of simple objects. Then, employing a device he invented, called a mirror stereoscope, he presented these line drawings together to the viewer: left view to left eye alone; right view to right eye alone. Imagine his astonishment—and delight!—when he saw the skeletal outline of the object spring into 3-D relief, looking like he could almost reach out and grab it. It must have been the same sense of wonder every child experiences when playing with a stereo viewer such as the familiar View-Master. It seems like magic.
But how exactly does the brain blend the two eyes’ slightly different pictures harmoniously into a single fused picture? And how does it measure and extract the differences to allow for seeing in stereo? On one hand, it needs to unify the pictures; on the other hand, it needs to preserve and measure their differences.
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Add CommentStereo vision is about "seeing" followed by interpretion reinforced by predictability ("believing") . Other information inputs to our brain can be combined and interpreted as "process dynamics." Reinforcement by belief or expeerience becomes "reality." This is the physiological basis of culture, along with other contributing ideas/experiences of "me" and "not me". Follow this "down the rabbit hole" and you find a basis for family (which may include pheromone reinforcement). I find it interesting to ponder the "reality" of our brain inputs to social beliefs that need stereo-like interpretations. " Ponder the extension of "me" to "us" with "not me/us" to "them". We have the illusiory foundation of war, community, and other social dynamics. In some sense, the boundry between "me" and "you" is an illusion. When I think about it, this is obvious -- not profound in the least. Labels are convenient artifacts.
Reply | Report Abuse | Link to thisViewing stereo pictures w/o a stereoscope is difficult for pictures wider than the span between your eyes, because we're not used to spreading our vision wider that parallel for looking at distant objects. But you can easily see large pictures in 3D by switching the pictures left and right and looking at them cross-eyed. Hold a pencil point between you and the pictures, adjusting its position forward/back, left/right and tipping your head from side to side, so the point lines up with the same object in both pictures. Concentrating for a few seconds on the scene in the pictures should lock them in to one view, at which point you can move the pencil away. Unless the pictures were taken simultaneously with a 3D camera, their positions will probably have to be adjusted relative to each other to line up. With practice, you won't need the pencil. Just look cross-eyed at them until you can get the two to line up. There used to be books of pictures that produced many 3D illusions this way. I don't know if they're still available.
Reply | Report Abuse | Link to thisAt about age 15, I was experimenting with prisms from an old pair of binoculars. Looking at nearby objects, one eye looking through two prisms in a certain orientation, I noticed an odd effect. Analyzing the effect and enhancing it, I thought I had invented a device to magnify depth perception, until I later saw an army version in a surplus store, called a stadiscope. Make a periscope (using mirrors and/or prisms). Turn the 'far' end of the periscope down so it sticks out to the side away from the other eye, while still looking forward. Adjust the reflectors so the view through the periscope is closely aligned with the straight view, which will take some experimentation. This in effect adds distance between the views of the two eyes, thereby increasing parallax and depth perception. The ideal is to make two periscopes, one sticking out to the side of each eye, on one base frame, which puts both eyes at the same optical distance from the object viewed. The greater the distance to the object, the wider the periscope system should be. The tricky part is getting all four the reflectors lined up so you see one view with both eyes.
Part of the effect I originally observed was that things seem to look smaller, I assume because my brain thinks if I have that much depth perception, the objects must be closer, therefore smaller.
Even though the position and nature of eyes of other species, like the ruminates for example, seem to make it impossible for them to perceive visual reality as we do, we can probably have no idea as to how their brains process the images they receive and what the final picture is.
Reply | Report Abuse | Link to thisIf what we perceive is to be called reality, then it is possible that their brains create a picture similar to what we see, except that the fixed portion of our vision is ahead of us, and we turn our heads for peripheral vision, whilst the fixed portion of their vision is on the two sides whilst they have to turn their heads to see what is in front of them. Not much different, but life saving for creatures who need to evade predators. The important thing is not so much the aperature through which we receive light, but what the brain does with the light it receives, as any cinematographer knows. Gestalt psychology also comes into play here, to explain some of these effects, like persistance of vision, for example, or rather processing delay.
