Chapter 4 Section 5

Chapter 4 Section 5 Perception Obj: Summarize the laws of sensory perception.

Rules of Perceptual Organization Gestalt psychologists applied the principle that “the whole is more than the sum of its parts” to the study of perception. Using this principle, they noted many different ways in which people make sense of sensory information. These ways are called the rules of perceptual organization and include closure, figure-ground perception, proximity, similarity, continuity, and common fate.

Closure – Do you see random blotches or a cat? If you perceive a cat, it is not just because of the visual sensations provided by the drawing. Those are actually quite confusing. The pattern is not very clear. Despite the lack of clarity, however, you can still see a cat. Why? The answer is that you are familiar with cats and that you try to fit the pieces of information into a familiar pattern. What you are doing with this picture is filling in the blanks. Gestalt psychologists refer to this as the principle of closure. Closure is the tendency to perceive a complete or whole figure even when there are gaps in what your senses tell you.

Figure-Ground Perception – What do you see? In the center of the drawing, you probably see a vase. If you look again, though, you may see more than a vase. Can you see two profiles? Figure-ground perception is the perception of figures against a background. When you saw a vase, it was a light-colored figure against a dark background. The profiles, on the other hand, were dark figures against a light background.

Other Rules of Organization -

If you saw the wavy line and the straight line, you were probably organizing your perceptions according to the rule of continuity. People usually prefer to see smooth, continuous patterns (like lines and waves), not disrupted ones.

Finally, there is the law of common fate. Have you ever noticed how when you see things moving together, you perceive them as belonging together? For example, a group of people running in the same direction would appear to have the same purpose. You assume that they are part of the same group and that they are all running to the same place-that they have a common fate.

Perception of Movement The next time you are in a car or a bus that is stopped at a traffic light, pay attention to what happens when the vehicle in the next lane begins to more forward. Do you think at first that your vehicle, not the other one, is moving? Is it unclear whether your car or bus is moving backward or the other one is moving forward?

To be able to sense movement, humans need to see an object change its position relative to other objects. We all know that Earth is moving, but do we really feel it? To early scientists, whose only instrument for visual observation was the naked eye, it seemed logical that the sun circled Earth. After all, that is what they seemed to be seeing. To observe that it is Earth that moves around the sun, we would have to be somewhere in outer space. We cannot observe it while standing on Earth itself.

So how do you decide which vehicle is moving at the traffic light? One way is to look for objects that you know are stable, like structures on the side of the road-buildings, signs, or trees. If you are steady in relation to them, then your vehicle is not moving. This also sheds light on how participants in events perceive the action from different points of view, physically and psychologically. Those favoring one side over another tend to be biased in their reporting of events. For example, the viewpoint of someone driving a car that runs a red light may differ from that of someone driving a car in another direction.

Stroboscopic Motion Along with perception of real movement, psychologists have also studied illusions of movement. One such illusion of movement is called stroboscopic motion. In stroboscopicmotion, the illusion of movement is produced by showing the rapid progression of images or objects that are not moving at all. Have you ever seen one of those little books designed to be flipped through quickly so that the figures on the pages appear to move? These books work because of stroboscopic motion.

Movies work in a similar way. Despite the name movie, movies do not consist of images that move. Instead, the audience is shown 16 to 22 pictures, or frames, per second. Each frame is just slightly different from the previous one. Showing the frames in rapid succession creates the illusion of movement. Why? Because of the law of continuity, humans prefer to see things as one continuous image. Perception smoothes over the interruptions and fills in the gaps.

Depth Perception Imagine trying to go through life without being able to judge depth or distance. You would have trouble going up or down stairs without stumbling. Depth, in this case, has little to do with the way people sometimes use the word deep. It is not, for instance, the depth of a lake or a hole. Depth here means “distance away.” for example, without really thinking about it, you decide how far away a glass of juice is from you. Can you just reach out and pick it up, or do you have to get out of your chair? You perceive the depth of objects through both monocular and binocular cues.

Monocular cues for Depth – Monocularcues need only one eye to be perceived. Artists use monocular cues to create an illusion of depth. These cues create the illusion of three dimensions, or depth, on two-dimensional, or flat, surfaces. Monocular cues cause certain objects to appear more distant from the viewer than others. These cues include perspective, clearness, overlapping, shadow, and texture gradient.

