Depth perception ap psychology definition

Deborah C. Escalante

Perceptual Organization

In earlier study guides, we learned how we are able to convert what we see (lightwaves) into neural impulses and how we detect color and shapes. In other words, how we sense visual stimuli, which is also called bottom-up processing. In this study guide, we will learn how we perceive that visual stimuli or make sense of and interpret the images we see (top-down processing). More specifically, we will learn how we perceive form, depth, and motion.

Form perception

Early in the 20th century, a group of German psychologists discovered that we have a natural tendency to organize our sensations into wholes or what they called gestalts. This means that our brains don’t just take information in, but we interpret and actually construct our perceptions. The following are some examples of how we do this when perceiving form.


This is the idea that we naturally organize what we see into objects (figures) that stand out from their backgrounds. For example, in the image below, if you consider the black in the image to be the background, you will see the white as the figure (a woman’s face). If you consider the white as the background, you will a saxophone player as the figure.

Image Courtesy of Wikimedia Commons.

Group Principles

In addition to organizing objects in our visual field into figure and ground, we also naturally organize objects into groups. Some of the most common examples of these principles are:

  • similarity: We group similar figures together, for example we perceive a group of baseball players wearing the same color jersey as one team

  • proximity: We group nearby figures together, for example, we perceive players sitting together on a bench as one team

  • continuity: We perceive smooth, continuous patterns rather than discontinuous ones

  • closure: We fill in gaps to create a complete, whole object

  • connectedness: We perceive items that are uniform and linked as a single unit

Photo courtesy of Pixabay.

Depth Perception

Depth perception is our ability to perceive objects in 3 dimensions and to judge distance. It also enables us to avoid falling down stairs and off cliffs, as Gibson and Walk demonstrated in their famous study with infants and a make-believe visual cliff (see below). All species, by the time they are mobile, have this ability as it is essential for our survival. In order to perceive depth, we use both monocular (one eye) and binocular (two eyes) cues to perceive depth and judge distance.

Image courtesy of Wikipedia Commons and the National Institute of Health.

👁 Monocular Cues

Monocular cues are available to either eye alone and include:

  • Relative Height

    • We perceive objects that are higher to be farther away from us. In the image below, it looks like the house is farther away because of this monocular cue.

Image Courtesy of Micky and Marissa.

  • Relative Size

    • If two images are, in reality, the same size, we would perceive the one closest to us to be larger. For example, if you place one teddy bear close to the camera and another really far away from the camera, the one closer would look larger. This happens even though the two teddy bears are really the same size.

  • Interposition

    • If one image covers another, it looks closer/larger.

Image Courtesy of Sjsu.

  • Relative Motion/Motion Parallax

    • When we are in motion, the objects we are looking at look like they are moving, as well. You could see this happening when you’re in a car looking out the window.

  • Linear Perspective

    • Parallel lines look like they come together in the distance.

Image Courtesy of @Psych_Review.

  • Light and Shadow

    • When there are shadows involved, there is a perception of depth.

Image Courtesy of Jim Foley.

👀 Binocular Cues

Binocular cues depend on the use of both eyes. The main binocular cue is retinal disparity, the difference between the two retinal images that result due to your eyes being about 2.5 inches apart. Your brain judges distance by comparing these images; the greater the disparity (difference), the closer the image is.

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Motion Perception

Our ability to perceive motion is important because it allows us to do things like drive, walk and ride a bicycle. Generally, our brains perceive motion by assuming that shrinking objects are moving away and objects that are getting bigger are moving closer. Sometimes, however, we perceive motion when there really isn’t any. The phenomenon called stroboscopic movement creates this illusion when we perceive movement in slightly varying images shown in rapid succession. Did you ever create a flip book when you were a kid? That is an example of stroboscopic movement. The same concept applies to film animation; a super fast presentation of Mickey Mouse in slightly different positions will create an illusion of movement.

Image courtesy of Wikimedia Commons.

Another illusion of movement is called the phi phenomenon. This is created when adjacent lights blink on and off in quick succession, like you might see on Christmas lights along the edge of a roof or highway arrows telling us to keep to one side or the other.

Perceptual Constancy

Perceptual constancy is another example of how our brains play tricks on us as we interpret the objects in our visual field. Also called object constancy or constancy phenomenon, it refers to our tendency to see familiar objects as having consistent color, size and shape, regardless of changes in lighting, distance or angle of perspective. In short, our brains interpret stimuli as they are assumed to be, rather than as they actually are.

  • Color constancy is perceiving objects as having a consistent color, even as illumination changes. An apple will look red to us outside in the sunlight, and also inside with indoor lighting—despite different light wavelengths reflecting off the apple.

  • Brightness constancy, also called lightness constancy, refers to perceiving objects with the same level of brightness, even though the illumination may change. For example, we see snow as white at noon under sunlight and in the evening under moonlight.

  • Shape constancy is perceiving objects as having a consistent shape, even as our orientation to it changes. For example, when we look at a refrigerator we see a rectangle. When we open the door, we continue to perceive the door as a rectangle even though technically the image that is being cast on our retina is a trapezoid.

  • Size constancy refers to our tendency to perceive objects as unchanging in size, even though our closeness or distance from the object may change. 

Perceptual Adaptation

One last concept in the area of visual perception is perceptual adaptation. This refers to our remarkable ability to adjust to changing sensory input. If you wear glasses, you can probably relate to this example. When you get a new prescription, initially you may feel a little dizzy or out of sorts. But after a day or so, everything seems back to normal. A more extreme case of this phenomenon can be experienced when wearing visual distortion goggles that can actually turn your world upside down! But even in this extreme case, people will adjust to this inverted world and even be able to throw a football with some precision.

Courtesy of Dmitry HOH, via Wikimedia Commons.

Practice AP Question

The following question is from the 2011 AP Psychology Exam . It’s great review for both this unit and unit 1!

  1. A researcher designs a study to investigate the effect of feedback on perception of incomplete visual figures. Each participant stares at the center of a screen while the researcher briefly projects incomplete geometric figures one at a time at random positions on the screen. The participant’s task is to identify each incomplete figure. One group of participants receives feedback on the accuracy of their responses. A second group does not. The researcher compares the mean number of figures correctly identified by the two groups.

A. Identify the independent and dependent variables in the study.

B. Identify the role of each of the following psychological terms in the context of the research

  • Foveal Vision

  • Feature Detectors

  • Gestalt principle of closure

C. Describe how each of the following terms relates to the conclusions that can be drawn based on the research.

  • Random Assignment

  • Statistical Significance

🎥 Watch: AP Psychology – Visual Anatomy and Perception


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.
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Seeing a dog running and knowing how close it is.

depth perception examples

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.

Lack of depth perception for donkey and chicken

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.

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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.

Depth perception test

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:

  • Eyeglasses
  • Vision therapy
  • Muscle surgery

For Amblyopia, the treatment includes an eye patch over the afflicted eye until it is corrected.

Lesson Summary

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.

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