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Perception (psychology), process by which organisms interpret and organize sensation to produce a meaningful experience of the world. Sensation usually refers to the immediate, relatively unprocessed result of stimulation of sensory receptors in the eyes, ears, nose, tongue, or skin. Perception, on the other hand, better describes one's ultimate experience of the world and typically involves further processing of sensory input. In practice, sensation and perception are virtually impossible to separate, because they are part of one continuous process.
Our sense organs translate physical energy from the environment into electrical impulses processed by the brain. For example, light, in the form of electromagnetic radiation, causes receptor cells in our eyes to activate and send signals to the brain. But we do not understand these signals as pure energy. The process of perception allows us to interpret them as objects, events, people, and situations.
Without the ability to organize and interpret sensations, life would seem like a meaningless jumble of colors, shapes, and sounds. A person without any perceptual ability would not be able to recognize faces, understand language, or avoid threats. Such a person would not survive for long. In fact, many species of animals have evolved exquisite sensory and perceptual systems that aid their survival.
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II. Principles Of Perceptual Organization
Organizing raw sensory stimuli into meaningful experiences involves cognition, a set of mental activities that includes thinking, knowing, and remembering. Knowledge and experience are extremely important for perception, because they help us make sense of the input to our sensory systems. To understand these ideas, try to read the following passage:
You could probably read the text, but not as easily as when you read letters in their usual orientation. Knowledge and experience allowed you to understand the text. You could read the words because of your knowledge of letter shapes, and maybe you even have some prior experience in reading text upside down. Without knowledge of letter shapes, you would perceive the text as meaningless shapes, just as people who do not know Chinese or Japanese see the characters of those languages as meaningless shapes. Reading, then, is a form of visual perception.
Note that in the example above, you did not stop to read every single letter carefully. Instead, you probably perceived whole words and phrases. You may have also used context to help you figure out what some of the words must be. For example, recognizing upside may have helped you predict down, because the two words often occur together. For these reasons, you probably overlooked problems with the individual letters—some of them, such as the n in down, are mirror images of normal letters. You would have noticed these errors immediately if the letters were right side up, because you have much more experience seeing letters in that orientation.
How people perceive a well-organized pattern or whole, instead of many separate parts, is a topic of interest in Gestalt psychology. According to Gestalt psychologists, the whole is different than the sum of its parts.
A. Gestalt Laws of Grouping
The three founders of Gestalt psychology were German researchers Max Wertheimer, Kurt Koffka, and Wolfgang Köhlerrts. Gestalt is a German word meaning configuration or pattern.
These men identified a number of principles by which people organize isolated parts of a visual stimulus into groups or whole objects. There are five main laws of grouping: proximity, similarity, continuity, closure, and common fate. A sixth law, that of simplicity, encompasses all of these laws.
Although most often applied to visual perception, the Gestalt laws also apply to perception in other senses. When we listen to music, for example, we do not hear a series of disconnected or random tones. We interpret the music as a whole, relating the sounds to each other based on how similar they are in pitch, how close together they are in time, and other factors. We can perceive melodies, patterns, and form in music. When a song is transposed to another key, we still recognize it, even though all of the notes have changed.
1. Proximity
The law of proximity states that the closer objects are to one another, the more likely we are to mentally group them together. In the illustration below, we perceive as groups the boxes that are closest to one another. Note that we do not see the second and third boxes from the left as a pair, because they are spaced farther apart.
2. Similarity
The law of similarity leads us to link together parts of the visual field that are similar in color, lightness, texture, shape, or any other quality. That is why, in the following illustration, we perceive rows of objects instead of columns or other arrangements.
3. Continuity
The law of continuity leads us to see a line as continuing in a particular direction, rather than making an abrupt turn. In the drawing on the left below, we see a straight line with a curved line running through it. Notice that we do not see the drawing as consisting of the two pieces in the drawing on the right.
4. Closure
According to the law of closure, we prefer complete forms to incomplete forms. Thus, in the drawing below, we mentally close the gaps and perceive a picture of a duck. This tendency allows us to perceive whole objects from incomplete and imperfect forms.
5. Common Fate
The law of common fate leads us to group together objects that move in the same direction. In the following illustration, imagine that three of the balls are moving in one direction, and two of the balls are moving in the opposite direction. If you saw these in actual motion, you would mentally group the balls that moved in the same direction. Because of this principle, we often see flocks of birds or schools of fish as one unit.
6. Simplicity
Central to the approach of Gestalt psychologists is the law of prägnanz, or simplicity. This general notion, which encompasses all other Gestalt laws, states that people intuitively prefer the simplest, most stable of possible organizations. For example, look at the illustration below. You could perceive this in a variety of ways: as three overlapping disks; as one whole disk and two partial disks with slices cut out of their right sides; or even as a top view of three-dimensional, cylindrical objects. The law of simplicity states that you will see the illustration as three overlapping disks, because that is the simplest interpretation.
B. Figure and Ground
Not only does perception involve organization and grouping, it also involves distinguishing an object from its surroundings. Notice that once you perceive an object, the area around that object becomes the background. For example, when you look at your computer monitor, the wall behind it becomes the background. The object, or figure, is closer to you, and the background, or ground, is farther away.
Gestalt psychologists have devised ambiguous figure-ground relationships—that is, drawings in which the figure and ground can be reversed—to illustrate their point that the whole is different from the sum of its parts. Consider the accompanying illustration entitled “Figure and Ground.” You may see a white vase as the figure, in which case you will see it displayed on a dark ground. However, you may also see two dark faces that point toward one another. Notice that when you do so, the white area of the figure becomes the ground. Even though your perception may alternate between these two possible interpretations, the parts of the illustration are constant. Thus, the illustration supports the Gestalt position that the whole is not determined solely by its parts. The Dutch artist M. C. Escher was intrigued by ambiguous figure-ground relationships.
Although such illustrations may fool our visual systems, people are rarely confused about what they see. In the real world, vases do not change into faces as we look at them. Instead, our perceptions are remarkably stable. Considering that we all experience rapidly changing visual input, the stability of our perceptions is more amazing than the occasional tricks that fool our perceptual systems. How we perceive a stable world is due, in part, to a number of factors that maintain perceptual constancy.
