SENSATION AND PERCEPTION
Glance around you. Notice the incredible variety of colors, shades, shadows, and images. Listen carefully to the diversity of sounds, loud and soft, near and far. Focus on everything that’s touching you—your clothes, your shoes, the chair you’re sitting on. Now, inhale deeply through your nose and identify the aromas in the air.
With these simple observations you have exercised four of your senses: vision, hearing, touch, and smell. The primary function of the nervous system is communication—the transmission of information from one part of the body to the other. Where does that information come from? Put simply, your senses are the gateway through which your brain receives all its information about the environment. It’s a process that is so natural and automatic that we typically take it for granted until it is disrupted by illness or injury. Nevertheless, as the stories of Paul and Warren demonstrate, people with one nonfunctional sense are amazingly adaptive. Often, they learn to compensate for the missing environmental information by relying on their other senses. In this chapter, we will explore the overlapping processes of sensation and perception. Sensation refers to the detection and basic sensory experience of environmental stimuli, such as sounds, objects, and odors. Perception occurs when we integrate, organize, and interpret sensory information in a way that is meaningful. Here’s a simple example to contrast the two terms. Your eyes’ physical response to light, splotches of color, and lines reflects sensation. Integrating and organizing those sensations so that you interpret the light, splotches of color, and lines as a painting, a flag, or some other object reflects perception. Where does the process of sensation leave off and the process of perception begin? There is noclear boundary line between the two processes as we actually experience them. In fact, many researchers in this area of psychology regard sensationand perception as a single process. Although the two processes overlap, in this chapter we will present sensation and perception as separate discussions. In the first half of the chapter, we’ll discuss the basics of sensation— how our sensory receptors respond to stimulation and transmit that information in usable form to the brain. In the second half of the chapter, we’ll explore perception—how the brain actively organizes and interprets the signals sent from our sensory receptors. We’re accustomed to thinking of the senses as being quite different from one another.
However, all our senses involve some common processes. All sensation is a result of the stimulation of specialized cells, called sensory receptors, by some form of energy. Imagine biting into a crisp, red apple. Your experience of hearing the apple crunch is a response to the physical energy of vibrations in the air, or sound waves.The sweet taste of the apple is a response to the physical energy of dissolvable chemicals in your mouth, just as the distinctive sharp aroma of the apple is a response to airborne chemical molecules that you inhale through your nose. The smooth feel of the apple’s skin is a response to the pressure of the apple against your hand. And the mellow red color of the apple is a response to the physical energy of light waves reflecting from the irregularly shaped object you’ve just bitten into. Sensory receptors convert these different forms of physical energy into electrical impulses that are transmitted via neurons to the brain. The process by which a form of physical energy is converted into a coded neural signal thatcan be processed by the nervous system is called transduction. These neural signals are sent to the brain, where the perceptual processes of organizing and interpreting the coded messages occur. The basic steps involved in sensation and perception. We are constantly being bombarded by many different forms of energy. For instance, at this very moment radio and television waves are bouncing aroundthe atmosphere and passing through your body. However, sensory receptors are so highly specialized that they are sensitive only to very specific types of energy.
Sensing the World: Some Basic Principles
To study sensation is to study an ageless question: How does the world out there get represented in here, inside our heads? Put another way, how are the external stimuli that strike our bodies transformed into messages that our brains comprehend?
Thresholds
Each species comes equipped with sensitivities that enable it to survive and thrive. We sense only a portion of the sea of energy that surrounds us, but to this portion we are exquisitely sensitive. Our absolute threshold for any stimulus is the minimum stimulatioecessary for us to detect it 50 percent of the time. Signal detection researchers report that our individual absolute thresholds vary with our psychological state.
Experiments reveal that we can process some information from stimuli too weak to recognize. But the restricted conditions under which this occurs would not enable unscrupulous opportunists to exploit us with subliminal messages.
To survive and thrive, an organism must have difference thresholds low enough to detect minute changes in important stimuli. In humans, a difference threshold (also called a just noticeable difference, or jnd) increases in proportion to the size of the stimulus—a principle known as Weber’s law.
Along with being specialized as to the types of energy that can be detected, our senses are specialized in other ways as well. We do not have an infinite capacity to detect all levels of energy. To be sensed, a stimulus must first be strong enough to be detected—loud enough to be heard, concentrated enough to be smelled, bright enough to be seen. The point at which a stimulus is strong enough to be detected because it activates a sensory receptor cell is called a threshold. There are two general kinds of sensory thresholds for each sense—the absolute threshold and the difference threshold.
The absolute threshold refers to the smallest possible strength of a stimulus that can be detected half the time. Why just half the time? Because the minimum level of stimulation that can be detected varies from person to person and from trial to trial. Because of this human variability, researchers have arbitrarily set the limit as the minimum level of stimulation that can be detected half the time. Under ideal conditions (which rarely occur iormal daily life), our sensory abilities are far more sensitive than you might think. Can stimuli that are below the absolute threshold affect us?
The other important threshold involves detecting the difference between two stimuli. The difference threshold is the smallest possible difference between two stimuli that can be detected half the time. Another term for the difference threshold is just noticeable difference, which is abbreviated jnd. The just noticeable difference will vary depending on its relation to the original stimulus. This principle of sensation is called Weber’s law, after the German physiologist Ernst Weber (1795–1878).
