- Define visual perception and the Gestalt Laws of Perceptual Organization
- Understand perception of movement, depth perception, and other aspects of visual perception
[SLIDE 1]
Visual perception is the process by which we organize or make sense of the sensory impressions caused by the light that strikes our eyes. Visual perception involves our knowledge, expectations, and motivations. Sensation may be thought of as a mechanical process, such as light stimulating the rods and cones of the retina, and perception is an active process through which we interpret the world.
We organize visual information into meaningful wholes by means of general knowledge and desire to fit incoming information into familiar patterns. The textbook shows an example of the principle of closure, where disconnected pieces of information are integrated into a meaningful whole. In other words, there is a tendency to perceive a complete or whole figure even when there are gaps in the sensory input.
[SLIDE 2]
Early in the 20th century, Gestalt psychologists noted certain consistencies in the way we integrate bits and pieces of sensory stimulation into meaningful wholes. They proposed rules that govern these processes. These rules are known as the laws of perceptual organization or the tendency to integrate perceptual elements into meaningful patterns.
One example is Figure–Ground Perception. For instance, individual cars seen against the background of the street are easier to pick out than cars piled on top of one another. When figure–ground relationships are ambiguous, or capable of being interpreted in various ways, our perceptions tend to be unstable and shift back and forth.
Figure 3.9 below shows an example of this by illustrating a vase against various backgrounds. Your perception changes as you view the vase placed over various images in the background.
[SLIDE 3]
Above: Figure 3.9 - The Rubin Vase
[SLIDE 4]
Gestalt psychologists have noted that our perceptions are also guided by rules or laws of proximity, similarity, continuity, and common fate.
When Part A of Figure 3.10 (below) is viewed, you may say it consists of six lines or of three groups of two parallel lines. If you said three sets of lines, you were influenced by the proximity or nearness, of some of the lines. There is no other reason for perceiving them in pairs or subgroups because all lines in the image are parallel and equal in length.
[SLIDE 5]
Above: Figure 3.10 - Some Gestalt Laws of Perceptual Organization
[SLIDE 6]
Again, refer to Figure 3.10 above. In part B, you may perceive the figure as a six-by-six grid or as three columns of Xs and three columns of Os. According to the law of similarity, we perceive similar objects as belonging together. For this reason, you may have been more likely to describe part B in terms of columns than in terms of rows or a grid.
In part C, you may see a circle with two lines stemming from it or you may see it as a broken line that goes through a circle. If you saw it as a single broken line, you were probably organizing your perceptions according to the rule of continuity. That is, we perceive a series of points or a broken line as having unity.
According to the law of common fate, elements seen moving together are perceived as belonging together. Thus, a group of people running in the same direction appears unified in purpose. Part D of Figure 3.10 provides another example of the law of closure, where arcs tend to be perceived as a circle, rather than as just a series of arcs.
[SLIDE 7]
Consider putting together a jigsaw puzzle. If you use the picture on the box as a guide, you are using top-down processing. That is, you are using the completed image to search for the proper pieces. With bottom-up processing, we begin with bits and pieces of information and try to assemble them in a pattern. This example holds true with the jigsaw puzzles and even when being placed into a social or vocational situation. In bottom-up processing, we do not have a clear idea of where we are going as a goal.
[SLIDE 8]
Have you ever been in a moving train when you get the feeling you are stationary and the images outside the window, such as another train, are moving instead? Or you might not be sure whether your train is moving forward or the other train is moving back.
The visual perception of movement is based on change of position relative to other objects. How can you be certain which train is moving? One way is to look for objects that you know are still, such as platform columns, houses, signs, or trees. If your position does not change in relation to them, your train is not moving. You might also try to sense the motion of the train in your body. These are all perceptions of real movement.
Psychologists have also studied several types of apparent movement, or illusions - sensations that give rise to misperceptions. One of these illusions is stroboscopic motion. So-called motion pictures do not really consist of images that move. Instead, the audience is shown 16–22 pictures, or frames, per second. Each frame differs slightly from the preceding one. Showing the frames in rapid succession provides the illusion of movement. This illusion of motion is called stroboscopic motion. Figure 3.11 above describes this further.
[SLIDE 9]
Monocular and binocular cues help us perceive the depth of objects or, in other words, their distance from us. Artists use monocular cues called pictorial cues to create an illusion of depth. These cues can be perceived by one eye and include perspective, relative size, clearness, overlapping, shadows, and texture gradient, and they cause some objects to seem more distant than others even though they are all drawn or painted on a flat surface.