III. Perceptual Constancy
As we view an object, the image it projects on the retinas of our eyes changes with our viewing distance and angle, the level of ambient light, the orientation of the object, and other factors. Perceptual constancy allows us to perceive an object as roughly the same in spite of changes in the retinal image. Psychologists have identified a number of perceptual constancies, including lightness constancy, color constancy, shape constancy, and size constancy.
A. Lightness Constancy
Lightness constancy means that our perception of an object's lightness or darkness remains constant despite changes in illumination. To understand lightness constancy, try the following demonstration. First, take a plain white sheet of paper into a brightly lit room and note that the paper appears to be white. Then, turn out a few of the lights in the room. Note that the paper continues to appear white. Next, if it will not make the room pitch black, turn out some more lights. Note that the paper appears to be white regardless of the actual amount of light energy that enters the eye.
Lightness constancy illustrates an important perceptual principle: Perception is relative. Lightness constancy may occur because the white piece of paper reflects more light than any of the other objects in the room—regardless of the different lighting conditions. That is, you may have determined the lightness or darkness of the paper relative to the other objects in the room. Another explanation, proposed by 19th-century German physiologist Hermann von Helmholtz, is that we unconsciously take the lighting of the room into consideration when judging the lightness of objects.
B. Color Constancy
Color constancy is closely related to lightness constancy. Color constancy means that we perceive the color of an object as the same despite changes in lighting conditions. You have experienced color constancy if you have ever worn a pair of sunglasses with colored lenses. In spite of the fact that the colored lenses change the color of light reaching your retina, you still perceive white objects as white and red objects as red. The explanations for color constancy parallel those for lightness constancy. One proposed explanation is that because the lenses tint everything with the same color, we unconsciously “subtract” that color from the scene, leaving the original colors.
C. Shape Constancy
Another perceptual constancy is shape constancy, which means that you perceive objects as retaining the same shape despite changes in their orientation.
To understand shape constancy, hold a book in front of your face so that you are looking directly at the cover. The rectangular nature of the book should be very clear. Now, rotate the book away from you so that the bottom edge of the cover is much closer to you than the top edge. The image of the book on your retina will now be quite different. In fact, the image will now be trapezoidal, with the bottom edge of the book larger on your retina than the top edge. (Try to see the trapezoid by closing one eye and imagining the cover as a two-dimensional shape.) In spite of this trapezoidal retinal image, you will continue to see the book as rectangular. In large measure, shape constancy occurs because your visual system takes depth into consideration.
D. Size Constancy
Depth perception also plays a major role in size constancy, the tendency to perceive objects as staying the same size despite changes in our distance from them. When an object is near to us, its image on the retina is large. When that same object is far away, its image on the retina is small. In spite of the changes in the size of the retinal image, we perceive the object as the same size. For example, when you see a person at a great distance from you, you do not perceive that person as very small. Instead, you think that the person is of normal size and far away. Similarly, when we view a skyscraper from far away, its image on our retina is very small—yet we perceive the building as very large.
Psychologists have proposed several explanations for the phenomenon of size constancy. First, people learn the general size of objects through experience and use this knowledge to help judge size. For example, we know that insects are smaller than people and that people are smaller than elephants. In addition, people take distance into consideration when judging the size of an object. Thus, if two objects have the same retinal image size, the object that seems farther away will be judged as larger. Even infants seem to possess size constancy.
Another explanation for size constancy involves the relative sizes of objects. According to this explanation, we see objects as the same size at different distances because they stay the same size relative to surrounding objects. For example, as we drive toward a stop sign, the retinal image sizes of the stop sign relative to a nearby tree remain constant—both images grow larger at the same rate.
IV. Depth Perception
Depth perception is the ability to see the world in three dimensions and to perceive distance. Although this ability may seem simple, depth perception is remarkable when you consider that the images projected on each retina are two-dimensional. From these flat images, we construct a vivid three-dimensional world. To perceive depth, we depend on two main sources of information: binocular disparity, a depth cue that requires both eyes; and monocular cues, which allow us to perceive depth with just one eye.
A. Binocular Disparity
Because our eyes are spaced about 7 cm (about 3 in) apart, the left and right retinas receive slightly different images. This difference in the left and right images is called binocular disparity. The brain intFor a demonstration of binocular disparity, fully extend your right arm in front of you and hold up your index finger.
Now, alternate closing your right eye and then your left eye while focusing on your index finger. Notice that your finger appears to jump or shift slightly—a consequence of the two slightly different images received by each of your retinas. Next, keeping your focus on your right index finger, hold your left index finger up much closer to your eyes. You should notice that the nearer finger creates a double image, which is an indication to your perceptual system that it is at a different depth than the farther finger. When you alternately close your left and right eyes, notice that the nearer finger appears to jump much more than the more distant finger, reflecting a greater amount of binocular disparity.
You have probably experienced a number of demonstrations that use binocular disparity to provide a sense of depth. A stereoscope is a viewing device that presents each eye with a slightly different photograph of the same scene, which generates the illusion of depth. The photographs are taken from slightly different perspectives, one approximating the view from the left eye and the other representing the view from the right eye. The View-Master, a children's toy, is a modern type of stereoscope.
Filmmakers have made use of binocular disparity to create 3-D (three-dimensional) movies. In 3-D movies, two slightly different images are projected onto the same screen. Viewers wear special glasses that use colored filters (as for most 3-D movies) or polarizing filters (as for 3-D IMAX movies). The filters separate the image so that each eye receives the image intended for it. The brain combines the two images into a single three-dimensional image. Viewers who watch the film without the glasses see a double image.egrates these two images into a single three-dimensional image, allowing us to perceive depth and distance.
Another phenomenon that makes use of binocular disparity is the autostereogram. The autostereogram is a two-dimensional image that can appear three-dimensional without the use of special glasses or a stereoscope. Several different types of autostereograms exist. The most popular, based on the single-image random dot stereogram, seemingly becomes three-dimensional when the viewer relaxes or defocuses the eyes, as if focusing on a point in space behind the image. The two-dimensional image usually consists of random dots or lines, which, when viewed properly, coalesce into a previously unseen three-dimensional image. This type of autostereogram was first popularized in the Magic Eye series of books in the early 1990s, although its invention traces back to 1979. Most autostereograms are produced using computer software. The mechanism by which autostereograms work is complex, but they employ the same principle as the stereoscope and 3-D movies. That is, each eye receives a slightly different image, which the brain fuses into a single three-dimensional image.