Weber’s law holds that for each sense, the size of a just noticeable difference is a constant proportion of the size of the initial stimulus. So, whether we can detect a change in the strength of a stimulus depends on the intensity of the original stimulus. For example, if you are holding a pebble (the original stimulus), you will notice an increase in weight if a second pebble is placed in your hand. But if you start off holding a very heavy rock (the original stimulus), you probably won’t detect an increase in weight when the same pebble is balanced on it. What Weber’s law underscores is that our psychological experience of sensation is relative. There is no simple, one-to-one correspondence between the objective characteristics of a physical stimulus, such as the weight of a pebble, and our psychological experience of it.
Sensory Adaptation
Suppose your best friend has invited you over for a spaghetti dinner. As you walk in the front door, you’re almost overwhelmed by the odor of onions and garlic cooking on the stove. However, after just a few moments, you no longer notice the smell. Why? Because your sensory receptor cells become less responsive to a constant stimulus. This gradual decline in sensitivity to a constant stimulus is called sensory adaptation. Once again, we see that our experience of sensation is relative—in this case, relative to the duration of exposure.
Sensory adaptation refers to our ability to adapt to unchanging stimuli. For example, when we smell an odor in a room we’ve just entered and remain in that room for a period of time, the odor will no longer be easily detected. The phenomenon of sensory adaptation focuses our attention on informative changes in stimulation by diminishing our sensitivity to constant or routine odors, sounds, and touches.
Because of sensory adaptation, we become accustomed to constant stimuli, which allows us to quickly notice new or changing stimuli. This makes sense. If we were continually aware of all incoming stimuli, we’d be so overwhelmed with sensory information that we wouldn’t be able to focus our attention. So, for example, once you manage to land your posterior on the sofa, you don’t need to be constantly reminded that the sofa is beneath you.
Vision
A lone caterpillar on the screen door, the pile of dirty laundry in the closet corner, a spectacular autumn sunset, the intricate play of color, light, and texture in a painting by Monet. The sense organ for vision is the eye, which contains receptor cells that are sensitive to the physical energy of light. Before we can talkabout how the eye functions, we need to briefly discuss some characteristics of light as the visual stimulus.
Each sense receives stimulation, transduces it into neural signals, and sends these neural messages to the brain. We have glimpsed how this happens with vision.
The energies we experience as visible light are a thin slice from the broad spectrum of electromagnetic radiation. The hue and brightness we perceive in a light depend on the wavelength and intensity.
After entering the eye and being focused by a camera-like lens, light waves strike the retina. The retina’s light-sensitive rods and color-sensitive cones convert the light energy into neural impulses, which are coded by the retina before traveling along the optic nerve to the brain.
The visual system includes the eyes, the accessory structures, and the optic nerves (II), tracts, and pathways. The eyes respond to light and initiate afferent action potentials, which are transmitted from the eyes to the brain by the optic nerves and tracts. The accessory structures, such as eyebrows, eyelids, eyelashes, and tear glands, help protect the eyes from direct sunlight and damaging particles.
Much of the information about the world around us is detected by the visual system. Our education is largely based on visual input and depends on our ability to read words and numbers.Visual input includes information about light and dark, color and hue.
Accessory Structures
Accessory structures protect, lubricate,move, and in other ways aid in the function of the eye. These structures include the eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles. The eyebrows protect the eyes by preventing perspiration, which can irritate the eyes, from running down the forehead and into them, and they help shade the eyes from direct sunlight. The eyelids, also called palpebrae, with their associated lashes, protect the eyes from foreign objects. The space between the two eyelids is called the palpebral fissure, and the angles where the eyelids join at the medial and lateral margins of the eye are called canthi. The medial canthus contains a small reddish-pink mound called the caruncle. The caruncle contains some modified sebaceous and sweat glands. The eyelids consist of five layers of tissue. From the outer to the inner surface, they are (1) a thin layer of integument on the external surface; (2) a thin layer of areolar connective tissue; (3) a layer of skeletal muscle consisting of the orbicularis oculi and levator palpebrae superioris muscles; (4) a crescent-shaped layer of dense connective tissue called the tarsal plate, which helps maintain the shape of the eyelid; and (5) the palpebral conjunctiva (described in the next section), which lines the inner surface of the eyelid and the anterior surface of the eyeball.
If an object suddenly approaches the eye, the eyelids protect the eye by rapidly closing and then opening (blink reflex). Blinking, which normally occurs about 25 times per minute, also helps keep the eye lubricated by spreading tears over the surface of the eye. Movements of the eyelids are a function of skeletal muscles. The orbicularis oculi muscle closes the lids, and the levator palpebrae superioris elevates the upper lid.
The eyelids also help regulate the amount of light entering the eye. Eyelashes are attached as a double or triple row of hairs to the free edges of the eyelids. Ciliary glands are modified sweat glands that open into the follicles of the eyelashes to keep them lubricated.When one of these glands becomes inflamed, it’s called a sty. Meibomian glands are sebaceous glands near the inner margins of the eyelids and produce sebum, which lubricates the lids and restrains tears from flowing over the margin of the eyelids. An infection or blockage of a meibomian gland is called a chalazion, or meibomian cyst. The conjunctiva is a thin, transparent mucous membrane. The palpebral conjunctiva covers the inner surface of the eyelids, and the bulbar conjunctiva covers the anterior surface of the eye. The points at which the palpebral and bulbar conjunctivae meet are the superior and inferior conjunctival fornices.
