Visual Perception
© 2010
This eText is the property of Toru Sato. All rights reserved © 2010. This eText is not to be copied, distributed, or downloaded without permission of the author. Any violation of copyright found in this eText is unintentional. Please notify the author if copyrighted material is found and not appropriately referenced.
Color Perception
The two most commonly discussed topics in visual perception are color perception and depth perception. We will first focus on color perception. As you have learned from Dr. Boeree's the chapter on the senses, the retina (the inside of the eyeball; see picture below) contains rods and cones. Rods are sensitive even in relative darkness but cannot sense color. Cones are not very effective in relative darkness but they allow us to see color.
Image Courtesy: National Eye Institute, National Institutes of Health
How do these cones allow us to see color? Back in the in the 19th century Thomas Young and Hermann von Helmholtz examined how the eye works and developed a theory. It is now known as the Young-Helmholtz trichromatic theory. This theory eventually allowed us to discover that there are three types of cones, each sensitive to a certain frequency range on the color spectrum. One kind of cone responds a frequency range that roughly corresponds to the color blue, another one to green, and yet another one to red. The relative strengths of the signals from the three types of cones are interpreted by the brain and this makes us experience all of the colors of our visual spectrum and not just blue, green and red (Goldstein, 2002). For example, yellow light is experienced when the cones responsible for red and green are activated but the ones responsible for blue are not.
Even though the trichromatic theory of color vision helps us understand how color vision works, it does not explain all aspects of color vision. Later work by Ewald Hering has shown that color vision is also processed in another part of the brain called the thalamus. When the cones send messages to the brain about what colors you might be seeing, the message first goes to the thalamus. Hering found that, in the thalamus, the information about what color you are seeing is processed in relation to its opposites (red-green, blue-yellow, & black-white). This theory, known as the opponent-process theory of color vision, helped us understand why we never see color certain color combinations, such as reddish-green or yellowish-blue. This is because opposing colors do not mix and our thalamus analyzes color with that assumption (Goldstein, 2002).
Depth Perception
Another important element of visual perception is depth perception. Depth perception is the visual ability to perceive the world in three dimensions. Even though our retina is a curved surface that is closer to two dimensions than three (like the inside half of a ball), there are many mechanisms at work that allow us to see things three dimensionally. These mechanisms are typically classified into two different kinds of depth cues, binocular and monocular depth cues. Binocular depth cues are cues that require input from both eyes and monocular depth cues are the ones that require the input from only one eye (Goldstein, 2002). Let us begin with binocular depth cues.
Binocular Depth Cues
The first one is called retinal (or binocular) disparity. As you know, humans typically have two eyes on different locations on the face (left & right side). This means that the two eyes are looking at things from a slightly different angle and sending messages of slightly different images to the brain. By using these two images of the same scene from a slightly different angle, the brain automatically calculates how far an object is. If the object is far away, the two images should be very similar. If the object is very close, the two images from our two eyes should be very different. This is known as retinal (or binocular) disparity.
The second binocular depth cue is called convergence. When we focus our two eyes on one object, our two eyes converge (they are directed inward at an angle). If the object is very close, our eyeballs will converge quite a lot. If it is far away, it will converge less. By sensing how much our eyes are converging, the brain automatically calculates how far an object is. This is depth cue is known as convergence.
Even though binocular depth cues are helpful, they are other ways to perceive depth. If you are fortunate enough to have good vision in both eyes, you can close one of your eyes and notice that you can still see depth. This is because we have monocular depth cues, cues of depth using only one eye.
Monocular Depth Cues
Let us begin with some simple monocular depth cues. One is called relative size. This depth cue is based on the idea that objects that are closer look bigger. The further away it is, the smaller it will look. In the picture below, the children in at the front look much larger than the children in the back. Another simple one is called interposition (or overlap). When two objects overlap in a visual image, we know that the one that covers and hides the other is closer to us. In the picture below, the boy in the white shirt in the third row is partially hidden behind the girl in the orange jacket in the second row. This, among other things, tells us that the girl is closer than the boy. Another depth cue that can be used when looking at the picture below is relative height. We know that the children lower on the picture are closer than the children higher up on the image. Relative height is a depth cue allowing us to understand that, when we are looking at things below our eye level, objects that are higher up on the visual image are further away. We can also use another depth cue when we look at the picture below. If you look closely, you will notice that the children sitting closer look brighter (reflect more light) and the children sitting in the back look darker. This is a depth cue known as relative brightness. In general, more light reaches our eyes when we look at objects that are closer to us. Therefore, in many cases, the further away an object is the darker it looks.
Image Courtesy: www.public-domain-image.com
There also other depth cues we are using when we look at a visual scene. When we look at the edges of the road in the image below, we assume that they are parallel to each other even though they look like they are converging. Even though, it looks like the road is becoming narrower, we assume that it is the same width all along and the narrower it looks, the further away it is. This is an example of a depth cue known as linear perspective. When two parallel lines converge, we sometimes interpret that as depth.
Image Courtesy: www.public-domain-image.com
Although you can use this next depth cue in the picture of the road above, it might be easier to see it in the picture of the flowers below. If you look at the photo of the flowers below, you will notice the nice texture that the flowers create in the meadow. You will also notice that the texture of the flowers turn from coarse to fine as you move your eyes from the flowers close to you to the flowers that are in the back. This is a depth cue known as texture gradient. When we see some texture in an image, we assume that the finer the texture becomes, the further away it is.
Image Courtesy: www.public-domain-image.com
Let's consider another depth cue, if you look at the object on the wall in the picture below, would you say that the dark round thing in the middle is bulging out or caving in? Most people would say that it is bulging out. This is because it is bright at the top and dark on the bottom. Even though there are exceptions, light usually comes from above in our everyday lives (e.g., sunlight, street lights, ceiling lights). Therefore, if we see some object on the wall that is bulging out, the top part reflects the light from above and looks bright. The bottom part, however, should look dark because it is in the shade. In contrast, if we see some object on the wall that is caving in, the bottom part reflects the light from above and looks bright. The top part should look dark because it is in the shade. This is a depth cue known as Light & Shadow (or Shadowing).
Image Courtesy: www.public-domain-image.com
Finally, the last monocular depth cue we will discuss is one that requires the observer to be moving. This depth cue is called motion parallax (or relative motion). When an observer moves, the relative movement of various motionless objects against a background tells us about their relative distance. If you are riding a bus and look out the side window, things closer than what your eyes are focusing on look like they are moving backwards. The closer it is, the faster is seems to move backwards. In contrast, things further away than what your eyes are focusing on look like they are moving forwards. The further away it is, the faster it seems to move forward. This is the main reason why the moon seems to follow us when we look outside a moving vehicle. Most things we can focus our eyes on are closer to us than the moon is. Therefore, the moon seems to be moving forward with us.
References
Goldstein, E. B. (2002). Sensation and perception (6th ed.). Pacific Grove CA: Wadsworth.