Although binocular disparity is a very useful depth cue, it is only effective over a fairly short range—less than 3 m (10 ft). As our distance from objects increases, the binocular disparity decreases—that is, the images received by each retina become more and more similar. Therefore, for distant objects, your perceptual system cannot rely on binocular disparity as a depth cue. However, you can still determine that some objects are nearer and some farther away because of monocular cues about.
B. Monocular Cues
Close one eye and look around you. Notice the richness of depth that you experience. How does this sharp sense of three-dimensionality emerge from input to a single two-dimensional retina? The answer lies in monocular cues, or cues to depth that are effective when viewed with only one eye.
The problem of encoding depth on the two-dimensional retina is quite similar to the problem faced by an artist who wishes to realistically portray depth on a two-dimensional canvas. Some artists are amazingly adept at doing so, using a variety of monocular cues to give their works a sense of depth.
Although there are many kinds of monocular cues, the most important are interposition, atmospheric perspective, texture gradient, linear perspective, size cues, height cues, and motion parallax.
1. Interposition
Probably the most important monocular cue is interposition, or overlap.
When one object overlaps or partly blocks our view of another object, we judge the covered object as being farther away from us. This depth cue is all around us—look around you and notice how many objects are partly obscured by other objects. To understand how much we rely on interposition, try this demonstration. Hold two pens, one in each hand, a short distance in front of your eyes. Hold the pens several centimeters apart so they do not overlap, but move one pen just slightly farther away from you than the other. Now close one eye. Without binocular vision, notice how difficult it is to judge which pen is more distant. Now, keeping one eye closed, move your hands closer and closer together until one pen moves in front of the other. Notice how interposition makes depth perception much easier.
2. Atmospheric Perspective
The air contains microscopic particles of dust and moisture that make distant objects look hazy or blurry. This effect is called atmospheric perspective or aerial perspective, and we use it to judge distance. In the song “America the Beautiful,” the line that speaks of “purple mountains' majesty” is referring to the effect of atmospheric perspective, which makes distant mountains appear bluish or purple. When you are standing on a mountain, you see brown earth, gray rocks, and green trees and grass—but little that is purple. When you are looking at a mountain from a distance, however, water droplets suspended in the air bend the light so that the rays that reach your eyes lie in the blue or purple part of the color spectrum. This same effect makes the sky appear blue.
3. Texture Gradient
An influential American psychologist, James J. Gibson, was among the first people to recognize the importance of texture gradient in perceiving depth. A texture gradient arises whenever we view a surface from a slant, rather than directly from above. Most surfaces—such as the ground, a road, or a field of flowers—have a texture. The texture becomes denser and less detailed as the surface recedes into the background, and this information helps us to judge depth. For example, look at the floor or ground around you. Notice that the apparent texture of the floor changes over distance. The texture of the floor near you appears more detailed than the texture of the floor farther away. When objects are placed at different locations along a texture gradient, judging their distance from you becomes fairly easy.
4. Linear Perspective
Artists have learned to make great use of linear perspective in representing a three-dimensional world on a two-dimensional canvas. Linear perspective refers to the fact that parallel lines, such as railroad tracks, appear to converge with distance, eventually reaching a vanishing point at the horizon. The more the lines converge, the farther away they appear.
5. Size Cues
Another visual cue to apparent depth is closely related to size constancy. According to size constancy, even though the size of the retinal image may change as an object moves closer to us or farther from us, we perceive that object as staying about the same size. We are able to do so because we take distance into consideration. Thus, if we assume that two objects are the same size, we perceive the object that casts a smaller retinal image as farther away than the object that casts a larger retinal image. This depth cue is known as relative size, because we consider the size of an object's retinal image relative to other objects when estimating its distance.
Another depth cue involves the familiar size of objects. Through experience, we become familiar with the standard size of certain objects, such as houses, cars, airplanes, people, animals, books, and chairs. Knowing the size of these objects helps us judge our distance from them and from objects around them.
6. Height Cues
We perceive points nearer to the horizon as more distant than points that are farther away from the horizon. This means that below the horizon, objects higher in the visual field appear farther away than those that are lower. Above the horizon, objects lower in the visual field appear farther away than those that are higher. For example, in the accompanying picture entitled “Relative Height,” the animals higher in the photo appear farther away than the animals lower in the photo. But above the horizon, the clouds lower in the photo appear farther away than the clouds higher in the photo. This depth cue is called relative elevation or relative height, because when judging an object's distance, we consider its height in our visual field relative to other objects.
7. Motion Parallax
The monocular cues discussed so far—interposition, atmospheric perspective, texture gradient, linear perspective, size cues, and height cues—are sometimes called pictorial cues, because artists can use them to convey three-dimensional information. Another monocular cue cannot be represented on a canvas. Motion parallax occurs when objects at different distances from you appear to move at different rates when you are in motion. The next time you are driving along in a car, pay attention to the rate of movement of nearby and distant objects. The fence near the road appears to whiz past you, while the more distant hills or mountains appear to stay in virtually the same position as you move. The rate of an object's movement provides a cue to its distance.
V. Motion Perception
Although motion plays an important role in depth perception, the perception of motion is an important phenomenon in its own right. It allows a baseball outfielder to calculate the speed and trajectory of a ball with extraordinary accuracy. Automobile drivers rely on motion perception to judge the speeds of other cars and avoid collisions. A cheetah must be able to detect and respond to the motion of antelopes, its chief prey, in order to survive.
Initially, you might think that you perceive motion when an object's image moves from one part of your retina to another part of your retina. In fact, that is what occurs if you are staring straight ahead and a person walks in front of you. Motion perception, however, is not that simple—if it were, the world would appear to move every time we moved our eyes. Keep in mind that you are almost always in motion. As you walk along a path, or simply move your head or your eyes, images from many stationary objects move around on your retina. How does your brain know which movement on the retina is due to your own motion and which is due to motion in the world? Understanding that distinction is the problem that faces psychologists who want to explain motion perception.
One explanation of motion perception involves a form of unconscious inference. That is, when we walk around or move our head in a particular way, we unconsciously expect that images of stationary objects will move on our retina. We discount such movement on the retina as due to our own bodily motion and perceive the objects as stationary.