Lacrimal Apparatus
The lacrimal apparatus consists of a lacrimal gland situated in the superolateral corner of the orbit and a nasolacrimal duct beginning in the inferomedial corner of the orbit. The lacrimal gland is innervated by parasympathetic fibers from the facial nerve (VII). The gland produces tears, which leave the gland through several ducts and pass over the anterior surface of the eyeball.
Tears are produced constantly by the gland at the rate of about 1 mL/day to moisten the surface of the eye, lubricate the eyelids, and wash away foreign objects. Tears are mostly water, with some salts,mucus, and lysozyme, an enzyme that kills certain bacteria.Most of the fluid produced by the lacrimal glands evaporates from the surface of the eye, but excess tears are collected in the medial corner of the eye by the lacrimal canaliculi. The opening of each lacrimal canaliculus is called a punctum. The upper and lower eyelids each have a punctum near the medial canthus. Each punctum is located on a small lump called the lacrimal papilla. The lacrimal canaliculi open into a lacrimal sac, which in turn continues into the nasolacrimal duct. The nasolacrimal duct opens into the inferior meatus of the nasal cavity beneath the inferior nasal concha.
Optic system of eyeball
Cornea allows light to enter the eyeball. Aqueous humor fills anterior and posterior chambers in front of lens. Crystalline lens is a transparent elastic and biconcave lens, which refracts light and focuses it on retina. Vitreous body is a transparent gel enclosed by vitreous membrane, which fills eyeball behind lens.
The Cornea
The central part of the cornea receives oxygen from the outside air. Soft plastic contact lenses worn for long periods must therefore be permeable to air so that air can reach the cornea. The most common eye injuries are cuts or tears of the cornea caused by foreign objects like stones or sticks hitting the cornea. Extensive injury to the cornea may cause connective tissue deposition, thereby making the cornea opaque. The cornea was one of the first organs transplanted. Several characteristics make it relatively easy to transplant: It’s easily accessible and relatively easily removed; it’s avascular and therefore does not require as extensive circulation as do other tissues; and it’s less immunologically active and therefore less likely to be rejected than other tissues.
The middle tunic of the eyeball is called the vascular tunic because it contains most of the blood vessels of the eyeball. The arteries of the vascular tunic are derived from a number of arteries called short ciliary arteries, which pierce the sclera in a circle around the optic nerve. These arteries are branches of the ophthalmic artery, which is a branch of the internal carotid artery. The vascular tunic contains a large number of melanin-containing pigment cells and appears black in color. The portion of the vascular tunic associated with the sclera of the eye is the choroid. The term choroid means membrane and suggests that this layer is relatively thin (0.1–0.2 mm thick). Anteriorly, the vascular tunic consists of the ciliary body and iris. The ciliary body is continuous with the choroid, and the iris is attached at its lateral margins to the ciliary body. The ciliary body consists of an outer ciliary ring and an inner group of ciliary processes, which are attached to the lens by suspensory ligaments. The ciliary body contains smooth muscles called the ciliary muscles, which are arranged with the outer muscle fibers oriented radially and the central fibers oriented circularly. The ciliary muscles function as a sphincter, and contraction of these muscles can change the shape of the lens.
The ciliary processes are a complex of capillaries and cuboidal epithelium that produces aqueous humor. The iris is the “colored part” of the eye, and its color differs from person to person. Brown eyes have brown melanin pigment in the iris. Blue eyes are not caused by a blue pigment but result from the scattering of light by the tissue of the iris, overlying a deeper layer of black pigment. The blue color is produced in a fashion similar to the scattering of light as it passes through the atmosphere to form the blue skies from the black background of space. The iris is a contractile structure, consisting mainly of smooth muscle, surrounding an opening called the pupil. Light enters the eye through the pupil, and the iris regulates the amount of light by controlling the size of the pupil. The iris contains two groups of smooth muscles: a circular group called the sphincter pupillae and a radial group called the dilator pupillae. The sphincter pupillae are innervated by parasympathetic fibers from the oculomotor nerve (III).When they contract, the iris decreases or constricts the size of the pupil. The dilator pupillae are innervated by sympathetic fibers. When they contract, the pupil is dilated. The ciliary muscles, sphincter pupillae, and dilator pupillae are sometimes referred to as the intrinsic eye muscles.
Lens
The lens is an unusual biologic structure. Transparent and biconvex, with the greatest convexity on its posterior side, the lens consists of a layer of cuboidal epithelial cells on its anterior surface and a posterior region of very long columnar epithelial cells called lens fibers. Cells from the anterior epithelium proliferate and give rise to the lens fibers at the equator of the lens. The lens fibers lose their nuclei and other cellular organelles and accumulate a special set of proteins called crystallines. This crystalline lens is covered by a highly elastic transparent capsule. The lens is suspended between the two eye compartments by the suspensory ligaments of the lens, which are connected from the ciliary body to the lens capsule.
The eye functions much like a camera. The iris allows light into the eye, and the lens, cornea, and humors focus the light onto the retina. The light striking the retina is converted into action potentials that are relayed to the brain.
The electromagnetic spectrum is the entire range of wavelengths or frequencies of electromagnetic radiation from very short gamma waves at one end of the spectrum to the longest radio waves at the other end. Visible light is the portion of the electromagnetic spectrum that can be detected by the human eye. Light has characteristics of both particles (photons) and waves, with a wavelength between 400 and 700 nm. This range sometimes is called the range of visible light or, more correctly, the visible spectrum. Within the visible spectrum, each color has a different wavelength.
