7 Sensation and Perception
STEP 4 Review the Knowledge You Need to Score High
What you perceive is an active construction of reality. Perception results from the interaction of many neuron systems, each performing a simple task. Natural selection favors a perceptual system that is very efficient at picking up information needed for survival in a three-dimensional world in which there are predators, prey, competitors, and limited resources. According to the nativist direct-perception theory of James Gibson, inborn brain mechanisms enable even babies to create perceptions directly from information supplied by the sense organs. For visual perception, your visual cortex transmits information to association areas of your parietal and temporal lobes that integrate all the pieces of information to make an image you recognize. Your brain looks for constancies and simplicity, making a huge number of perceptual decisions, often without your conscious awareness, in essentially two different ways of processing. The particular stimuli you select to process greatly affect your perceptions.
Attention is the set of processes by which you choose from among the various stimuli bombarding your senses at any instant, allowing some to be further processed by your senses and brain. You focus your awareness on only a limited aspect of all you are capable of experiencing, which is selective attention. You are often blind to stimuli you do not focus on, which can lead to accidents. For example, if you are paying attention to a magician performing a trick, you may miss a gorilla sitting in the background which is inattentional blindness. Another example is if you are focused on what to order for dinner, you may not notice if your server changes shirts, this is called change blindness.
In data-driven bottom-up processing, your sensory receptors detect external stimulation and send these raw data to the brain for analysis. Hubel and Wiesel’s feature-detector theory assumes that you construct perceptions of stimuli from activity in neurons of the brain that are sensitive to specific features of those stimuli, such as lines, angles, even a letter or face. In his constructionist theory, Hermann von Helmholtz maintained that we learn through experience to convert sensations into accurate perceptions. Anne Treisman’s feature-integration theory proposes that detection of individual features of stimuli and integration into a whole occur sequentially in two different stages. First, the detection of features involves bottom-up parallel processing, and second, the integration of features involves less automatic, partially top-down serial processing.
Concept-driven top-down processing takes what you already know about particular stimulation, what you remember about the context in which it usually appears, and how you label and classify it, to give meaning to your perceptions. Your expectations, previous experiences, interests, and biases give rise to different perceptions. Where you perceive a conflict among senses, vision usually dominates, which is called visual capture. That accounts for why you think the voice is coming from a ventriloquist’s wooden pal when the puppet’s mouth moves.
Gestalt Organizing Principles of Form Perception
Max Wertheimer, Kurt Koffka, and Wolfgang Kohler studied how the mind organizes sensations into perceptions of meaningful patterns or forms, called a gestalt in German. These Gestalt psychologists concluded that in perception, the whole is different from, and can be greater than, the sum of its parts. Unlike structuralists of the early 1900s, they thought that forms are perceived not as combinations of features but as wholes.
This is exemplified by the phi phenomenon, which is the illusion of movement created by presenting visual stimuli in rapid succession. Videos consist of slightly different frames projected rapidly one after another, giving the illusion of movement. Gestaltists also noted that we see objects as distinct from their surroundings. The figure is the dominant object, and the ground is the natural and formless setting for the figure. This is called the figure—ground relationship. Gestaltists claimed that the nervous system is innately predisposed to respond to patterns of stimuli according to rules or principles. Their most general principle was the law of Pragnanz, or good form, which claimed that we tend to organize patterns in the simplest way possible. Other principles of organization include proximity, closure, similarity, and continuity or continuation. Consider the following: DEMON DAY BREAK FAST. Looking at the groups of letters, you probably read the four words demon, day, break, and fast, rather than Monday, daybreak, or breakfast. Proximity, the nearness of objects to each other, is an organizing principle. We perceive objects that are close together as parts of the same pattern. Do you know someone who writes letters without quite closing the letter o or crossing the t? You probably still know what the letter is. The principle of closure states that we tend to fill in gaps in patterns. The closure principle is not limited to vision. For example, if someone started singing, “Happy Birthday to . . . ,” you might finish it in your mind. The principle of similarity states that like stimuli tend to be perceived as parts of the same pattern. The principle of continuity or continuation states that we tend to group stimuli into forms that follow continuous lines or patterns.
Optical or visual illusions are discrepancies between the appearance of a visual stimulus and its physical reality. Visual illusions, such as reversible figures, illustrate the mind’s tendency to separate figure and ground in the absence of sufficient cues for deciding which is which. Illusory contours illustrate the tendency of the perceptual system to fill in missing elements to perceive whole patterns.