In contrast, when we are moving and the image of an object does not move on our retina, we perceive that object as moving. Consider what happens as a person moves in front of you and you track that person's motion with your eyes. You move your head and your eyes to follow the person's movement, with the result that the image of the person does not move on your retina. The fact that the person's image stays in roughly the same part of the retina leads you to perceive the person as moving.
Psychologist James J. Gibson thought that this explanation of motion perception was too complicated. He reasoned that perception does not depend on internal thought processes. He thought, instead, that the objects in our environment contain all the information necessary for perception. Think of the aerial acrobatics of a fly. Clearly, the fly is a master of motion and depth perception, yet few people would say the fly makes unconscious inferences. Gibson identified a number of cues for motion detection, including the covering and uncovering of background. Research has shown that motion detection is, in fact, much easier against a background. Thus, as a person moves in front of you, that person first covers and then uncovers portions of the background.
People may perceive motion when none actually exists. For example, motion pictures are really a series of slightly different still pictures flashed on a screen at a rate of 24 pictures, or frames, per second. From this rapid succession of still images, our brain perceives fluid motion—a phenomenon known as stroboscopic movement.
VI. The Role of Experience
Experience in interacting with the world is vital to perception. For instance, kittens raised without visual experience or deprived of normal visual experience do not perceive the world accurately. In one experiment, researchers reared kittens in total darkness, except that for five hours a day the kittens were placed in an environment with only vertical lines. When the animals were later exposed to horizontal lines and forms, they had trouble perceiving these forms.
Philosophers have long debated the role of experience in human perception.
In the late 17th century, Irish philosopher William Molyneux wrote to his friend, English philosopher John Locke, and asked him to consider the following scenario: Suppose that you could restore sight to a person who was blind. Using only vision, would that person be able to tell the difference between a cube and a sphere, which she or he had previously experienced only through touch? Locke, who emphasized the role of experience in perception, thought the answer was no. Modern science actually allows us to address this philosophical question, because a very small number of people who were blind have had their vision restored with the aid of medical technology.
Two researchers, British psychologist Richard Gregory and British-born neurologist Oliver Sacks, have written about their experiences with men who were blind for a long time due to cataracts and then had their vision restored late in life. When their vision was restored, they were often confused by visual input and were unable to see the world accurately. For instance, they could detect motion and perceive colors, but they had great difficulty with complex stimuli, such as faces. Much of their poor perceptual ability was probably due to the fact that the synapses in the visual areas of their brains had received little or no stimulation throughout their lives. Thus, without visual experience, the visual system does not develop properly.
VII. The Role of Context
Visual experience is useful because it creates memories of past stimuli that can later serve as a context for perceiving new stimuli. Thus, you can think of experience as a form of context that you carry around with you.
Ordinarily, when you read, you use the context of your prior experience with words to process the words you are reading. Context may also occur outside of you, as in the surrounding elements in a visual scene. When you are reading and you encounter an unusual word, you may be able to determine the meaning of the word by its context. Similarly, when looking at the world, you routinely make use of context to interpret stimuli. For instance, look at Example A in the illustration called “Context Effects.” Note that you can perceive an identical stimulus as either a B or an 8, depending on whether you read the row of letters or the column of numbers. Your perception depends on the context.
Although context is useful most of the time, on some rare occasions context can lead you to misperceive a stimulus. Look at Example B in the “Context Effects” illustration. Which of the green circles is larger? You may have guessed that the green circle on the right is larger. In fact, the two circles are the same size. Your perceptual system was fooled by the context of the surrounding red circles.
VIII. Visual Illusions
A visual illusion occurs when your perceptual experience of a stimulus is substantially different from the actual stimulus you are viewing. In the previous example, you saw the green circles as different sizes, even though they were actually the same size. To experience another illusion, look at the illustration entitled “Zöllner Illusion.” What shape do you see? You may see a trapezoid that is wider at the top, but the actual shape is a square. Such illusions are natural artifacts of the way our visual systems work. As a result, illusions provide important insights into the functioning of the visual system. In addition, visual illusions are fun to experience.
Consider the pair of illusions in the accompanying illustration, “Illusions of Length.” These illusions are called geometrical illusions, because they use simple geometrical relationships to produce the illusory effects. The first illusion, the Müller-Lyer illusion, is one of the most famous illusions in psychology. Which of the two horizontal lines is longer? Although your visual system tells you that the lines are not equal, a ruler would tell you that they are equal. The second illusion is called the Ponzo illusion. Once again, the two lines do not appear to be equal in length, but they are.
Hearing
I. Introduction
Ear, organ of hearing and balance. Only vertebrates, or animals with backbones, have ears. Invertebrate animals, such as jellyfish and insects, lack ears, but have other structures or organs that serve similar functions. The most complex and highly developed ears are those of mammals.
II. Structure of the Human Ear
Like the ears of other mammals, the human ear consists of three sections: the outer, middle, and inner ear. The outer and middle ears function only for hearing, while the inner ear also serves the functions of balance and orientation.
A. Outer Ear
The outer ear is made up of the auricle, or pinna, and the outer auditory canal. The auricle is the curved part of the ear attached to the side of the head by small ligaments and muscles. It consists largely of elastic cartilage, and its shape helps collect sound waves from the air. The earlobe, or lobule, which hangs from the lower part of the auricle, contains mostly fatty tissue.
The outer auditory canal, which measures about 3 cm (about 1.25 in) in length, is a tubular passageway lined with delicate hairs and small glands that produce a wax-like secretion called cerumen.
The canal leads from the auricle to a thin taut membrane called the eardrum or tympanic membrane, which is nearly round in shape and about 10 mm (0.4 in) wide. It is the vibration of the eardrum that sends sound waves deeper into the ear, where they can be processed by complex organs and prepared for transmission to the brain. The cerumen in the outer auditory canal traps and retains dust and dirt that might otherwise end up on the eardrum, impairing its ability to vibrate.
The inner two-thirds of the outer auditory canal is housed by the temporal bone, which also surrounds the middle and inner ear. The temporal bone protects these fragile areas of the ear.
B. Middle Ear
The eardrum separates the outer ear from the middle ear. A narrow passageway called the eustachian tube connects the middle ear to the throat and the back of the nose. The eustachian tube helps keep the eardrum intact by equalizing the pressure between the middle and outer ear. For example, if a person travels from sea level to a mountaintop, where air pressure is lower, the eardrums may cause pain because the air pressure in the middle ear becomes greater than the air pressure in the outer ear. When the person yawns or swallows, the eustachian tube opens, and some of the air in the middle ear passes into the throat, adjusting the pressure in the middle ear to match the pressure in the outer ear. This equalizing of pressure on both sides of the eardrum prevents it from rupturing.
