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Depth Perception

This essay will describe the contribution made to a general understanding of depth perception by analyzing visual illusions. Firstly it is important to discuss how important depth perception is. Depth perception is the visual ability to perceive the world in three dimensions. It is a trait common to many higher animals. Depth perception allows the beholder to measure the distance to an object accurately. Depth perception is important to people in everyday life because when you drive, depth is used to measure the distance of an approaching car. Another reason for the importance of depth perception is when one calls out to a friend walking down the street, one will determine how loudly to call depending on how far away the friend is perceived to be. If we as humans and animals did not have the ability of depth perception, the world would not be as it is today; it would be much more dysfunctional; for instance, car crashes would be frequent and talking louder than necessary.1

There is no depth in the eye; therefore, it must construct in the brain. The brain can distinguish between eye movements, which do the signal movement of objects (real movement), and those (head movements) which don’t. As will be discussed in further detail later in this essay, it is important to distinguish which cues the brain uses to construct depth or the third dimension. The activity of the brain is where the depth comes from, using cues. An area in the brain creates depth from visual images (retina, what the eye sees). In images, there are cues, which allow depth to construct a three-dimensional visual experience. Although this depth perception ability may seem simple, it is remarkable when you consider that the images projected on each retina are two-dimensional. From these flat images, a vivid three-dimensional world is constructed. To perceive depth when it is effective, we depend on two main sources of information: binocular disparity, a depth cue that requires both eyes; and monocular cues, which allow us to perceive depth with just one eye.2

Retinal disparity, stereopsis, accommodation, and convergence are all non-pictorial (primary) cues. These are all binocular and oculomotor cues to a depth that require both eyes to be used together. 3 Retinal disparity refers to the difference between two images because our eyes are nearly three inches apart; each retina receives a slightly different image of the world. When detected by the brain, it provides an important cue to distance. Our minds can perceive the world in three dimensions primarily because we have binocular vision. Binocular vision occurs when two eyes look at the same thing at a slightly different angle, resulting in two slightly different images. The blue squares below show images of the same block that each eye might see. It’s simple to confirm that we have binocular vision by placing your hand a foot in front of your face and alternate closing each eye; your hand will appear to jump back and forth. 4

The slight difference between viewpoints of your two eyes is called binocular disparity. Binocular disparity is the form of depth perception used by the human brain and is the most easily manipulated for perception tasks. The brain can take these two different views and put them together to form a solid, three-dimensional object. Stereopsis is the brain’s process to combine two images, so we do not see double images. This then allows us to experience one 3-D sensation rather than two different images. 5. It is the only true binocular cue. Although it is more powerful than convergence and accommodation, it’s less effective at extensive distances. Stereoscopic vision is used when looking at “magic eye” pictures. 6

Accommodation is a muscular cue; the eye’s lens changes shape when focused on an object, thickening for nearby objects and flattening for distant objects. Accommodation is known as an oculomotor. 7 Convergence is an oculomotor cue based on distance; it is the process by which the eyes point more and more inward as an object gets closer. 8. By noting the angle of convergence, the brain provides us with depth information over distances from about 6 to 20 feet (Hochberg, 1971). Convergence is one of two binocular cues of visual depth perception, in which kinaesthetic information on the amount of ocular convergence identifies the distance of objects being fixated. 9 Monocular Cues, on the other hand, involve those cues that exist for a single eye. This is one of the major categories for depth perception, as several different monocular cues help in-depth perception.

An important monocular cue for depth is provided by motion. As the world moves, whether that movement is caused by the perceiver or by his surrounding environment, objects in the environment move in distinct, predictable patterns. This concept is known as the motion parallax. 10 When an observer moves, the apparent relative motion of several stationary objects against a background gives hints about their relative distance. This effect can be seen clearly when driving in a car close by; things pass quickly, while far-off objects appear still. Some animals that lack binocular vision due to wide placement of the eyes employ parallax more explicitly than humans for depth cueing (e.g. some types of birds, which bob their heads to achieve motion parallax, and squirrels, which move in lines orthogonal to an object of interest to do the same).