Light Refraction and Reflection
An important characteristic of light is that it can be refracted (bent). As light passes from air to a denser substance like glass or water, its speed is reduced. If the surface of that substance is at an angle other than 90 degrees to the direction the light rays are traveling, the rays are bent as a result of variation in the speed of light as it encounters the new medium. This bending of light is called refraction. If the surface of a lens is concave, with the lens thinnest in the center, the light rays diverge as a result of refraction. If the surface is convex, with the lens thickest in the center, the light rays tend to converge. As light rays converge, they finally reach a point at which they cross. This point is called the focal point, and causing light to converge is called focusing. No image is formed exactly at the focal point, but an inverted, focused image can form on a surface located some distance past the focal point. How far past the focal point the focused image forms depends on a number of factors. A biconvex lens causes light to focus closer to the lens than does a lens with a single convex surface. Furthermore, the more nearly spherical the lens, the closer to the lens the light is focused; the more flattened the biconcave lens, the more distant is the point where the light is focused.
If light rays strike an object that is not transparent, they bounce off the surface. This phenomenon is called reflection. If the surface is very smooth, such as the surface of a mirror, the light rays bounce off in a specific direction. If the surface is rough, the light rays are reflected in several directions and produce a more diffuse reflection. We can see most solid objects because of the light reflected from their surfaces.
Focusing of Images on the Retina
The focusing system of the eye projects a clear image on the retina. Light rays converge as they pass from the air through the convex cornea. Additional convergence occurs as light encounters the aqueous humor, lens, and vitreous humor. The greatest contrast in media density is between the air and the cornea; therefore, the greatest amount of convergence occurs at that point. The shape of the cornea and its distance from the retina are fixed, however, so that no adjustment in the location of the focal point can be made by the cornea. Fine adjustment in focal point location is accomplished by changing the shape of the lens. In general, focusing can be accomplished in two ways. One is to keep the shape of the lens constant and move it nearer or farther from the point at which the image will be focused, such as occurs in a camera, microscope, or telescope. The second way is to keep the distance constant and to change the shape of the lens, which is the technique used in the eye.
As light rays enter the eye and are focused, the image formed just past the focal point is inverted. Action potentials that represent the inverted image are passed to the visual cortex of the cerebrum, where they are interpreted by the brain as being right side up.
The focal point (FP) is where light rays cross. (a) Distant image. The lens is flattened, and the image is focused on the retina. (b) Accommodation for near vision. The lens is more rounded, and the image is focused on the retina.
When the ciliary muscles are relaxed, the suspensory ligaments of the ciliary body maintain elastic pressure on the lens, thereby keeping it relatively flat and allowing for distant vision. The condition in which the lens is flattened so that nearly parallel rays from a distant object are focused on the retina is referred to as emmetropia and is the normal resting condition of the lens. The point at which the lens does not have to thicken for focusing to occur is called the far point of vision and normally is
When an object is brought closer than
Accommodation
When focusing on a nearby object, the ciliary muscles contract as a result of parasympathetic stimulation from the oculomotor nerve (III). This sphincterlike contraction pulls the choroid toward the lens to reduce the tension on the suspensory ligaments. This allows the lens to assume a more spherical form because of its own elastic nature. The more spherical lens then has a more convex surface, causing greater refraction of light. This process is called accommodation. As light strikes a solid object, the rays are reflected in every direction from the surface of the object. Only a small portion of the light rays reflected from a solid object, however, pass through the pupil and enter the eye of any given person. An object far away from the eye appears small compared to a nearby object because only nearly parallel light rays enter the eye from a distant object. Converging rays leaving an object closer to the eye can also enter the eye, and the object appears larger.
When rays from a distant object reach the lens, they don’t have to be refracted to any great extent to be focused on the retina, and the lens can remain fairly flat.When an object is closer to the eye, the more obliquely directed rays must be refracted to a greater extent to be focused on the retina. As an object is brought closer and closer to the eye, accommodation becomes more and more difficult because the lens cannot become any more convex. At some point, the eye no longer can focus the object, and it’s seen as a blur. The point at which this blurring occurs is called the near point of vision, which is usually about 2–3 inches from the eye for children, 4–6 inches for a young adult,
Vision Charts
When a person’s vision is tested, a chart is placed
Pupil constriction
Another factor involved in focusing is the depth of focus, which is the greatest distance through which an object can be moved and still remain in focus on the retina. The main factor affecting the depth of focus is the size of the pupil. If the pupillary diameter is small, the depth of focus is greater than if the pupillary diameter is large.With a smaller pupillary opening, an object may therefore be moved slightly nearer or farther from the eye without disturbing its focus. This is particularly important when viewing an object at close range because the interest in detail is much greater, and therefore the acceptable margin for error is smaller.When the pupil is constricted, the light entering the eye tends to pass more nearly through the center of the lens and is more accurately focused than light passing through the edges of the lens. Pupillary diameter also regulates the amount of light entering the eye.
The dimmer the light, the greater the pupil diameter must be. As the pupil constricts during close vision, therefore, more light is required on the object being observed.
Extrinsic Eye Muscles
Six extrinsic muscles of the eye cause the eyeball to move. Four of these muscles run more or less straight anteroposteriorly. They are the superior, inferior, medial, and lateral rectus muscles. Two muscles, the superior and inferior oblique muscles, are placed at an angle to the globe of the eye. The movements of the eye can be described graphically by a figure resembling the letter H. The clinical test for normal eye movement is therefore called the H test. A person’s inability to move his eye toward one part of the H may indicate dysfunction of an extrinsic eye muscle or the cranial nerve to the muscle. The superior oblique muscle is innervated by the trochlear nerve (IV). The nerve is so named because the superior oblique muscle goes around a little pulley, or trochlea, in the superomedial corner of the orbit. The lateral rectus muscle is innervated by the abducens nerve (VI), so named because the lateral rectus muscle abducts the eye. The other four extrinsic eye muscles are innervated by the oculomotor nerve (III).