The middle ear is a narrow, air-filled chamber that extends vertically for about 15 mm (about 0.6 in) and for nearly the same distance horizontally. Inside this chamber is a linked chain of three ossicles, or very small bones. Both the Latin and common names of these bones are derived from their shapes. They are called the malleus, or hammer; the incus, or anvil; and the stapes, or stirrup, which is the tiniest bone in the body, being smaller than a grain of rice.
The hammer is partly embedded in the eardrum, and the stirrup fits into the oval window, a membrane that fronts the inner ear. Vibrations of the eardrum move the hammer. The motion of the hammer moves the anvil, which in turn moves the stirrup. As sound vibrations pass from the relatively large area of the eardrum through the chain of bones, which have a smaller area, their force is concentrated. This concentration amplifies, or increases, the sound just before it passes through the oval window and into the inner ear. When loud noises produce violent vibrations, two small muscles, called the tensor tympani and the stapedius, contract and limit the movement of the ossicles, thus protecting the middle and inner ear from damage.
C. The Inner Ear
The chain of bones in the middle ear leads into the convoluted structures of the inner ear, or labyrinth, which contains organs of both hearing and balance. The three main structures of the inner ear are the cochlea, the vestibule, and the three semicircular canals.
The cochlea is a coiled tube that bears a close resemblance to the shell of a snail, which is what the word means in Greek. Along its length the cochlea is divided into three fluid-filled canals: the vestibular canal, the cochlear canal, and the tympanic canal. The partition between the cochlear canal and the tympanic canal is called the basilar membrane. Embedded in the basilar membrane is the spiral-shaped organ of Corti. The sensory cells in the organ of Corti have thousands of hairlike projections that receive sound vibrations from the middle ear and send them on to the brain via the auditory nerve. In the brain they are recognized and interpreted as specific sounds.
The vestibule, the second main structure of the inner ear, helps the body maintain balance and orientation by monitoring the sensations of movement and position. Without a sense of balance, even simple functions like walking would pose impossible challenges. With no sense of orientation, people would not know if they were in a normal position, upside down, or lying on their sides. Both balance and orientation depend on nerve impulses to reach the brain when the body is unbalanced or disoriented. The brain, in turn, sends messages to appropriate muscles, causing them to correct the imbalance or reposition the body.
The vestibule is made up of two sacs, the utriculus and the sacculus. Special sensory areas in the walls of the utriculus send impulses to the brain indicating the position of the head. These sensory areas consist of hairlike projections embedded in gelatin. Covering the surface of the gelatin are small mineral particles. Depending on the position of the head, the gelatin and mineral particles exert varying pressures on the sensory cells. The cells, in turn, send particular patterns of stimulation to the brain, where the patterns are interpreted.
For example, when the head is upright, the gelatin and mineral particles press down on all the hairlike cells equally. When the head is tilted straight forward by dropping the chin, the gelatin and mineral particles pull on all the hairlike cells equally. If the head is tilted to one side or the other, the cells receive unequal stimulation, varying with the direction and amount of tilt. If the utriculus of both ears is destroyed by injury or disease, the head will hang down limply unless its position can be judged with the eyes. The utriculus is also used to detect the body's starting or stopping. If a person stops suddenly, the gelatin and mineral particles continue to move, exerting a forward pull on the hairlike cells. The cells then send a specific pattern of nerve impulses to the brain.
The structure of the sacculus is similar to that of the utriculus, but its function is not well understood. The sacculus may aid in determining body orientation, but it may also have a function in hearing.
Arising from the utriculus is the third main structure of the inner ear, the three semicircular canals. These canals direct body balance when the body moves in a straight line or rotates in any direction. Each canal also contains sensory areas with sensory hair cells that project into a cone-shaped cap of gelatin. Two of the semicircular canals are in a vertical position and are used to detect vertical movement, such as jumping or falling. The third canal is horizontal and detects horizontal movement, such as turning or spinning.
The action of the canals depends on the inertia of the fluid inside. When the motion of the body changes, the fluid lags behind, causing the hair cells in the canal to bend. The bending of the hair cells sends nerve impulses to the brain, which in turn informs the body of changes in the direction of movement.
III. Hearing
Sound is a series of vibrations moving as waves through air or other gases, liquids, or solids. A ringing bell, for example, sets off vibrations in the air. Detection of these vibrations, or sound waves, is called hearing. The detection of vibrations passing through the ground or water is also called hearing. Some animals can detect only vibrations passing through the ground, and others can hear only vibrations passing through water.
Humans, however, can hear vibrations passing through gases, solids, and liquids. Sometimes sound waves are transmitted to the inner ear by a method of hearing called bone conduction. For example, people hear their own voice partly by bone conduction. The voice causes the bones of the skull to vibrate, and these vibrations directly stimulate the sound-sensitive cells of the inner ear. Only a relatively small part of a normal person's hearing depends on bone conduction, but some totally deaf people can be helped if sound vibrations are transferred to the skull bones by a hearing aid.
Humans hear primarily by detecting airborne sound waves, which are collected by the auricles. The auricles also help locate the direction of sound. Although some people have auricular muscles so well-developed that they can wiggle their ears, human auricles, when compared to those of other mammals, have little importance. Many mammals, especially those with large ears, such as rabbits, can move their auricles in many directions so that sound can be picked up more easily.
After being collected by the auricles, sound waves pass through the outer auditory canal to the eardrum, causing it to vibrate. The vibrations of the eardrum are then transmitted through the ossicles, the chain of bones in the middle ear. As the vibrations pass from the relatively large area of the eardrum through the chain of bones, which have a smaller area, their force is concentrated. This concentration amplifies, or increases, the sound.
When the sound vibrations reach the stirrup, the stirrup pushes in and out of the oval window. This movement sets the fluids in the vestibular and tympanic canals in motion. To relieve the pressure of the moving fluid, the membrane of the oval window bulges out and in. The alternating changes of pressure in the fluid of the canals cause the basilar membrane to move. The organ of Corti, which is part of the basilar membrane, also moves, bending its hairlike projections. The bent projections stimulate the sensory cells to transmit impulses along the auditory nerve to the brain.
