Definition of Depth Perception
The Complex Challenge to Perceive Depth
The sense of sight is truly an extraordinary ability we have. To be able to see different colors, perceive motion, discern shapes, approximate the size of an object far away, and judge speed and distances is a gift we often take for granted.
Our ability to see objects in a 3 dimensional way, fill in blind spots, and correct images that appear distorted is an amazing and complex challenge we do almost every minute of every day. Our eyes are very closely located to our brain, probably because there is so much information that needs to be processed so quickly.
Our retinas contain about 150 million light sensitive cones and rods cells, as an outgrowth of the brain. In our brain, we have hundreds of millions of neurons that are dedicated to visual processing. These neurons take up nearly 30% of our cortex. In comparison, our sense of touch takes up 8%,and hearing takes up 3 %. There are a million fibers that make up the optic nerves, which send signals from the retina to the brain.
We Think We See with Our Eyes, But We Really See with Our Brain
Our eyes pick up millions of signals and electrical impulses from our retinas. Our brain puts together the information and assembles a model of the spatial image. We achieve this by inferring the light patterns that hit the 100 million photoreceptors in our retina. The optic nerve fibers processes it into an image that we define.
Our eyes do not see the images in front of us. What we see is lines and motion that our brains interpret and create what they might represent.
The images we think we see, are not perceived by our eyes, but in the cerebral cortex of the brain. The optic nerves send signals from the retinas to a relay station that processes our sensory messages. These signals proceed to the virtual cortex, called V1, a part of the brain at the back of the skull. They then feed into a second processing area, called V2, and branch out to areas that specialize in different functions, such as detecting depth, color, detail shape and movements. It is through these functions we can recognize faces.
There is much more we need to learn about the eyes and the nerve signals associated with them. It takes a tremendous amount of mental processing to perceive what is in our visual field. It is a complex and higher order brain function that takes up a tremendous percentage of our brain. It is believed seeing takes up the largest function in our brains.
What We Really See
Our Eyes Really See Two Dimensional Images
Stereoscopic vision works very effectively at distances up to 18 feet. Distances greater than this, our brain starts estimating relative sizes and motion to approximate depth.
The world is three dimensional. Objects have height, width, and depth. Although we see depth automatically, it is a complex process that our brain creates. In reality, we have no direct way of seeing three dimensions, because our eyes see things 2 dimensionally.
The distance and different images each retina perceives causes a three dimensional affect. We are fooled into perceiving depth, when we really don’t see it. If we can’t fuse the images, we will have double vision, known as diplopia.
Three Dimensional Images
What is Depth Perception?
Depth perception is the ability to perceive objects in space as being 3 dimensional.
Our brains interpret how far away objects are. This is our depth perception.
People interpret this through either monocular vision, which is using one eye, or binocular vision, which is using 2 eyes to gather the visual information.
Stereoscopic Vision and 3-d
Monocular vision works with the cues at distances of about 100 feet or more in distance, because at that distance, both retinas see an identical image. We get our cues using binocular and monocular vision. These cues help us perceive depth
Binocular vision gives us a greater sense of depth, especially when viewing objects that are closer than 100 feet.
Stereoscopic vision, is three dimensional vision. We are able to see in stereoscopic vision because of the space between our eyes, which averages about 8-10 centimeters apart. When we focus on an object, each eye gets a slightly different view.
Science knows that depth perception happens on V1, the primary visual cortex or even higher, where individual neurons receive the input from both retinas. Scientists have located these neurons, so we know they exist. We also know that the neurons are important to fuse and merging these images. What science doesn’t yet understand is how the brain accomplishes this.
What we do know, is our brain is able to take sparse clues and formulate a 3 dimensional image from a flat painting. The brain has an amazing ability to fill in the areas we don’t see.
We all have a natural blind spot in each of our eyes. The blind spot is located off center toward the side of our visual field, but we are unaware of it. That is because with both our eyes open, the blind spot gets cancelled out.
Also, because our eyes are in constant motion, we are prevented from seeing it. If you close one eye and stare, you might become aware of it, but then our mind starts to fill in and we rarely notice it. What our brain does is substitute the background that is likely to be in that blind spot.
Depth Perception is a Very Important Aspect of Sight
Our ability to perceive depth involves a complicated process that we often give little thought to. Each eye sees separate 2 dimensional images that the brain interprets into one 3 dimensional picture.
