You'd think that after 6 million years of evolution, our brain would be extremely accurate at identifying what we are looking at. Yet, it can still be tricked by so many visual illusions. The reason is that we can't perceive all of the information we receive from our visual environment directly. Our brain uses hints and shortcuts to extract information about information like depth, these hints are called visual cues and constancies, and they are the reason we can see three-dimensionally.
Look at the image beneath. Even though it looks like a standing 3D triangle, we know it is a 2D picture.
A 2D picture can be drawn in a particular way to make us think that it is a 3D object, pixabay.com
- We will start with defining visual cues to understand the visual cues meaning and importance better.
Next, we will focus on visual constancies to understand how they relate to 3D perception.
Then, we will explore the visual cues types, including pictorial depth cues.
Throughout the article, we will use examples of visual cues and constancies to help you understand what they are.
Finally, we will examine how visual cues trick us into perceiving illusions.
Visual cues and perception
When we look around, the information that we see is sent to the brain. Interestingly, the retina's input is two-dimensional, yet we can see the world in three dimensions. So, how does our brain know about depth?
The retina is a structure located at the back of our eye. Its function is to translate the visual information from light to electrical signals and pass it to the nerves that connect the eyes with the brain.
Visual cues and perception: Howe we see the world
Depth perception is our ability to see the world in 3D. It allows us not only to see how long and wide objects are but also how far away they are. Understanding the distance of different objects from us is crucial for interaction with the environment.
Without depth perception, we wouldn't be able to judge which objects, animals, or people are close enough for us to interact with. We wouldn't be able to tell whether an object is just tiny or whether it is a normal size but far away. And, the world would be impossible to navigate, and vision wouldn't be very useful. Fortunately, all humans who can see are capable of depth perception.
There are two different types of depth cues: monocular depth cues and binocular depth cues. Cues that can be processed with one eye are called monocular cues. So, if you wore an eye patch, you would still be able to create a 3D representation of the environment. In comparison, binocular cues are when both eyes receive visual information to allow depth perception.
Some people might be unable to perceive binocular depth cues due to conditions such as blindness in one of their eyes. However, they are still capable of depth perception to some extent because they can rely on monocular depth cues.
Visual constancies meaning
We can perceive certain features of physical objects as the same despite changes in how they appear, which are called visual constancies. Let's now unpack this definition.
The way that objects are presented on our retina changes constantly, yet we don't see. For example, the furniture in our room continually changes shape as we move and our perspective changes. Similarly, when the lighting changes, we can still perceive objects as having the same colour. We don't see changes in oranges or apple's colour when the sun goes down. This allows us to see a stable and constant image of the world.
We distinguish three types of visual constancies; shape, colour and size constancy.
Perhaps the most relevant visual constancies for depth perception are shape and size constancies.
When we look at a rectangle from different perspectives, its shape becomes slightly distorted, yet we still perceive it as a rectangle, not a trapeze.
Our brain correctly interprets the distortions of the rectangle's shape as being due to changes in our or the rectangles positioning and not as a change in the object's shape.
Size constancy is also very telling in terms of distance and depth. We have a sense of how tall or wide different objects are. Therefore we can use information about changes in their size as hints about their distance from us. For example, we generally know how tall a human is. Therefore if we see a human in our visual field looking much smaller, we can assume they are standing far away from us.
Haber and Levin (2001) asked nine college students to estimate the distance between them and differing objects. The objects were either objects with commonly known sizes, objects that can vary in size or cardboard cut-outs of geometric shapes.
The researchers found that participants were much better at estimating the size of objects they were familiar with and that were of consistent size. The results suggest that size constancy and relative size are important cues for depth perception.
Visual cues types and examples
Let's now take a look at the different types of visual cues and how we experience them in real life.
Pictorial depth cues
Pictorial depth cues are all considered monocular. They can be depicted on 2D images and don't require any movement.
The first cue we'll describe is the height in plane. Elements of the visual scene that are the most elevated are interpreted as being the furthest from us. In contrast, objects that are lower down are interpreted as being closer to us. Consider the image below, the tree that appears the furthest away is also the most elevated.