Distant objects stimulate smaller areas on the retina than nearby ones, even though they may be the same size. The distances between far-off objects also appear to be smaller than equivalent distances between nearby objects. For this reason, the phenomenon known as perspective occurs. That is, we tend to perceive parallel lines as coming closer together, or converging, as they recede from us. As we will see when we discuss size constancy, however, distant objects that look small are larger when they are close. In this way, their relative size also becomes a cue to their distance.
Artists normally use relative size to suggest depth in their works. The clearness of an object suggests its distance. Artists can suggest that objects are closer to the viewer by depicting them in greater detail. We also learn that nearby objects can block our view of more distant objects. Overlapping is the placing of one object in front of another. Figure 3.12 below illustrates these concepts.
[SLIDE 10]
Above: Figure 3.12 - Overlapping and Shadowing as Cues for Depth
[SLIDE 11]
Another monocular cue is texture gradient. Closer objects are perceived as having rougher textures. Motion cues are another kind of monocular cue. When driving in the country, you may notice that distant objects such as mountains and stars appear to move along with you. Objects at an intermediate distance seem to be stationary, but nearby objects such as roadside markers, rocks, and trees seem to go by quite rapidly.
The tendency of objects to seem to move backward or forward as a function of their distance is known as motion parallax. We learn to perceive objects that appear to move with us as being at greater distances. We noted that nearby objects cause the lens of the eye to accommodate or bend more in order to bring them into focus. The sensations of tension in the eye muscles also provide a monocular cue to depth, especially when we are within about four feet of the objects.
[SLIDE 12]
Binocular cues, or cues that involve both eyes, also help us perceive depth. Two binocular cues are retinal disparity and convergence.
Try an experiment. Hold your right index finger at arm’s length. Now hold your left index finger about a foot closer, but in a direct line. If you keep your eyes relaxed as you do so, you will see first one finger and then the other. An image of each finger will be projected onto the retina of each eye, and each image will be slightly different because the finger will be seen from different angles. The difference between the projected images is referred to as retinal disparity and serves as a binocular cue for depth perception. Closer objects have greater retinal disparity.
If we try to maintain a single image of the closer finger, our eyes must turn inward, or converge on it, making us cross-eyed. Convergence causes feelings of tension in the eye muscles and provides another binocular cue for depth. The binocular cues of retinal disparity and convergence are strongest when objects are close.
[SLIDE 13]
There are a number of perceptual constancies, including that of size constancy. The image of a dog seen from 20 feet away occupies about the same amount of space on your retina as an inch-long insect crawling on your hand. Yet you do not perceive the dog to be as small as the insect. Through your visual experiences you have acquired size constancy. That is, you perceive an object as the same size even though the size of its image on your retina varies as a function of its distance.
Experience teaches us about perspective -- that the same object seen at a distance appears to be smaller than when it is nearby. Figure 3.13 above illustrates this concept.
[SLIDE 14]
Color constancy is the tendency to perceive objects as retaining their color even though lighting conditions may alter their appearance. Your bright yellow car may edge toward gray evening arrives. But when you finally locate the car in the parking lot, you may still think of it as yellow. You expect to find a yellow car and still consider it to be more yellow than the nearby cars.
Brightness constancy is similar to color constancy. As depicted in Figure 3.14 above, the yellow–orange squares within the blue squares are equally bright, yet the one within the dark blue square is perceived as brighter. Why? Again, consider the role of experience. If it were nighttime, we would expect yellows and oranges to fade to gray. The fact that the yellow–orange within the dark square stimulates the eye with equal intensity suggests that it must be much brighter than the orange within the lighter square.
[SLIDE 15]
Shape constancy is the tendency to perceive objects as maintaining their shape, even if we look at them from different angles so that the shape of their image on the retina changes dramatically. You perceive the top of a coffee cup or a glass to be a circle even though it is a circle only when seen from above. When seen from an angle, it is an ellipse. When the cup or glass is seen on edge, its retinal image is the same as that of a straight line.
So why do you still describe the rim of the cup or glass as a circle? Perhaps for two reasons: First, experience has taught you that the cup will look circular when seen from above. Second, you may have labeled the cup as circular or round. Figure 3.15 above further illustrates this concept.
[SLIDE 16]
The principles of perceptual organization make it possible for our eyes to play tricks on us. That is, the perceptual constancies we have just discussed trick the eye through visual illusions. The Hering–Helmholtz and Müller–Lyer illusions are named after the people who devised them.
Figure 3.16 below shows examples of these illusions. In the Hering–Helmholtz illusion (part A), the horizontal lines are straight and parallel. However, the radiating lines cause them to appear to be bent outward near the center. The two lines in the Müller–Lyer illusion (part B) are the same length, but the line on the left, with its reversed arrowheads, looks longer.
[SLIDE 17]
Above: Figure 3.16 - The Hering-Helmholtz and Muller-Lyer Illusions