Even though the position and nature of eyes of other species, like the ruminates for example, seem to make it impossible for them to perceive visual reality as we do, we can probably have no idea as to how their brains process the images they receive and what the final picture is.
Reply | Report Abuse | Link to thisIf what we perceive is to be called reality, then it is possible that their brains create a picture similar to what we see, except that the fixed portion of our vision is ahead of us, and we turn our heads for peripheral vision, whilst the fixed portion of their vision is on the two sides whilst they have to turn their heads to see what is in front of them. Not much different, but life saving for creatures who need to evade predators. The important thing is not so much the aperature through which we receive light, but what the brain does with the light it receives, as any cinematographer knows. Gestalt psychology also comes into play here, to explain some of these effects, like persistance of vision, for example, or rather processing delay.
It has been demonstrated already that birds such as doves move their heads quickly to and fro, which makes them look silly but which creates a sense of depth for them. Motion also adds depth to a 2-dimensional picture, such as in a movie. When the camera stops moving sideways, the sense of depth is lost. So it is now proved that we are doves.
Reply | Report Abuse | Link to thisI think vision is like the perception of taste. It is interpreted. What we see and what is perceived can differ. That is why two individuals can see the exact same event or picture and two different things. A picture is worth a thousand words, but the language of those words may be different among others.
Reply | Report Abuse | Link to thisHaving been born with amblyopia or lazy eye, I have never had the experience of natural 3D vision. I have managed to compensate without ever knowing what it would be like to actually see the depth in the world. I have never been very good at hitting a baseball or judging distances in golf but somehow I survive. I doubt if there is any way now that I am in my fifties to ever know. I will go to my grave as a 2D man.
Reply | Report Abuse | Link to thisCheers
Having been born with amblyopia or lazy eye, I have never had the experience of natural 3D vision. I have managed to compensate without ever knowing what it would be like to actually see the depth in the world. I have never been very good at hitting a baseball or judging distances in golf but somehow I survive. I doubt if there is any way now that I am in my fifties to ever know. I will go to my grave as a 2D man.
Reply | Report Abuse | Link to thisCheers
Sorry, of course. But you may be able to compensate. If I close one eye and move my head slightly from left to right and back, a sense of depth occurs, due to fact that distant objekt will move less than those very close. The shifting angles will be much the same as when the brain combines pictures from two eyes. Propbably you have used such compensations already to get by. Or am I wrong? Yours&
Reply | Report Abuse | Link to this"Images of the letter project to the central part of each retina (the fovea)" ... but of all parts of the retina, the fovea is ill-equipped with receptors and resultant acuity is not of the best.
Reply | Report Abuse | Link to thisCan you see depth with one eye? Yes, you can. When I look with crossed eyes at a 2D picture (photograph, TV, painting, etc.), and at the same time cover one eye completely whith my hand, I see total depht in the picture. You must keep looking whith crossed-eyes. To get focus takes a little experiance. When I do this in front a mirror, the depht in the mirror has gone but pictures Ican see in the mirror are getting depht.
Reply | Report Abuse | Link to thisAt some point I made myself a sort of stadiscope ( mentioned in an earlier post) with four little mirrors and it worked. Other possible methods are with prismas or bended glas.
I too have a 'lazy eye', but see perfectly well in 3 dimensions because my 'leading' eye still can focus on objects to estimate distances. But I cannot anticipate the trajectories of balls and suffer from 'dropsy' .
Reply | Report Abuse | Link to thisWhat amazes me most, is the collossal 4D pixel representation of everything around me by the human brain. This image can extend for miles, something no computer can hope to match.
I was delighted to discover that with 3D glaseses that alternately flash an image to each eye I can experience a full stereo movie, for me a rivetting feeling!