Relative size is another important pictorial depth cue. The apparent size of an object relates to how far away it is from us. We know how big a tree typically is by comparing what we know about the tree size with the size that the tree appears to be. From this, we can estimate how far it is from us; this cue is also apparent in the image below. The tree that is the biggest appears to be the closest, while the tree that is the smallest appears the furthest away.
Trees standing alongside a path, Alicja Blaszkiewicz, Vaia Originals
Now let's look at the photo of the sunflower field. One cue we can use to assess the depth in this picture is occlusion. The sunflower that is the closest partially overlaps the one behind it. Typically, the view of objects that are further away will be mostly or entirely obstructed by nearer objects.
Another aspect of the picture that varies depending on distance is the texture gradient. We can see the fine texture in the centres of the sunflowers closest to us; this texture becomes smaller and less visible when looking at the sunflowers furthest away.
Sunflower field, freepik.com
Let's move on to the highway photo, where we can clearly see linear perspective. Since we are used to seeing how parallel lines look from a distance, we can quickly interpret the lines converging on this picture as a sign of distance rather than the road becoming more narrow.
Linear perspective on the example of a highway, freepik.com
Binocular depth cues
Binocular depth cues use the compares the information received from each eye. The differing images allow us to extract more information about the object's distance. Even though our eyes are close together, each looks at the world from a slightly different perspective.
You can experience the disparity between what your eyes see yourself. Place an object like your hand or your phone close to your face. Now, look at it with your right eye and then with your left eye only. You may notice some slight differences.
This retinal disparity, so the difference in information to the retina in each eye helps us perceive the relative distance of objects. This results in stereopsis, which is the perception of depth due to binocular vision.
Let's try another experiment. Try to look at an object that is very close to your face. You can use your finger or try to look at the tip of your nose. Then look at something very far away.
You can feel that the muscles of your eyes move when you look at things at different distances; this is called convergence. As your eyes converge to focus on the object in front of you, the muscles that control eye movement stretch. This muscle feedback is also a binocular cue that helps us estimate the object's distance.
Examples of visual cues and constancies in visual illusions
All these cues make a lot of sense and seem to be accurate. So, why do they sometimes mislead us? Every so often, the cues can be ambiguous or in conflict with each other, leading to visual illusions. While pictorial cues are mostly accurate in real-life settings, artificially created images can use them to trick our perception.
Let's consider the Kanizsa Triangle illusion. The placement of the figures in this image makes us see a white triangle in the middle, which is not actually there because we fill the contours with another figure.
The illusory interpretation of this image is also consistent with the occlusion cue.
Kanizsa triangle, Fibonacci, CC BY-SA 3.0 , via Wikimedia Commons
On the other hand, the Ponzo illusion is caused by our reliance on pictorial depth cues. The two yellow lines that you can see on the image below are actually of the same length, even though the line, which is higher up in the picture, appears much larger.
The illusion is caused because we have different pictorial cues like apparent size, height in plane, texture gradient and linear perspective, all of which suggest that the yellow line higher up is relatively much longer compared to the other one.
One of the lines is much shorter than the black lines parallel to it, and the other is longer; this gives us information about their apparent size.
Another cue is the height in plane; the difference between the height of the two lines suggests that one is further away.
The texture gradient created by the shorter black lines becomes smaller higher up on the picture, signalling information about distance. This also suggests that one of the yellow lines is much further away than the other.
We can also see linear perspective created by the convergence of the two lines perpendicular to the yellow lines, indicating depth.
If this were a scene we saw in real life, we would be right, and the lines wouldn't be the same size!
Ponzo illusion, Tony Philips, National Aeronautics and Space Adm., Public domain, via Wikimedia Commons
Visual Cues and Constancies - Key takeaways
- Depth perception refers to the ability to see the world in three dimensions. It is possible because of visual depth cues.
- Depth cues are all the elements of the visual scene that reliably signal information about depth and distance. We can distinguish between monocular depth cues and binocular depth cues.
- We can perceive certain features of physical objects as the same despite changes in how they appear; these features are called visual constancies. We distinguish three types of visual constancies; shape, colour and size constancy.
- Pictorial depth cues are all considered monocular and can be depicted on 2D images. Pictorial depth cues include height in plane, relative size, occlusion, and texture gradient.
- Binocular cues include retinal disparity and convergence.
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