Depth Cues in the Human Visual System
Author: Marko Teittinen
The human visual system interprets depth in sensed images using bothphysiological and psychological cues. Some physiological cues requireboth eyes to be open (binocular), others are available also whenlooking at images with only one open eye (monocular). Allpsychological cues are monocular. In the real world the human visualsystem automatically uses all available depth cues to determinedistances between objects. To have all these depth cues available ina VR system some kind of a stereo display is required to takeadvantage of the binocular depth cues. Monocular depth cues can beused also without stereo display.
The physiological depth cues are accommodation, convergence,binocular parallax, and monocular movement parallax. Convergenceand binocular parallax are the only binocular depth cues, all othersare monocular. The psychological depth cues are retinal image size,linear perspective, texture gradient, overlapping, aerialperspective, and shades and shadows.
Accommodation is the tension of the muscle that changes the focallength of the lens of eye. Thus it brings into focus objects atdifferent distances. This depth cue is quite weak, and it iseffective only at short viewing distances (less than 2 meters) andwith other cues.
When watching an object close to us, our eyes point slightly inward.This difference in the direction of the eyes is called convergence.This depth cue is effective only on short distances (less than 10meters).
As our eyes see the world from slightly different locations, theimages sensed by the eyes are slightly different. This difference inthe sensed images is called binocular parallax. Human visual systemis very sensitive to these differences, and binocular parallax is themost important depth cue for medium viewing distances. The sense ofdepth can be achieved using binocular parallax even if all other depthcues are removed.
Monocular Movement Parallax
If we close one of our eyes, we can perceive depth by moving our head.This happens because human visual system can extract depth informationin two similar images sensed after each other, in the same way it cancombine two images from different eyes.
Retinal Image Size
When the real size of the object is known, our brain compares thesensed size of the object to this real size, and thus acquiresinformation about the distance of the object.
When looking down a straight level road we see the parallel sides ofthe road meet in the horizon. This effect is often visible in photosand it is an important depth cue. It is called linear perspective.
The closer we are to an object the more detail we can see of itssurface texture. So objects with smooth textures are usuallyinterpreted being farther away. This is especially true if thesurface texture spans all the distance from near to far.
When objects block each other out of our sight, we know that theobject that blocks the other one is closer to us. The object whoseoutline pattern looks more continuous is felt to lie closer.
The mountains in the horizon look always slightly bluish or hazy. Thereason for this are small water and dust particles in the air betweenthe eye and the mountains. The farther the mountains, the hazier theylook.
Shades and Shadows
When we know the location of a light source and see objects castingshadows on other objects, we learn that the object shadowing the otheris closer to the light source. As most illumination comes downward wetend to resolve ambiguities using this information. The threedimensional looking computer user interfaces are a nice example onthis. Also, bright objects seem to be closer to the observer thandark ones.
Okoshi, T., Three-Dimensional Imaging Techniques, Academic Press, NewYork, 1976.
Human Interface Technology Laboratory
Imagine you’re in a car and you see a tree in the distance. How is it that as we drive closer the tree begins to look bigger? Trees obviously aren’t growing. So what is causing this? I’ll give you a hint… it’s our brain and eyes using depth cues. Depth perception refers to the ability to see the world in 3D and judge how far away/close objects are from and to us. This judgement is very important for navigating everyday life. How we move from one point to another relies quite heavily on our ability to perceive depth, and even picking up an object, such as your pencil, relies on the ability to judge depth.
For example, if we were crossing the road and couldn’t judge how far away a car was, it would be a bit of a disaster.
Let’s take a look at depth cues in psychology!
- We will start by taking a look at monocular depth cues definition psychology and binocular depth cues psychology.
- We will then move on to look at monocular depth cues examples whilst exploring aspects such as height in plane, relative size, occlusion and linear perspective.
- Moving along to do the same and looking at binocular depth cues examples, focusing on convergence and retinal disparity.
Finally, we will highlight the difference between monocular and binocular depth cues.
Monocular Depth Cues – Definition in Psychology
Monocular depth cues in psychology can be defined as:
Monocular depth cues: information about the depth that can be judged using only one eye. Monocular depth cues can be used in pictures, so a lot of monocular depth cues are used in art to give viewers a sense of depth.