Survival in a three-dimensional world requires adaptations for determining the distances of objects around you. Depth perception is the ability to judge the distance of objects. You interpret visual cues that tell you how near or far away objects are. Cues are either monocular or binocular. Monocular cues are clues about distance based on the image of one eye, whereas binocular cues are clues about distance requiring two eyes.
Binocular cues include retinal disparity and convergence. Your principal binocular cue is retinal disparity, which is the slightly different view the two eyes have of the same object because the eyes are a few centimeters apart. You can experience retinal disparity by extending your arm directly in front of you with your thumb up. Close one eye while looking at your thumb with the other. Then close the open eye and open the closed eye. Your thumb appears to move with respect to the background. If you follow the same procedure with your thumb closer, you’ll notice that your thumb appears to move more. The degree of retinal disparity decreases with distance. With both eyes open, your brain fuses the two images, resulting in perception of depth. Convergence is the inward turning of your eyes that occurs when you look at an object that is close to you; the closer an object, the more convergence. You can experience convergence by looking at the tip of your nose with both eyes. Convergence is a less important distance cue than retinal disparity and cannot be used for objects beyond about 8 meters (about 25 feet).
Monocular cues include motion parallax, accommodation, interposition or overlap, relative size, relative clarity, texture gradient, relative height or elevation, linear perspective, and relative brightness. Motion parallax and accommodation require active use of your eye in viewing, whereas the other monocular cues are pictorial depth cues that can be given in a flat picture. Motion parallax involves images of objects at different distances moving across the retina at different rates. Closer objects appear to move more than distant objects when you move your head. When riding in a moving vehicle, you see very close objects move rapidly in the opposite direction; more distant objects move more slowly past you; extremely far away objects, such as the moon, seem to move with you. Accommodation of the lens increases as an object gets closer.
Look outside your window to notice all the pictorial cues:
• Interposition or overlap can be seen when a closer object cuts off the view of part or all of a more distant one.
• Relative size of familiar objects provides a cue to their distance when the closer of two same-size objects casts a larger image on your retina than the farther one.
• Relative clarity can be seen when closer objects appear sharper than more distant, hazy objects.
• Texture gradient provides a cue to distance when closer objects have a coarser, more distinct texture than faraway objects that appear more densely packed or smooth.
• Relative height or elevation can be seen when the objects closest to the horizon appear to be the farthest from you. The lowest objects in our field of vision generally seem the closest.
• Linear perspective provides a cue to distance when parallel lines, such as edges of sidewalks, seem to converge in the distance.
• Relative brightness can be seen when the closer of two identical objects reflects more light to your eyes.
• Optical illusions, such as the Müller-Lyer illusion and the Ponzo illusion, in which two identical horizontal bars seems to differ in length, may occur because distance cues lead one line to be judged as farther away than the other. Similarly, the moon illusion may occur because the moon, when near the horizon, is judged to be farther away than when it is high in the sky, although in both positions it casts the same image on the retina.
At the website www.optics4kids.org, you can see many examples of visual illusions and discover several uses for optics.
As a car approaches, you know that it’s not growing in size, even though the image it casts on your retina gets larger, because you impose stability on the constantly changing sensations you experience. This phenomenon is called perceptual constancy. Three perceptual constancies are size constancy, by which an object appears to stay the same size despite changes in the size of the image it casts on the retina as it moves farther away or closer; shape constancy, by which an object appears to maintain its normal shape regardless of the angle from which it is viewed; and brightness constancy, by which an object maintains a particular level of brightness regardless of the amount of light reflected from it. The real shape, orientation, size, brightness, and color are perceived as remaining relatively constant even when there are significant variations in the image it projects. This enables you to identify objects no matter what your viewing angle is, how far away you are, or how dim the lights are.
Perceptual Adaptation and Perceptual Set
Have you ever looked through a periscope or displacement goggles and tried to reach for an object only to find it wasn’t where you thought it was? If you repeated your actions, after a short period of time you were probably able to reach the item easily. You adapted to the changed visual input. Newly sighted people who had been blind from birth are immediately able to distinguish colors and to separate figure from ground, but only gradually become able to visually recognize shapes. Visual perception can also be influenced by cultural factors, assumptions, and beliefs. To make use of the cue of relative size, you need to be familiar with the object and have been exposed to viewing objects in the distance.
Culture and Experience
Your perceptual set or mental predisposition can influence what you perceive when you look at ambiguous stimuli. Your perceptual set is determined by the schemas you form as a result of your experiences. Schemas are concepts or frameworks that organize and interpret information. This can account for people’s interpretations of unidentified flying objects, the Loch Ness monster, or seeing a cloud of dust in a movie.