A. Loudness, Pitch, and Tone
Human ears are capable of perceiving an extraordinarily wide range of changes in loudness, the tiniest audible sound being about 1 trillion times less intense than a sound loud enough to cause the ear pain. The loudness or intensity of a noise is measured in a unit called the decibel. The softest audible sound to humans is 0 decibels, while painful sounds are those that rise above 140 decibels.
Besides loudness, the human ear can detect a sound's pitch, which is related to a sound's vibration frequency, or the number of sound waves passing into the ear in a given period. The greater the frequency, the higher the pitch. The maximum range of human hearing includes sound frequencies from about 15 to about 18,000 waves, or cycles, per second. Because the human ear cannot hear very low frequencies, the sound of one's own heartbeat is inaudible. At the other end of the scale, a highly pitched whistle producing 30,000 cycles per second is not audible to the human ear, but a dog can hear it.
The third characteristic of sound detected by the human ear is tone. The ability to recognize tone enables humans to distinguish a violin from a clarinet when both instruments are playing the same note. The least noticeable change in tone that can be picked up by the ear varies with pitch and loudness.
Another sonic phenomenon, known as masking, occurs because lower-pitched sounds tend to deafen the ear to higher-pitched sounds. To overcome the effects of masking in noisy places, people are forced to raise their voices.
IV. Diseases of the Human Ear
Some diseases of the ear can cause partial or total deafness. In addition, most diseases of the inner ear are associated with a disturbance of balance. Ear problems should be evaluated by specially trained physicians called otolaryngologists, who treat conditions ranging from eardrum injuries caused by physical trauma to bony deposits in the inner ear caused by the aging process.
The auricle and the opening into the outer auditory canal may be missing at birth. Acquired malformations of the outer ear include scarring from cuts and other wounds. Othematoma, known popularly as cauliflower ear, is a common result of injury to the ear cartilage followed by internal bleeding and excessive production of ear tissue.
A. Middle Ear Disorders
Diseases of the middle ear include perforation of the eardrum and infection. Perforation of the eardrum may be caused by injury from a sharp object, a blow to the ear, or by sudden changes in atmospheric pressure.
Infection of the middle ear, whether acute or chronic, is called otitis media. Acute otitis media with effusion includes all acute infections of the middle ear caused by pus-forming bacteria, which usually reach the middle ear by way of the eustachian tube. Bacterial infection of the mastoid process, a cone-shaped, honeycombed projection of bone behind the auricle, may occur as a complication of middle ear infections. Hearing impairment often follows because newly malformed tissues affect the mobility of the eardrum and the ossicles. Painful swelling of the eardrum may require a surgical incision to permit drainage of the middle ear. Since the use of penicillin and other antibiotics became widespread, mastoid complications have become much less frequent. Sometimes acute otitis media with effusion leads to a chronic infection that does not respond readily to antibacterial agents.
Sight
Eye (anatomy), light-sensitive organ of vision in animals. The eyes of various species vary from simple structures that are capable only of differentiating between light and dark to complex organs, such as those of humans and other mammals, that can distinguish minute variations of shape, color, brightness, and distance. The actual process of seeing is performed by the brain rather than by the eye. The function of the eye is to translate the electromagnetic vibrations of light into patterns of nerve impulses that are transmitted to the brain.
Eye (anatomy), light-sensitive organ of vision in animals. The eyes of various species vary from simple structures that are capable only of differentiating between light and dark to complex organs, such as those of humans and other mammals, that can distinguish minute variations of shape, color, brightness, and distance. The actual process of seeing is performed by the brain rather than by the eye. The function of the eye is to translate the electromagnetic vibrations of light into patterns of nerve impulses that are transmitted to the brain.
. The Human Eye
The entire eye, often called the eyeball, is a spherical structure approximately 2.5 cm (about 1 in) in diameter with a pronounced bulge on its forward surface.
The outer part of the eye is composed of three layers of tissue. The outside layer is the sclera, a protective coating. It covers about five-sixths of the surface of the eye. cornea. The middle layer of the coating of the eye is the choroid, a vascular layer lining the posterior three-fifths of the eyeball. The choroid is continuous with the ciliary body and with the iris, which lies at the front of the eye. The innermost layer is the light-sensitive retina.
The cornea is a tough, five-layered membrane through which light is admitted to the interior of the eye. Behind the cornea is a chamber filled with clear, watery fluid, the aqueous humor, which separates the cornea from the crystalline lens. The lens itself is a flattened sphere constructed of a large number of transparent fibers arranged in layers. It is connected by ligaments to a ringlike muscle, called the ciliary muscle, which surrounds it. The ciliary muscle and its surrounding tissues form the ciliary body. This muscle, by flattening the lens or making it more nearly spherical, changes its focal length.
The pigmented iris hangs behind the cornea in front of the lens, and has a circular opening in its center. The size of its opening, the pupil, is controlled by a muscle around its edge. This muscle contracts or relaxes, making the pupil larger or smaller, to control the amount of light admitted to the eye.
Behind the lens the main body of the eye is filled with a transparent, jellylike substance, the vitreous humor, enclosed in a thin sac, the hyaloid membrane. The pressure of the vitreous humor keeps the eyeball distended.
The retina is a complex layer, composed largely of nerve cells. The light-sensitive receptor cells lie on the outer surface of the retina in front of a pigmented tissue layer. These cells take the form of rods or cones packed closely together like matches in a box. Directly behind the pupil is a small yellow-pigmented spot, the macula lutea, in the center of which is the fovea centralis, the area of greatest visual acuity of the eye. At the center of the fovea, the sensory layer is composed entirely of cone-shaped cells. Around the fovea both rod-shaped and cone-shaped cells are present, with the cone-shaped cells becoming fewer toward the periphery of the sensitive area. At the outer edges are only rod-shaped cells.
Where the optic nerve enters the eyeball, below and slightly to the inner side of the fovea, a small round area of the retina exists that has no light-sensitive cells. This optic disk forms the blind spot of the eye.
III. Functioning of the Eye
In general the eyes of all animals resemble simple cameras in that the lens of the eye forms an inverted image of objects in front of it on the sensitive retina, which corresponds to the film in a camera.