Binocular Depth Cues – Definition in Psychology
Binocular depth cues in psychology can be defined as:
Binocular depth cues: information about depth that uses both eyes to see and understand 3D space; this is a lot easier for our brains to comprehend than monocular depth cues.
The difference between monocular and binocular depth cues is that monocular depth cues use one eye to judge depth, and binocular depth cues use both eyes to perceive depth.
Monocular Depth Cues – Types and Examples
There are four monocular depth cues you will need to know for GCSE psychology. These are:
- Height in plane
- Relative size
- Linear perspective.
Height in plane
Height in plane is when objects placed higher up appear or would be interpreted as further away. Have a look at the monocular depth cues example below, note that the house that is placed higher would be interpreted as being further away from us, and the house lower down would be seen as being closer to us.
Example of height in plane, Erika Hae, StudySmarter Originals
If there are two objects that are the same size (e.g., two trees of the same size), the object that is closer will look larger. Have a look at the monocular depth cues example below, tree number 1 seems closer because it is larger, and tree number 2 seems further away because it is smaller.
Example of relative size, Erika Hae, StudySmarter Originals
This is when one object partially hides another object. The object in front that is overlapping the other is perceived to be closer than the one that is being partially hidden. Look at the monocular depth cues example below; the rectangle appears closer as it overlaps and partially hides the triangle.
How is Depth Perception Created?
Depth perception is created when the eyes and the brain work together in an effort to perceive the depth, or the length, width, and height, of the world around us. Humans have two eyes. Having two eyes to see through is called binocular vision. Binocular vision helps to create a stronger sense of depth perception than monocular vision or having one eye. This is because the brain can get a view from two different angles, thus seeing the same object or room from a slightly different length, width, and height, through both eyes. When the images are compiled within the brain and one image is produced for us to comprehend or ‘see’ then we can perceive depth. When looking at a small object, humans have the ability to turn both of their eyes in slightly. This effect is called convergence and it allows for a closer look at small objects, which allows the brain to better perceive the length, width, and height of the object within space. Depth perception examples include:
- Knowing how close someone is when they are walking toward us.
- Having a pencil and a mug on the desk and being able to tell which one is closer.
- Seeing a dog running away and knowing how far away it is.
Seeing a dog running and knowing how close it is.
Binocular and Monocular Depth Cues
There are a variety of visual cues to help a person determine the depth of the world around them and have special awareness both in the monocular and binocular sense. Binocular depth cues are all of the ways that both eyes can help to perceive the world around us. Monocular depth cues are all the ways that just one eye can see the world around us and help us to perceive it. There are a few important terms to know when discussing depth cues.
Binocular depth cues include:
- Retinal disparity which is the slightly different images a person’s two eyes send to the brain.
- Fusion is where the brain combines two different images to make it into one.
Monocular depth cues include:
- Shadow stereopsis refers to the perceptions of areas that are in the shadows and how they are perceived by people with normal binocular vision. These areas are perceived differently by the eyes because they do not have defined outlines, but instead have gradients.
- Relative size of an object refers to the size that the object looks. Objects that are farther away look smaller to the eye, while objects that are closer up look larger.
- Texture gradient is an example of linear perspective. Objects that are farther away or extend farter away from us such as a cornfield will appear to have a finer, smoother texture the farther out it is. The texture will be more defined with close-up objects.
- Interposition is the perception that one object is covering another object because it is in front of it. It is a position cue.
- Motion parallax refers to objects that appear to move faster if they are closer to a person, and objects appearing to move slower if they are farther away from a person. This is due to the perceived distance that the object is traveling.
Evolution of Depth Perception
Humans have good depth perception because their eyes are close together and face in front of them. This allows the vision that they see through binocular eyes to overlap. When the vision overlaps it improves the brain’s ability to perceive depth.
Many animals such as chickens, fish, and horses have eyes on the sides of their heads, this gives them a good panoramic view of the world, but it does not give them very good depth perception. Since their eyes have two different views of the world, and the images do not overlap, the brain is only processing each eye according to monocular depth cues, or one-eye depth.