Convergence
Because the light rays entering the eyes from a distant object are nearly parallel, both pupils can pick up the light rays when the eyes are directed more or less straight ahead. As an object moves closer, however, the eyes must be rotated medially so that the object is kept focused on corresponding areas of each retina. Otherwise the object appears blurry. This medial rotation of the eyes is accomplished by a reflex which stimulates the medial rectus muscle of each eye. This movement of the eyes is called convergence.
Convergence can easily be observed. Have someone stand facing you. Have the person reach out one hand and extend an index finger as far in front of his face as possible.While the person keeps his gaze fixed on the finger, have him slowly bring the finger in toward his nose until he finally touches it. Notice the movement of his pupils during this movement.What happens?
Retina
The retina is the innermost, nervous tunic of the eye. It consists of the outer pigmented retina, which is pigmented simple cuboidal epithelium, and the inner sensory retina, which responds to light. The sensory retina contains 120 million photoreceptor cells called rods and another 6 or 7 million cones, as well as numerous relay neurons. The retina covers the inner surface of the eye posterior to the ciliary body.
The pupil appears black when you look into a person’s eye because of the pigment in the choroid and the pigmented portion of the retina. The eye is a closed chamber, which allows light to enter only through the pupil. Light is absorbed by the pigmented inner lining of the eye; thus looking into it is like looking into a dark room. If a bright light is directed into the pupil, however, the reflected light is red because of the blood vessels on the surface of the retina. This is why the pupils of a person looking directly at a flash camera often appear red in a photograph. People with albinism lack the pigment melanin, and the pupil always appears red because no melanin is present to absorb light and prevent it from being reflected from the back of the eye. The diffusely lighted blood vessels in the interior of the eye contribute to the red color of the pupil.
When the posterior region of the retina is examined with an ophthalmoscope, several important features can be observed.Near the center of the posterior retina is a small yellow spot approximately
Visual Information Processing
In the cortex, individual neurons called feature detectors, respond to specific features of a visual stimulus, and their information is pooled for interpretation by higher-level brain cells. Sub-dimensions of vision (color, movement, depth, and form) are processed separately and simultaneously, illustrating the brain’s capacity for parallel processing. The visual pathway faithfully represents retinal stimulation, but the brain’s representation incorporates our assumptions, interests, and expectations.
The retina of each eye, which gives us the potential to see the whole world, is about the size and thickness of a postage stamp. The retina consists of a pigmented retina and a sensory retina. The sensory retina contains three layers of neurons: photoreceptor, bipolar, and ganglionic. The cell bodies of these neurons form nuclear layers separated by plexiform layers, where the neurons of adjacent layers synapse with each other. The outer plexiform (plexuslike) layer is between the photoreceptor and bipolar cell layers. The inner plexiform layer is between the bipolar and ganglionic cell layers.
The pigmented retina, or pigmented epithelium, consists of a single layer of cells. This layer of cells is filled with melanin pigment and, together with the pigment in the choroid, provides a black matrix, which enhances visual acuity by isolating individual photoreceptors and reducing light scattering. Pigmentation is not strictly necessary for vision, however. People with albinism (lack of pigment) can see, although their visual acuity is reduced because of some light scattering.
The layer of the sensory retina nearest the pigmented retina is the layer of rods and cones. The rods and cones are photoreceptor cells, which are sensitive to stimulation from “visible” light. The light-sensitive portion of each photoreceptor cell is adjacent to the pigmented layer.
Rods
Rods are bipolar photoreceptor cells involved in noncolor vision and are responsible for vision under conditions of reduced light. The modified, dendritic, light-sensitive part of rod cells is cylindrical, with no taper from base to apex. This rod-shaped photoreceptive part of the rod cell contains about 700 double-layered membranous discs. The discs contain rhodopsin, which consists of the protein opsin covalently bound to a pigment called retinal (derived from vitamin A). In the resting (dark) state, the shape of opsin keeps 11-cis-retinal tightly bound to the internal surface of opsin. As light is absorbed by rod cells, opsin changes shape from 11-cis-retinal to all-trans-retinal. These changes activate the attached G protein, called transducin, which closes Na+ channels, resulting in hyperpolarization of the cell.
Cones
Color vision and visual acuity are functions of cone cells. Color is a function of the wavelength of light, and each color results from a certain wavelength within the visible spectrum. Even though rods are very sensitive to light, they cannot detect color, and sensory input that ultimately reaches the brain from these cells is interpreted by the brain as shades of gray. Cones require relatively bright light to function. As a result, as the light decreases, so does the color of objects that can be seen until, under conditions of very low illumination, the objects appear gray. This occurs because as the light decreases, fewer cone cells respond to the dim light.
Cones are bipolar photoreceptor cells with a conical lightsensitive part that tapers slightly from base to apex. The outer segments of the cone cells, like those of the rods, consist of double-layered discs. The discs are slightly more ous and more closely stacked in the cones than in the rods. Cone cells contain a visual pigment, iodopsin, which consists of retinal combined with a photopigment opsin protein. Three major types of color-sensitive opsin exist: blue, red, and green; each closely resembles the opsin proteins of rod cells but with somewhat different amino acid sequences. These color photopigments function in much the same manner as rhodopsin, but whereas rhodopsin responds to the entire spectrum of visible light, each iodopsin is sensitive to a much narrower spectrum.