There are various monocular cues to depth: Linear perspective, Texture, Occlusion, Light and shadow, Familiar size, and motion parallax. Linear perspective is a cue to depth based on the convergence of parallel lines in two-dimensional representations, e.g. railway tacks. 11. The lines appear to converge as they move away into the distance. (Gross 2005) The texture is a cue to depth based on a nearer surface having an increased texture density, e.g. Textured surfaces, such as pebbles on a beach, or waves on the sea, look rougher closer up than from a distance. Texture gradient is used as a clue to distance. (Eysenck 2004)

Occlusion is a cue to depth based on a closer object hiding part of a more distant one. This does not provide information about the distance of an object from us. Instead, it indicates relative depth, we know the partially covered object is farther away than another object, but we don’t know how much farther away it is.  Paul Signac’s painting, Place des Lices, St. Tropez (1893), makes extensive use of occlusion, showing how difficult it is to tell which branches in the picture are in front and back. (Goldtein 1999) 12 Light and shadow show a pattern of light and dark on and around an object. 3-D objects produce variations in light and shade. (Gross 2005). Highlights and shadows can provide information about an object’s dimensions and depth. Because our visual system assumes the light comes from above, a totally different perception is obtained if the image is viewed upside down. (Eysenck 2004)

Familiar size shows a collection of different-sized objects; smaller ones are usually seen as more distant, especially if they’re known to have a constant size. (Gross 2005). As the car drives away, the retinal image becomes smaller and smaller. We interpret this as the car getting further and further away. This is referred to as size constancy. A retinal image of a small car is also interpreted as a distant car. (Eysenck 2004) Motion parallax is a cue to depth based on the tendency of closer objects to move faster than images of more distant objects. Helmholtz (1866/1911) described how, as we walk along, nearby objects appear to glide rapidly past us, but more distant objects appear to move more slowly, e.g. trees seen from a moving train window flash by when close to the track.

We have considered a range of depth cues monocular, binocular, and oculomotor. Monocular cues can be used with one eye or both eyes, whereas binocular cues require both eyes to be used together. Binocular cues are only effective at close range, as are oculomotor cues. However, we do not see multiple copies of the world. More than one depth cue is available to us simultaneously; the cues must be combined in some way. It makes sense to combine information from depth cues, as most cues can sometimes provide inaccurate information, so relying on only one may lead to error. (Eysenck 2004) It is now important to look at visual illusions; to do this, we must first understand the concept ‘perception’. Perception refers to understanding what we take in through our senses; in terms of illusions, this means our eyes. Illusions occur because our brain is trying to understand what we see and make sense of the world around us. Illusions trick our brain into seeing things, which may or may not be genuine.13.

We can now look at examples of what happens when depth perception is ineffective. Although the perception is usually reliable, our perceptions sometimes misrepresent the world. We experience an illusion when our perception of an object doesn’t match its true physical characteristics. The brain processes information collected by the eye to give a percept that does not tally with the physical capacity of the stimulus causes. There are two main types of illusion – physiological illusions. These are the effects on the eyes and brain of too much stimulation of a specific type – brightness, tilt, color, movement, and cognitive illusions where the eye and brain make unconscious assumptions. 14 Visual illusions are often present in nature.

For instance, have you ever placed a stick in water and wondered why the part in the water appeared to be bent, or perhaps you’ve remembered a time when you watched a car’s tires spinning, and they appeared to spin in the opposite direction? Then the car was moving. These are just a few examples of illusions that occur in the world around us. 15 Physiological illusions, such as the afterimages following bright lights or adapting stimuli (a stimulus is a detectable change in the internal or external environment) of exceptionally longer flashing patterns, are supposed to be the effects on the eyes or brain of excessive stimulation of a specific type – brightness, tilt, color, movement, etc.

The theory is that stimuli have individual devoted neural paths in the early stages of visual processing and that repetitive stimulation of only one or a few channels causes a physiological imbalance that changes perception. 16 The Hermann grid illusion and Mach bands are two illusions that are best explained using a biological approach. Lateral inhibition, wherein the receptive field of the retina light and dark receptors compete to become active, has been used to explain why we see bands of increased brightness at the edge of a color difference when viewing Mach bands.

