Visual illusions
Visual illusions happen when our visual perception is “tricked” into seeing something inaccurately as the brain uses inappropriate strategies for interpreting sensory information received.
There are a number of reasons this happens such as:
- Misinterpreting depth cues – We incorrectly apply the rules of depth perception.
The Ponzo illusion
Depth cues help us to identify distance but with line drawings, we can be easily misled.
The Ponzo illusion above relies on the depth cue of linear perspective with the two outer lines of the drawing creating an illusion of perspective.
We therefore unconsciously see the top lines being further away and thus perceive it as being longer.
Measuring the lines however you can see they are both the same length.
The Muller-Lyer illusion
A similar effect is perceived with the Muller-Lyer illusion where we perceive the length of the lines as different dependent on whether the arrow points outwards or inwards – in truth the lines are of equal length:
Ambiguity – When an image could well be one thing or another.
Ambiguity is another cause for visual illusions.
When there are two equally possible explanations, the brain focuses on one explanation rather than the other.
The Rubin’s Vase illusion
The Rubin’s vase illusion is an example of this; we see either a vase or it could be two faces seen from the side.
We are able to see either the vase or the faces but not both at the same time as the other disappears.
The Necker Cube
The Necker Cube is another example of ambiguity.
The image of the Necker Cube is so ambiguous, the brain cannot decide where the front is as the image is perfectly balanced. It can, therefore, be seen in different ways as there is more than one possible viewpoint for it.
Fiction – Creating something that isn’t really there in order to complete the image.
The third type of illusion created by our minds is fiction.
The Kanizsa Triangle
The Kanizsa triangle is a good example of this below. We see a triangle in the middle but in reality, it is not there and a form of fiction our perception has created which has been influenced by the shapes around it.
Our perceptual system generates an image which fills the gap to create something plausible for us.
Size constancy – keeping the original perception of the size of an object even when information received by the eyes changes.
Perceptual constancy is about how we perceive objects as being the same (constant) even when the visual image we receive is different.
For example, if we look at a cup from different angles, the shapes we receive on the retina of our eyes is very different but we still see the same shape because we are applying constancy scaling which helps us make allowances for these changes. This is known as shape constancy.
A similar process occurs when we see people who are in the distance.
As they are further away we see them as smaller but as they approach we do not see them growing larger even though this is what is happening in the visual image. We apply size constancy which enables us to see them as the same size in reality.
The Ames Room illusion
The Ames room uses size constancy to produce a visual illusion.
It does this because we look at it from a specific viewpoint and the result is we see one person as being much larger than another.
This happens because although the room looks square it isn’t in reality.
The two people are also at different distances with the person who appears smaller actually being further away however the lines of the room are carefully designed to mask this from the observer.
See the image above and video below on how the Ames room actually works.
The Muller-Lyer illusion is a well-known optical illusion in which two lines of the same length appear to be of different lengths. The illusion was first created by a German psychologist named Franz Carl Muller-Lyer in 1889.
What Do You See?
In the top half of image above, which line appears the longest? For most people, the line with the fins of the arrow protruding outward (the center line) appears to be the longest, while the line with the arrow fins pointing inwards appears shorter. While your eyes might tell you that line in the middle is the longest, the shafts of both lines are exactly the same length, as shown in the bottom half of the image.
Like other optical illusions, the Muller-Lyer illusion has become the subject of considerable interest in psychology over the years. Different theories have emerged to explain the phenomenon.
How the Muller-Lyer Illusion Works
Optical illusions can be fun and interesting. But they also serve as an important tool for researchers. By looking at how we perceive these illusions, we can learn more about how the brain and perceptual process work. However, experts do not always agree on exactly what causes optical illusions, as is the case with the Muller-Lyer illusion.
The Size Constancy Explanation
According to psychologist Richard Gregory, this illusion occurs because of a misapplication of size constancy scaling. In most cases, size constancy allows us to perceive objects in a stable way by taking distance into account.
In the three-dimensional world, this principle allows us to perceive a tall person as tall whether they are standing next to us or off in the distance. When we apply this same principle to two-dimensional objects, Gregory suggests, errors can result.
Other researchers contend that Gregory’s explanation does not sufficiently explain this illusion. For example, other versions of the Muller-Lyer illusion utilize two circles at the end of the shaft. While there are no depth cues, the illusion still occurs. It has also been demonstrated that the illusion can even occur when viewing three-dimensional objects.
The Depth Cue Explanation
Depth plays an important role in our ability to judge distance. One explanation of the Muller-Lyer illusion is that our brains perceive the depths of the two shafts based upon depth cues. When the fins are pointing in toward the shaft of the line, we perceive it as sloping away much like the corner of a building. This depth cue leads us to see that line as further away and therefore shorter.
When the fins are pointing outward away from the line, it looks more like the corner of a room sloping toward the viewer. This depth cue leads us to believe that this line is closer and therefore longer.
The Conflicting Cues Explanation
An alternative explanation proposed by R. H. Day suggests that the Muller-Lyer illusion occurs because of conflicting cues. Our ability to perceive the length of the lines depends on the actual length of the line itself and the overall length of the figure. Since the total length of one figure is longer than the length of the lines themselves, it causes the line with the outward-facing fins to be seen as longer.
Researchers from the University of London suggest that the illusion demonstrates how the brain reflexively judges information about length and size before anything else.
“Many visual illusions might be so effective because they tap into how the human brain reflexively processes information. If an illusion can capture attention in this way, then this suggests that the brain processes these visual clues rapidly and unconsciously. This also suggests that perhaps optical illusions represent what our brains like to see,” explained researcher Dr. Michael Proulx.
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:
- 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.