Most people have one red pigment gene and one or more green pigment genes located in a tandem array on each X chromosome. An enhancer gene on the X chromosome apparently determines that only one color opsin gene is expressed in each cone cell. Only the first or second gene in the tandem array is expressed in each cone cell, so that some cone cells express only the red pigment gene and others express only one of the green pigment genes.
Although considerable overlap occurs in the wavelength of light to which these pigments are sensitive, each pigment absorbs light of a certain range of wavelengths. As light of a given wavelength, representing a certain color, strikes the retina, all cone cells containing photopigments capable of responding to that wavelength generate action potentials. Because of the overlap among the three types of cones, especially between the green and red pigments, different proportions of cone cells respond to each wavelength, thus allowing color perception over a wide range. Color is interpreted in the visual cortex as combinations of sensory input originating from cone cells. For example, when orange light strikes the retina, 99% of the red-sensitive cones respond, 42% of the green-sensitive cones respond, and no blue cones respond.When yellow light strikes the retina, the response is shifted so that a greater number of green-sensitive cones respond. The variety of combinations created allows humans to distinguish several million gradations of light and shades of color.
Distribution of Rods and Cones in the Retina
Cones are involved in visual acuity, in addition to their role in color vision. The fovea centralis is used when visual acuity is required, such as for focusing on the words of this page. The fovea centralis has about 35,000 cones and no rods. The 120 million rods are 20 times more plentiful than cones over most of the remaining retina, however. They are more highly concentrated away from the fovea and are more important in low-light conditions.
Inner Layers of the Retina
The middle and inner nuclear layers of the retina consist of two major types of neurons: bipolar and ganglion cells. The rod and cone photoreceptor cells synapse with bipolar cells, which in turn synapse with ganglion cells. Axons from the ganglion cells pass over the inner surface of the retina, except in the area of the fovea centralis, converge at the optic disc, and exit the eye as the optic nerve (II). The fovea centralis is devoid of ganglion cell processes, resulting in a small depression in this area; thus the name fovea, meaning small pit. As a result of the absence of ganglion cell processes in addition to the concentration of cone cells mentioned previously, visual acuity is further enhanced in the fovea centralis because light rays don’t have to pass through as many tissue layers before reaching the photoreceptor cells.
Rod and cone cells differ in the way they interact with bipolar and ganglion cells. One bipolar cell receives input from numerous rods, and one ganglion cell receives input from several bipolar cells so that spatial summation of the signal occurs and the signal is enhanced, thereby allowing awareness of stimulus from very dim light sources but decreasing visual acuity in these cells. Cones, on the other hand, exhibit little or no convergence on bipolar cells so that one cone cell may synapse with only one bipolar cell. This system reduces light sensitivity but enhances visual acuity.
Within the inner layers of the retina, associatioeurons are present also, which modify the signals from the photoreceptor cells before the signal ever leaves the retina. Horizontal cells form the outer plexiform layer and synapse with photoreceptor cells and bipolar cells. Amacrine cells form the inner plexiform layer and synapse with bipolar and ganglion cells. Interplexiform cells form the bipolar layer and synapse with amacrine, bipolar, and horizontal cells to form a feedback loop. Associatioeurons are either excitatory or inhibitory on the cells with which they synapse. These association cells enhance borders and contours, thereby increasing the intensity at boundaries, such as the edge of a dark object against a light background.
Color Vision
Research on how we see color supports two nineteenth-century theories. First, as the Young-Helmholtz trichromatic (three-color) theory suggests, the retina contains three types of cones. Each is most sensitive to the wavelengths of one of the three primary colors of light (red, green, or blue). Second, as opponent-process theory maintains, the nervous system codes the color-related information from the cones into pairs of opponent colors, as demonstrated by the phenomenon of afterimages and as confirmed by measuring opponent processes within visual neurons of the thalamus. The phenomenon of color constancy under varying illumination shows that our brains construct our experience of color.
Color vision has interested scientists for hundreds of years. The first scientific theory of color vision, proposed by Hermann von Helmholtz (1821–1894) in the mid-1800s, was called the trichromatic theory. A rival theory, the opponent-process theory, was proposed in the late 1800s. Each theory was capable of explaining some aspects of color vision, but neither theory could explain all aspects of color vision. Technological advances in the last few decades have allowed researchers to gather direct physiological evidence to test both theories. The resulting evidence indicates that both theories of color vision are accurate. Each simply describes color vision at a different stage of visual processing (Hubel, 1995).
The Trichromatic Theory As you’ll recall, only the cones are involved in color vision. According to the trichromatic theory of color vision, there are three varieties of cones. Each type of cone is especially sensitive to certain wavelengths— red light (long wavelengths), green light (medium wavelengths), and blue light (short wavelengths). For the sake of simplicity, we will refer to red-sensitive, green-sensitive, and blue-sensitive cones, but keep in mind that there is some overlap in the wavelengths to which a cone is sensitive (Abramov & Gordon, 1994).