Donkeys and chickens have less depth perception than cats or dogs.
Many of these discoveries were noted first by Charles Wheatstone in the Victorian era. He invented the stereoscope in the 1830s which allowed for the study of binocular vision to begin.
Wheatstone worked with retinal disparities to test the limits of the brain to see how different the images seen through each eye were. He also did experiments changing what each eye saw using the stereoscope to determine if the brain could process the images separately or together. He concluded that a person was able to view the different images easily. There are multiple theories on depth perception which include the Law of Newton-Muller-Gudden, and the Eye-Forelimb EF Hypothesis which will be discussed within this lesson.
The Law of Newton-Muller-Gudden
Isaac Newton first theorized that the side of the body the eye is on sends signals to the corresponding side of the brain, specifically the right and left hemispheres. The right eye would send signals to the right hemisphere. The left eye would send signals to the left hemisphere. The Law of Newton-Muller-Gudden involves the scientific principles that show how the structure of the brain, eyes, and nerves are interconnected. The Law of Newton-Muller-Gudden states that “the retinohypothalamic nerve, a neural input pathway, obeys the principle that the degree of optic fiber decussation in the eye cavity is inversely related to the front-facing portion of the optical axes of the eyes.” The term decussation means that what is seen if there is a flaw in the fibers on one side of the eye will have an effect on the other side of the body.
This Law has been disputed as recently as 2016 by a variety of scientists who have studied 23 species types from 11 different orders to discover that the opposite could be true. This theory is heavily debated in ocular science.
The Eye-Forelimb EF Hypothesis
Another theory on depth perception is the Eye-Forelimb EF Hypothesis, which suggests that the development of depth perception and make up of visual structures needed for depth perception stemmed from a need to better control forelimbs.
E.J. Gibson and R.D. Walk developed an experiment to test when depth perception develops in babies and animals. They discovered that it was developed around the time a baby could craw, or when a baby needs better control of his or her limbs. The experiment was called the visual cliff test. Plexiglass was placed over a drop-off. The babies were placed on one side, and the caregiver on the other side. Walk and Gibson hypothesized that if depth perception had already developed then the babies would be hesitant to cross over the plexiglass. They were proven correct.
Poor Depth Perception
People and other living organisms experience problems with poor depth perception. Some animals, like pigeons, use head movement to compensate for issues related to poor depth perception. There is a potential danger when humans have bad depth perception. Humans are supposed to have good depth perception to navigate the world around them. Poor depth perception can cause problems when driving, working, or just walking around the world.
Testing Depth Perception
One way to test depth perception is to put a photo of a golf ball on your wall about 6 inches in front of your eyes. Then, using your finger, hover it in front of the golf ball.
Then, focus on the golf ball and you will see the ball clearly, but you will also see two slightly blurry images of your finger on either side of the tennis ball.
After you are finished, then focus on your finger, and the ball should appear to be cut in half.
Golf ball for testing depth perception.
Disorders and Causes
There are several disorders that can cause a person to have depth perception issues, and these include:
- Strabismus: both eyes do not line up in the same direction, also known as cross-eyed.
- Amblyopia: a lazy eye
These disorders cause poor depth perception because they change the view that is coming from one or both eyes. When the view is not overlapping correctly, to give the brain two similar images that it can process, then the resulting image the person comprehends will lack depth perception.
Blindness in one eye or the loss of an eye can also create poor depth perception because it leaves a person to rely only on monocular depth cues. Though there is no cure for blindness, there are some common treatments for the disorders listed above. For strabismus, the treatments include one of the following:
- Vision therapy
- Muscle surgery
For Amblyopia, the treatment includes an eye patch over the afflicted eye until it is corrected.
In summary, depth perception is created by the brain working closely with the eyes. There are two different types of depth perception cues which include:
- Binocular depth perception cues
- Monocular depth perception cues
Depth perception has been studied for many years, and tests for depth perception were used as of the 1930s when the stereoscope was invented. Depth perception occurs in babies around the time they learn to crawl, as it helps babies to perceive their environment. Poor depth perception can be caused by a few disorders which include a lazy eye and issues with crossed eyes. Some of these issues can be corrected with the appropriate therapies.