Depth perception is an extremely important feature of our vision. In order to gain depth perception, both eyes have to be focused on the same image, and move at the same time. When both retinas work together the brain gets 2 images that it combines into one. When one eye is working better than another, the brain will pick the better image and block out the lesser quality image.
If the eyes are not working properly, someone can also have double vision. Loss of depth perception can cause clumsiness, anxiety and accidents. Poor depth perception may be a sign of other vision problems in old and young people.
It takes coordination between our left and right eyes to create subtle differences between the images that are received by the retinas. This allows us to perceive stereoscopic depth and to have a visual 3 dimensional image. The eyes are strategically separated so that each obtains a separate image of the world from a different vantage point.
The difference between these images is called binocular disparity. It is a demanding and challenging task on the visual system of the brain that takes a tremendous amount of energy from the brain to convert these disparities from our 2 separate eyes to form 1 image that has dimension, depth, and accuracy to it.
The central nervous system needs to register the difference between both eyes. In addition, object recognition, needs sensory input so that it can be matched against to our memory.
Binocular and Monocular Vision
It is binocular vision that gives us depth perception. The middle temporal area of the brain constructs an image with depth by comparing the two pictures our eyes perceive. Perception allows us to judge distances.
Even people who see with one eye can see some depth because the middle temporal area processes information it receives from the visual cues we see and the motion of our eyes. When we use one eye, we are using monocular vision.
This area of our brain can pick up the speed of the objects as it crosses our path of vision.
Monocular vision relies on cues from one eye to judge the depth and distance, using relative size, shadows, interposition, height, lights, texture gradient, lights, and relative size. Binocular visions uses both eyes to get an idea about depth using binocular disparity and convergence.
Animals and Depth Perception
Most predatory animals can see in 3d so when they are hunting, they can gauge how far away their prey is. Most herbivores have eyes on the sides of their heads so they can see as much of their environment as they possibly can.
The further apart a predator’s eyes, the greater the depth perception, as long as both eyes are viewing the same image at the same time. For example, Hammerhead sharks and lions have great depth perception, which assists them in catching fast moving prey.
Herbivores, like cow, sheep and rabbits, deer, and other hooved animals, have two eyes that focus on different things. Each eye is made to see a separate image.
Herbivores don’t see the depth in the same way. Their eyes are on the sides of their head. Each eye gives separate and independent images and can span over a large field of vision, giving them almost a panoramic view of their surroundings.
The Tortoise and Depth Perception
To see with binocular vision (both eyes), we use 2 main cues known as disparity and convergence. Each eye sees a different image and our brain merges into one 3-d image.
Our eyes angle inward slightly towards each other when we look at an object that is very close to us. Our brain can decipher how far away an object due to the greater effort it takes to see something too close, than further away. Binocular vision works well when seeing up to 6m away. Further than that, our eye separation doesn’t give us enough difference to help put together the cues we see about the image.
The Distance Between Our Eyes Serves a Purpose
Our eyes are separated horizontally and each eye sees a slightly different image of the world around us. Retinal disparities are in proportion to how far the object is from us.
Put two fingers up, one in front of the other. Then focus on the closer finger and close one eye while opening the other. Notice that the farther away the far finger is from you, without moving the near finger, the bigger the shift it its position is as you alternately open and close each eye. You will see that the images in the two eyes are different. The brain can sense the difference and create stereoscopic depth.
Leonardo da Vinci tried to explain it through his observation that because the eyes naturaly receive different views, it is not possible to create a 3 dimensional image on a flat canvas. In 1838, Charles Wheatstone, an English physicist, invented a mirror stereoscope that clearly showed images in 3-d, somewhat similar to the viewmaster toy.
When we focus on an image, our brain creates corresponding retinal points to create a single object from 2 images. Two neuroscientists, from Harvard University, David Hubel and Torsten Wiesel conducted new experiments in the 1960’s and found that we have binocular cells in our visual cortex that receive input from both of our eyes that create a fusion of the images and that is what gives us perception.
Further experiments by an Australian medical student named John Pettigrew, found that our brain has a kind of map of each eye’s image. Neurons trigger the brain to measure the horizontal scatter between the images from both eyes.
Our eyes are strategically set apart and placed on the front of our face to allow us to see in stereoscopic vision. Each eye produces images that are slightly different. The differences in the images is in direct proportion to the depth of the objects that we are looking at. When our brain receives the images, they are interpreted as depth.
Stereopsis is the ability to see well with both eyes. Stereopsis gives important binocular cues about depth perception. It uses retinal disparity and Panum’s fusional space.