Focusing the eye, as mentioned above, is accomplished by a flattening or thickening (rounding) of the lens. The process is known as accommodation. In the normal eye accommodation is not necessary for seeing distant objects. The lens, when flattened by the suspensory ligament, brings such objects to focus on the retina. For nearer objects the lens is increasingly rounded by ciliary muscle contraction, which relaxes the suspensory ligament. A young child can see clearly at a distance as close as 6.3 cm (2.5 in), but with increasing age the lens gradually hardens, so that the limits of close seeing are approximately 15 cm (about 6 in) at the age of 30 and 40 cm (16 in) at the age of 50. In the later years of life most people lose the ability to accommodate their eyes to distances within reading or close working range. This condition, known as presbyopia, can be corrected by the use of special convex lenses for the near range.
Structural differences in the size of the eye cause the defects of hyperopia, or farsightedness, and myopia, or nearsightedness. See Eyeglasses; Vision.
As mentioned above, the eye sees with greatest clarity only in the region of the fovea; due to the neural structure of the retina. The cone-shaped cells of the retina are individually connected to other nerve fibers, so that stimuli to each individual cell are reproduced and, as a result, fine details can be distinguished. The rodshaped cells, on the other hand, are connected in groups so that they respond to stimuli over a general area. The rods, therefore, respond to small total light stimuli, but do not have the ability to separate small details of the visual image. The result of these differences in structure is that the visual field of the eye is composed of a small central area of great sharpness surrounded by an area of lesser sharpness. In the latter area, however, the sensitivity of the eye to light is great. As a result, dim objects can be seen at night on the peripheral part of the retina when they are invisible to the central part.
The mechanism of seeing at night involves the sensitization of the rod cells by means of a pigment, called visual purple or rhodopsin, that is formed within the cells. Vitamin A is necessary for the production of visual purple; a deficiency of this vitamin leads to night blindness. Visual purple is bleached by the action of light and must be reformed by the rod cells under conditions of darkness. Hence a person who steps from sunlight into a darkened room cannot see until the pigment begins to form. When the pigment has formed and the eyes are sensitive to low levels of illumination, the eyes are said to be dark-adapted.
A brownish pigment present in the outer layer of the retina serves to protect the cone cells of the retina from overexposure to light. If bright light strikes the retina, granules of this brown pigment migrate to the spaces around the cone cells, sheathing and screening them from the light. This action, called light adaptation, has the opposite effect to that of dark adaptation.
Subjectively, a person is not conscious that the visual field consists of a central zone of sharpness surrounded by an area of increasing fuzziness. The reason is that the eyes are constantly moving, bringing first one part of the visual field and then another to the foveal region as the attention is shifted from one object to another. These motions are accomplished by six muscles that move the eyeball upward, downward, to the left, to the right, and obliquely. The motions of the eye muscles are extremely precise; the estimation has been made that the eyes can be moved to focus on no less than 100,000 distinct points in the visual field. The muscles of the two eyes, working together, also serve the important function of converging the eyes on any point being observed, so that the images of the two eyes coincide. When convergence is nonexistent or faulty, double vision results. The movement of the eyes and fusion of the images also play a part in the visual estimation of size and distance.
IV. Protective Structures
Several structures, not parts of the eyeball, contribute to the protection of the eye. The most important of these are the eyelids, two folds of skin and tissue, upper and lower, that can be closed by means of muscles to form a protective covering over the eyeball against excessive light and mechanical injury. The eyelashes, a fringe of short hairs growing on the edge of either eyelid, act as a screen to keep dust particles and insects out of the eyes when the eyelids are partly closed. Inside the eyelids is a thin protective membrane, the conjunctiva, which doubles over to cover the visible sclera. Each eye also has a tear gland, or lacrimal organ, situated at the outside corner of the eye. The salty secretion of these glands lubricates the forward part of the eyeball when the eyelids are closed and flushes away any small dust particles or other foreign matter on the surface of the eye. Normally the eyelids of human eyes close by reflex action about every six seconds, but if dust reaches the surface of the eye and is not washed away, the eyelids blink oftener and more tears are produced. On the edges of the eyelids are a number of small glands, the Meibomian glands, which produce a fatty secretion that lubricates the eyelids themselves and the eyelashes. The eyebrows, located above each eye, also have a protective function in soaking up or deflecting perspiration or rain and preventing the moisture from running into the eyes. The hollow socket in the skull in which the eye is set is called the orbit. The bony edges of the orbit, the frontal bone, and the cheekbone protect the eye from mechanical injury by blows or collisions.
V. Comparative Anatomy
The simplest animal eyes occur in the coelenterates and ctenophores, phyla comprising the jellyfish and somewhat similar primitive animals. These eyes, known as pigment eyes, consist of groups of pigment cells associated with sensory cells and often covered with a thickened layer of cuticle that forms a kind of lens. Similar eyes, usually having a somewhat more complex structure, occur in worms, insects, and mollusks.
Two kinds of image-forming eyes are found in the animal world, single and compound eyes. The single eyes are essentially similar to the human eye, though varying from group to group in details of structure. The lowest species to develop such eyes are some of the large jellyfish. Compound eyes, confined to the arthropods (see Arthropod), consist of a faceted lens, each facet of which forms a separate image on a retinal cell, creating a moasic field. In some arthropods the structure is more sophisticated, forming a combined image.
The eyes of other vertebrates are essentially similar to human eyes, although important modifications may exist. The eyes of such nocturnal animals as cats, owls, and bats are provided only with rod cells, and the cells are both more sensitive and more numerous than in humans. The eye of a dolphin has 7000 times as many rod cells as a human eye, enabling it to see in deep water. The eyes of most fish have a flat cornea and a globular lens and are hence particularly adapted for seeing close objects. Birds' eyes are elongated from front to back, permitting larger images of distant objects to be formed on the retina.
VI. Eye Diseases
Eye disorders may be classified according to the part of the eye in which the disorders occur.