If you take two objects that are exactly the same size and place one of them far away from you and the other nearby, the object that is farther away will stimulate a smaller area of your retina than the one that is near. Even though the objects are the same size, the amount of sensory input from the more distant object is smaller because it is farther away. The distances between far-off objects also appear to be smaller than the same distances between nearby objects.

For this reason, the phenomenon known as perspective occurs. Perspective is the tendency to see parallel lines as coming closer together, or converging, as they move away from us. However, experience teaches us that objects that look small when they are far away will seem larger when they are close, even though their size does not actually change. In this way, our perception of a familiar object’s size also becomes a cue to its distance from us.

The clearness of an object also helps in telling us how far away it might be. Nearby objects appear to be clearer, and we see more details. Faraway objects seem less clear and less detailed. Thus, the clearer a familiar object seems to be, the closer it is to us. Overlapping is another monocular cue that tells us which objects are far away and which ones are near. Overlapping is the perceiving of one object as being in front of another. Nearby objects can block our view of more-distant objects. Experience teaches us to perceive partly covered objects as being farther away than the objects that block them from view.

Shadows and highlights also give us information about objects’ three-dimensional shapes and where they are placed in relation to the source of light.

Still another monocular cue is texture gradient. Texture, of course, is the surface quality and appearance of an object. A gradient is a progressive change. Texture that is farther away from us appears to be denser than texture that is closer, and we see less detail. Therefore, closer objects are perceived as having a more varied texture than objects that are farther away.

The most complex of monocular cues of depth is called motion parallax. It is more complex because, as you have probably guessed by the name, it involves not a stationary picture but the image of something as the viewer moves. Motion parallax is the tendency of objects to seem to move forward or backward depending on how far away they are from the viewer. Examples: Moon following you at night or passing a stationary object that appears to be moving backwards

Binocular Cues for Depth – Whereas monocular cues can be perceived with just one eye, both eyes are required to perceive binocular cues for depth. Two binocular cues for depth are retinal disparity and convergence.

Hold a finger at arm’s length. Now slowly bring it closer until it almost touches your nose. If you keep your eyes relaxed as you do this, you will seem to see two fingers. An image of the finger will be projected into the retina of each eye. Each image will be slightly different because the finger will be seen at different angles. This difference is referred to as retinal disparity. The closer your finger comes, the farther apart the “two fingers” appear to be. Thus, the amount of retinal disparity we detect gives us a cue about the depth of an object. However, retinal disparity serves as a cue to depth only for objects that are within a few feet of us, not for objects that are farther away.

The other binocular cue we use is called convergence. Convergence is associated with feelings of tension in the eye muscle. When we try to maintain a single image of the approaching finger, our eyes must turn inward, or converge on it, giving us a cross-eyed look. The closer we feel our eyes moving toward each other, the nearer the objects they are looking at is. Like retinal disparity, convergence has stronger effects when objects are close.

Perceptual Constancies • Size Constancy – the image of a dog seen from a distance of 20 feet occupies about the same amount of space on the retina as an inch-long insect crawling in the palm of the hand. Yet we do not perceive the dog to be as small as the insect. Similarly, we may say that people on the ground look like ants when we are at the top of a tall building, but we know they remain people even if the details of their forms are lost in the distance.

Through experience, people acquire a sense of size constancy. Size constancy is the tendency to perceive an object as being of one size no matter how far away the object is, even though the size of its image on the retina varies with its distance. Through experience, humans learn about perspective-that the same object seen at a great distance from the viewer will appear much smaller than when it is nearby.

Color Constancy – the tendency to perceive objects as keeping their color even though different light might change the appearance of their color.

Brightness Constancy – brightness constancy is the tendency to perceive an object as being equally bright even when the intensity of the light around it changes. We judge the brightness of an object by the brightness of other objects around it. For example, a black object really looks almost gray in very bright sunlight, but we still perceive it as being black because everything else around it is also much brighter.

Shape Constancy – Take a glass and look at it from directly above. You see a circle, right? Now move back slightly; it becomes an ellipse. When you look at the glass straight on, the image on the top of it is a line. So why would you still describe the rim of the glass as being a circle? Because of shapeconstancy - the knowledge that an item has only one shape no matter what angle you view that item from.

Visual Illusions Do your eyes sometimes “play tricks on you?” Actually, your eyes are not to blame, but your brain’s use of perceptual constancies is responsible. Your brain can trick your eye through visual illusions. The rule of size constancy is that if two objects seem to be the same size and one is farther away, the farther object must be larger than it actually seems.

Chapter 4 Section 5