Once a receptor is active, it inhibits neighbouring receptors. This inhibition creates contrast, highlighting edges. In the Hermann grid illusion, the grey spots appear at the intersection because of the inhibitory response, which occurs due to the increased dark surround. Lateral inhibition has also been used to explain the Hermann grid illusion, but this has been disproved. 16 It is assumed that cognitive images arise by interaction with assumptions about the world, leading to “unconscious inferences,” an idea first suggested in the 19th century by Hermann Helmholtz. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions.16

  1. Ambiguous illusions are pictures or objects that elicit a perceptual ‘switch’ between the alternative interpretations. The Rubin vase is a well-known example.16
  2. Distorting illusions are characterized by distortions of size, length, or curvature. A famous example M�ller-Lyer illusion. The top horizontal line appears shorter than the bottom horizontal line, even though they are the same length. 16
  3. Paradox illusions are generated by objects that look ordinary at first. Still, after closer inspection, it’s obvious they cannot exist in reality, such as the Penrose triangle or impossible staircases, for example, in M. C. Escher’s Ascending and Descending and Waterfall. The triangle is an illusion dependent on a cognitive misunderstanding that adjacent edges must join. 16
  4. Fictional illusions are defined as the perception of objects that are genuinely not there to all but a single observer, such as those induced by schizophrenia or a hallucinogen known as hallucinations. Fictions help explain how we perceive that objects possess specific shapes; in the overlap of the triangle and disc illusion, there is no white triangle physically present, but we perceive the shape of a white triangle, which appears to be opaque and lighter the background. 16

Illusions can be based on an individual’s ability to see in three dimensions, even though the image hitting the retina is only two-dimensional. The Ponzo illusion is an example of an illusion that uses monocular depth perception cues to fool the eye.16 In the Ponzo illusion, the converging parallel lines tell the brain that the image higher in the visual field is further away. Therefore, the brain sees the image as larger. However, the two images hitting the retina are the same size. The Optical illusion is seen in a false perspective also exploits assumptions based on monocular cues of depth perception. The M. C. Escher painting Waterfall exploits rules of depth and proximity and our understanding of the physical world to create an illusion.16

Like depth perception, motion perception is responsible for several sensory illusions. Film animation is based on the illusion that the brain perceives a series of slightly varied images produced in rapid succession as a moving picture. When moving, as we would be while riding in a vehicle, steady surrounding objects may appear to move. We may also perceive a large object, like an airplane, to move more slowly than smaller objects, like a car, although the larger object is actually moving faster. The Phi phenomenon is another example of how the brain perceives motion, in which blinking lights are often created in close succession.16

In conclusion, this essay has described the contribution made to a general understanding of depth perception by analyzing the binocular and monocular cue approach and visual illusions. Brief examples have been demonstrated and discussed to explain depth perception. The cue approach to depth perception focuses on identifying information in the retinal image correlated with depth in the scene. E.g. if one object partially covers another object, as the blue circle does the red shown below, the partially covered object must be at a greater distance than the object covering it. This situation, called occlusion, is a signal, or cue, that one object is in front of another. According to the cue theory, we learn the connection between this cue and depth through our previous experience with the environment.

After this learning has occurred, the association between particular cues and depth becomes automatic, and when these depth cues are present, we experience the world in three dimensions. As discussed in Binocular and Monocular cues, several different types of cues have been identified that signal depth in the scene. These cues show how depth perception is effective and does take place by describing how our eyes receive images and how they are reflected on the brain from the eyes using different methods through binocular and monocular cue approaches to describe its impact on depth perception.

In contrast, studying visual illusions can help us to understand principles of depth perception. Thinking of an illusion as a phenomenon allows us to become knowingly aware of the complicated procedure that is always unfolding behind the scenes. The visual world that we perceive is always being created by an active mind continuously searching for patterns and explanations from the information it receives. 17 Illusions demonstrate the effects of depth perception going wrong and what happens when images are not clear and instead ambiguous.

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