The most common disease of the eyelids is hordeolum, known commonly as a sty, which is an infection of the follicles of the eyelashes, usually caused by infection by staphylococci. Internal sties that occur inside the eyelid and not on its edge are similar infections of the lubricating Meibomian glands. Abscesses of the eyelids are sometimes the result of penetrating wounds. Several congenital defects of the eyelids occasionally occur, including coloboma, or cleft eyelid, and ptosis, a drooping of the upper lid. Among acquired defects are symblepharon, an adhesion of the inner surface of the eyelid to the eyeball, which is most frequently the result of burns. Entropion, the turning of the eyelid inward toward the cornea, and ectropion, the turning of the eyelid outward, can be caused by scars or by spasmodic muscular contractions resulting from chronic irritation. The eyelids also are subject to several diseases of the skin such as eczema and acne, and to both benign and malignant tumors. Another eye disease is infection of the conjunctiva, the mucous membranes covering the inside of the eyelids and the outside of the eyeball. See Conjunctivitis; Trachoma.
Disorders of the cornea, which may result in a loss of transparency and impaired sight, are usually the result of injury but may also occur as a secondary result of disease; for example, edema, or swelling, of the cornea sometimes accompanies glaucoma.
The choroid, or middle coat of the eyeball, contains most of the blood vessels of the eye; it is often the site of secondary infections from toxic conditions and bacterial infections such as tuberculosis and syphilis. Cancer may develop in the choroidal tissues or may be carried to the eye from malignancies elsewhere in the body. The light-sensitive retina, which lies just beneath the choroid, also is subject to the same type of infections. The cause of retrolental fibroplasia, however—a disease of premature infants that causes retinal detachment and partial blindness—is unknown. Retinal detachment may also follow cataract surgery. Laser beams are sometimes used to weld detached retinas back onto the eye. Another retinal condition, called macular degeneration, affects the central retina. Macular degeneration is a frequent cause of loss of vision in older persons. Juvenile forms of this condition also exist.
The optic nerve contains the retinal nerve fibers, which carry visual impulses to the brain. The retinal circulation is carried by the central artery and vein, which lie in the optic nerve. The sheath of the optic nerve communicates with the cerebral lymph spaces. Inflammation of that part of the optic nerve situated within the eye is known as optic neuritis, or papillitis; when inflammation occurs in the part of the optic nerve behind the eye, the disease is called retrobulbar neuritis. When the pressure in the skull is elevated, or increased in intracranial pressure, as in brain tumors, edema and swelling of the optic disk occur where the nerve enters the eyeball, a condition known as papilledema, or chocked disk.
For disorders of the crystalline lens, see Cataract. See also Color Blindness.
VII. Eye Bank
Eye banks are organizations that distribute corneal tissue taken from deceased persons for eye grafts. Blindness caused by cloudiness or scarring of the cornea can sometimes be cured by surgical removal of the affected portion of the corneal tissue. With present techniques, such tissue can be kept alive for only 48 hours, but current experiments in preserving human corneas by freezing give hope of extending its useful life for months. Eye banks also preserve and distribute vitreous humor, the liquid within the larger chamber of the eye, for use in treatment of detached retinas. The first eye bank was opened in New York City in 1945. The Eye-Bank Association of America, in Rochester, New York, acts as a clearinghouse for information.
Nervous System, those elements within the animal organism that are concerned with the reception of stimuli, the transmission of nerve impulses, or the activation of muscle mechanisms.
Smell
Nose, organ of smell, and also part of the apparatus of respiration and voice. Considered anatomically, it may be divided into an external portion—the visible projection portion, to which the term nose is popularly restricted—and an internal portion, consisting of two principal cavities, or nasal fossae, separated from each other by a vertical septum, and subdivided by spongy or turbinated bones that project from the outer wall into three passages, or meatuses, with which various sinuses in the ethmoid, sphenoid, frontal, and superior maxillary bones communicate by The margins of the nostrils are usually lined with a number of stiff hairs (vibrissae) that project across the openings and serve to arrest the passage of foreign substances, such as dust and small insects, which might otherwise be drawn up with the current of air intended for respiration. narrow apertures.
The skeleton, or framework, of the nose is partly composed of the bones forming the top and sides of the bridge, and partly of cartilages. On either side are an upper lateral and a lower lateral cartilage, to the latter of which are attached three or four small cartilaginous plates, termed sesamoid cartilages. The cartilage of the septum separates the nostrils and, in association posteriorly with the perpendicular plate of the ethmoid and with the vomer, forms a complete partition between the right and left nasal fossae.
The nasal fossae, which constitute the internal part of the nose, are lofty and of considerable depth. They open in front through the nostrils and behind end in a vertical slit on either side of the upper pharynx, above the soft palate, and near the orifices of the Eustachian tubes, leading to the tympanic cavity of the ear.
In the olfactory region of the nose the mucous membrane is very thick and colored by a brown pigment. The olfactory nerve, or nerve of smell, terminates in the nasal cavity in several small branches; these ramify in the soft mucous membrane and end in tiny varicose fibers that in turn terminate in elongated epithelial cells projecting into the free surface of the nose.
Taste
Tongue (anatomy), muscular organ in the mouth, the primary organ of taste and important in the formation of speech and in the chewing and swallowing of food. The tongue, which is covered by a mucous membrane, extends from the hyoid bone at the back of the mouth upward and forward to the lips. Its upper surface, borders, and the forward part of the lower surface are free; elsewhere it is attached to adjacent parts of the mouth. The extrinsic muscles attach the tongue to external points, and the intrinsic muscle fibers, which run vertically, transversely, and longitudinally, allow it great range of movement. The upper surface is covered with small projections called papillae, which give it a rough texture. The color of the tongue, usually pinkish-red but discolored by various diseases, is an indication of health.
The tongue serves as an organ of taste, with taste buds scattered over its surface and concentrated toward the back of the tongue. In chewing, the tongue holds the food against the teeth; in swallowing, it moves the food back into the pharynx, and then into the esophagus when the pressure of the tongue closes the opening of the trachea, or windpipe. It also acts, together with the lips, teeth, and hard palate, to form word sounds.
Observations of cow tongues have recently revealed the presence of natural antibiotics on the tongue. The antibiotics are peptides that can prevent infection of cuts in the mouth by resident bacteria. Similar antibiotics are presumed to be produced by the human tongue as well.
Skin
The skin is considered the largest organ of the body and has many different functions. The skin functions in thermoregulation, protection, metabolic functions and sensation. The skin is divided into two main regions, the epidermis, and the dermis, each providing a distinct role in the overall function of the skin. The dermis is attached to an underlying hypodermis, also called subcutaneous connective tissue, which stores adipose tissue and is recognized as the superficial fascia of gross anatomy.