A given cone will be very sensitive to one of the three colors and only slightly responsive to the other two. When a color other than red, green, or blue strikes the retina, it stimulates a combination of cones. For example, if yellow light strikes the retina, both the redsensitive and green-sensitive cones are stimulated; purple light evokes strong reactions from red-sensitive and blue-sensitive cones. The trichromatic theory of color vision received compelling research support in 1964, when George Wald showed that different cones were indeed activated by red, blue, and green light. The trichromatic theory provides a good explanation for the most common form of color blindness: red–green color blindness. People with red–green color blindness cannot discriminate between red and green. That’s because they have normal blue-sensitive cones, but their other cones are either red-sensitive or green-sensitive. Thus, red and green look the same to them. Because red–green color blindness is so common, stoplights are designed so that the location of the light as well as its color provides information to drivers. In vertical stoplights the red light is always on top, and in horizontal stoplights the red light is always on the far left. The Opponent-Process Theory The trichromatic theory cannot account for all aspects of color vision. One important phenomenon that the theory does not explain is the afterimage. An afterimage is a visual experience that occurs after the original source of stimulation is no longer present. Afterimages can be explained by the opponent-process theory of color vision, which proposes a different mechanism of color detection from the one set forth in the trichromatic theory. According to the opponent-process theory of color vision, there are four basic colors, which are divided into two pairs of color-sensitive neurons: red–green and blue–yellow. The members of each pair oppose each other. If red is stimulated, green is inhibited; if green is stimulated, red is inhibited. Green and red cannot both be stimulated simultaneously. The same is true for the blue–yellow pair. In addition, black and white act as an opposing pair. Color, then, is sensed and encoded in terms of its proportion of red OR green, and blue OR yellow.
For example, red light evokes a response of RED-YES–GREEN-NO in the red–green opponent pair. Yellow light evokes a response of BLUE-NO–YELLOWYES. Colors other than red, green, blue, and yellow activate one member of each of these pairs to differing degrees. Purple stimulates the red of the red–green pair plus the blue of the blue–yellow pair. Orange activates red in the red–green pair and yellow in the blue–yellow pair. Afterimages can be explained when the opponent-process theory is combined with the general principle of sensory adaptation (Jameson & Hurvich, 1989). If you stare continuously at one color, sensory adaptation eventually occurs and your visual receptors become less sensitive to that color. What happens when you subsequently stare at a white surface? If you remember that white light is made up of the wavelengths for all colors, you may be able to predict the result. The receptors for the original color have adapted to the constant stimulation and are temporarily “off duty.” Thus they do not respond to that color. Instead, only the receptors for the opposing color will be activated, and you perceive the wavelength of only the opposing color. For example, if you stare at a patch of green, your green receptors eventually become “tired.” The wavelengths for both green and red light are reflected by the white surface, but since the green receptors are “off,” only the red receptors are activated.
An Integrated Explanation of Color Vision
At the beginning of this section we said that current research has shown that both the trichromatic theory and the opponent-process theory of color vision are accurate. How can both theories be right? It turns out that each theory correctly describes color vision at a different level of visual processing. As described by the trichromatic theory, the cones of the retina do indeed respond to and encode color in terms of red, green, and blue. But recall that signals from the cones and rods are partially processed in the ganglion cells before being transmitted along the optic nerve to the brain. Researchers now believe that an additional level of color processing takes place in the ganglion cells. As described by the opponent-process theory, the ganglion cells respond to and encode color in terms of opposing pairs (DeValois & DeValois, 1975). In the brain, the thalamus and visual cortex also encode color in terms of opponent pairs (Boynton, 1988; Engel, 1999). Consequently, both theories contribute to our understanding of the process of color vision. Each theory simply describes color vision at a different stage of visual processing (Hubel, 1995).
Neuronal Pathways for Vision
The optic nerve (II) leaves the eye and exits the orbit through the optic foramen to enter the cranial cavity. Just inside the vault and just anterior to the pituitary, the optic nerves are connected to each other at the optic chiasm. Ganglion cell axons from the nasal retina (the medial portion of the retina) cross through the optic chiasm and project to the opposite side of the brain. Ganglion cell axons from the temporal retina (the lateral portion of the retina) pass through the optic nerves and project to the brain on the same side of the body without crossing.
Beyond the optic chiasm, the route of the ganglionic axons is called the optic tract. Most of the optic tract axons terminate in the lateral geniculate nucleus of the thalamus. Some axons do not terminate in the thalamus but separate from the optic tract to terminate in the superior colliculi, the center for visual reflexes. Neurons of the lateral geniculate ganglion form the fibers of the optic radiations, which project to the visual cortex in the occipital lobe. Neurons of the visual cortex integrate the messages coming from the retina into a single message, translate that message into a mental image, and then transfer the image to other parts of the brain, where it is evaluated and either ignored or acted on.
The visual fields of the eyes partially overlap. The region of overlap is the area of binocular vision, seen with two eyes at the same time, and it is responsible for depth perception, the ability to distinguish betweeear and far objects and to judge their distance. Because humans see the same object with both eyes, the image of the object reaches the retina of one eye at a slightly different angle from that of the other.With experience, the brain can interpret these differences in angle so that distance can be judged quite accurately.
Hearing
The Stimulus Input: Sound Waves
The pressure waves we experience as sound vary in frequency and amplitude, and correspondingly in perceived pitch and loudness.
The Ear
Through a mechanical chain of events, sound waves traveling through the auditory canal cause minuscule vibrations in the eardrum. Transmitted via the bones of the middle ear to the fluid-filled cochlea, these vibrations create movement in tiny hair cells, triggering neural messages to the brain.
Research on how we hear pitch supports both the place theory, which best explains the sensation of high-pitched sounds, and frequency theory, which best explains the sensation of low-pitched sounds. We localize sound by detecting minute differences in the intensity and timing of the sounds received by each ear.