Fusion occurs when both of our eyes form a single image from the same object. When the objects are different, our mind suppresses one of the images to prevent confusion.
If someone does not have stereopsis, they need to rely on other visual clues to perceive depth.
Our eyes angle inward slightly towards each other when we look at an object that is very close to us.
Our brain can decipher how far away an object due to the greater effort it takes to see something too close, than further away.
Further than that, our eye separation doesn’t give us enough difference to help put together the cues we see about the image.
Some cues that help us see distance and depth are psychological cues. Some are physiological, coming about because of the way we move our eyes.
When we perceive depth through monocular clues, it is considered psychological, not physiological because we are using visual patterns instead of eye muscle adjustment or head movements. Generally, monocular cues are categorized as psychological because depth perception is achieved by clues from the visual pattern.
For example, a person can judge the distance of the car with monocular vision, because they know the relative size of a car. But a person may not be able to judge the relative distance of a cloud, only using size, because clouds come in different sizes.
We do not see as much depth perception with one eye, but we still use clues to help us see things 3 dimensionally. These are called monocular cues that allow us to estimate relative depth and distance. By using these clues, our mind converts 2 dimensional representations into 3 dimensional images.
Texture gradient - equally spaced items appear closer together as distance increase. The objects that are further away become less dominant, the further it is in the distance.
Linear Perspective: Objects that are further away look smaller and smaller. Artists use this to depict increasing distance as the lines converge leading toward the horizon.
Relative Size - By knowing how big something is and being able to compare objects to its size, approximate distance can be figured out by the viewer.
Aerial perspective - The relative color and contrast of objects gives us clues to their distance. When scattering light blurs the outlines of objects, the object is perceived as distant. Further away objects have a blue effect. Closer images look clearer.
Leonardo da Vinci originally described aerial perspective. Through his astute observations, he noticed that objects in the distant look bluer than objects closer up. The further away items tended to blend more into the atmosphere color. In teaching others to paint, his instructions state to paint the nearer object the true color and the one behind proportionately bluer. The next further object would be even bluer. “Thus if one is to be five times as distant, make it five times bluer” - Leonardo da Vinci.
Light And Shade: Shadows and highlights give us clues about the depth and dimension of objects. Closer objects are illuminated more than those that are further away. Shadows make an object appear that one is in front of another.
What Movement Tells Our Eyes
- Accommodation – Our eye muscles change the shape of the lens of our eyes to see things closer up, giving our brain information about distance. For example as a car drives away from us, the image gets smaller and smaller. Our mind interprets this as the image we are seeing is getting further and further from us. It is known as size constancy or relative size.
- Monocular Movement Parallax: When we move our heads side to side, we can tell that objects at various distances will move at a different relative speeds. We can tell that closer things move in the opposite direction of our head movement and distant things move in the same direction of our head movement. IIf you stand face to face with someone, and you move your head from side to side, the face in front of you, appears to move quickly, and the objects that are further away hardly move. With this information, our brain can calculate the distance of objects we see. Animals that move their head from side to side or up and down, will gain greater depth information about motion parallax and superposition. Try it for yourself.
How We Learn About Our World
It is believed that some ability to perceive depth is inherited, not learned. Some scientists think that we perceive space by sensing through visual cues that begin at birth. In studies done with crawling infants who see a visual cliff, they will avoid falling off. This has also been found in studies done with animals who are born with their eyes open and walking.
Some believe that we learn about our world through our senses and interpret these cues through touch and motor processes. For example, an item looks round because it feels round. It takes less effort to touch something closer than an item that is further away.
Medical Conditions that Affect Depth Perception
Some medical conditions can affect depth perception:
mild binocular disorder causes some discomfort when watching 3-d effects
optic nerve hypoplasia
macular degeneration which causes loss of color or contrast in an eye
cranial nerve palsay
loss of vision in one eye
anisometropia - each eye has a different refractive power
There may be some truth to helping people improve their depth perception. A book called Fixing My Gaze, by Susan R. Barr, a neuroscientist. According to her studies, people can learn to see with depth perception at any age. It used to be believed, if you didn’t take care of this issue before approximately age 7, the vision problem could never be corrected.
Take this Depth Perception Test
Depth Perception and Our World
We see the world through our vantage point. The ability to have depth perception helps us view the world three dimensionally.
Our brain works very hard to create the images we see. The world we witness, the experiences we see through sight, and world we create is done through the wonder of our minds. Sight is a marvelous and wonderful gift and as science learns more about this sense we have, we can learn more about how we interpret our surroundings.