Role of external ear
u External ear consists of auricle and external auditory meatus. Function of external ear is collection of sound waves.
u External auditory meatus conducts sound waves from auricle to tympanic membrane.
u External ear also helps in protection of middle and internal ear. It provides constant temperature and humidity near tympanic membrane and mechanical defense also.
Role of middle ear in sound perception
u Middle ear occupies tympanic cavity. Tympanic membrane forms lateral wall of tympanic cavity. Handle of malleus attaches to point of maximal concavity of tympanic membrane on its inner surface. Other end of malleus is bound to incus by ligaments. Opposite end of incus articulates with stem of stapes.
u Faceplate of stapes lies against membranous labyrinths in oval window, where sound waves are conducted into cochlear. Auditory ossicles increase pressure exerted by sound waves on fluid of cochlear. Thus provides impedance matching between sound waves in air and sound vibrations in fluid of cochlear.
u Tympanic cavity is filled with air. Besides auditory ossicles tympanic cavity also contains tensor tympani muscle and stapedius muscle. There are two windows in medial wall of tympanic cavity, round window and oval window.
Attenuation reflex and adaptation to sound
u It is a reflex that occurs when loud sounds low frequency sounds are transmitted through ossicular system into central nervous system. It occurs after latent period of 40-80 ms.
u Tensor tympani muscle pulls handle of malleus inwards. Stapedius muscle pulls stapes out of oval window. These two forces are opposite to each other. It causes entire ossicular system to become highly rigid.
u This mechanism reduces ossicular conduction of loud or low frequency sounds. As a result intensity of sound, which comes into inner ear reduces to 30-40 decibels.
Attenuation reflex importance
u Attenuation reflex protects basilar membrane of cochlear from damaging vibrations of loud sounds. It masks also low frequency sounds in loud environment. This phenomenon a person to concentrate on high frequency sounds.
u Attenuation reflex is actual also for decrease person’s hearing sensitivity to his own speech. Prolonged hearing of a loud sound also lead to transient loss of sensitivity of central auditory neurons because of inhibitory process. It is called central adaptation to sound.
Mechanism of conduction of sound waves
u Sound waves strike tympanic membrane. Ossicular system conducts this sound. Faceplate of stapes moves inward into scala media at oval window. So fluid moves inward into scala media. It causes vibration of basilar membrane. When basilar membrane bends upward to scala vestibuli, hair cells depolarize and generate action potential to nerve fibers of cochlear nerve.
u Place principle determines sound frequency by determining the position along basilar membrane that it most stimulated. Low frequency sounds cause maximal stimulation of basilar membrane near apex of cochlear. High frequency sounds cause maximal stimulation of basilar membrane near base of cochlear. Intermediate frequency sounds cause maximal stimulation of basilar membrane at middle of cochlear.
Determination of loudness of sounds
• There are three ways for determination of loudness of sounds during its perception. As sound becomes louder, amplitude of vibrations of basilar membrane increases. so hair cells produce impulses at rapid rate.
• Increase of sound amplitude cause rise of quantity of excited hair cells. Loud sound stimulates also special high-threshold hair cells, which appraise the central nervous system, that sound is loud.
Central division of auditory analyzer
• Hair cells are secondary sensitive cells, which give receptor potential to neurons of spiral ganglion of Corti. Then impulse puss to vestibulocochlear nerve – dorsal and ventral cochlear nuclei in upper medulla – trapezoid body – superior olivary nucleus – lateral lemniscus. Then fibers divide into three parts, which go to:
• – nucleus of lateral lemniscus;
• – higher centers;
• – inferior colliculi – medial geniculate nucleus – auditory cortex through auditory radiation.
• Auditory cortex lies in superior gyrus of temporal lobe and performs final processing of auditory information.
Binaural hearing
• Binaural hearing helps in determination of direction to sound origin. Binaural hearing provides detection of time-lag between entry of sound into one ear and into opposite ear. Medial superior olivary nucleus detects this information.
• Difference between intensities of sound in two ears also is important for determination of direction to sound origin. Lateral superior olivary nucleus detects it.
Hearing Loss and Deaf Culture
Hearing losses linked to conduction and nerve disorders can be caused by prolonged exposure to loud noise and by diseases and age-related disorders. Those who live with hearing loss face social challenges. Cochlear implants can enable some hearing for deaf children and most adults. But Deaf Culture advocates, noting that Sign is a complete language, question the enhancement. Additionally, deafness can lead to sensory compensation where other senses are enhanced. Advocates feel that this furthers their view that deafness is not a disability.
Other Important Senses
Touch
Our sense of touch is actually four senses—pressure, warmth, cold, and pain—that combine to produce other sensations, such as “hot.” One theory of pain is that a “gate” in the spinal cord either opens to permit pain signals traveling up small nerve fibers to reach the brain, or closes to prevent their passage. Because pain is both a physiological and a psychological phenomenon, it often can be controlled through a combination of physical and psychological treatments.
While vision, hearing, smell, and taste provide you with important information about your environment, another group of senses provides you with information that comes from a source much closer to home: your own body. In this section, we’ll first consider the skin senses, which provide essential information about your physical status and your physical interaction with objects in your environment. We’ll next consider the body senses, which keep you informed as to your position and orientation in space. own body. In this section, we’ll first consider the skin senses, which provide essential information about your physical status and your physical interaction with objects in your environment.
We’ll next consider the body senses, which keep you informed as to your position and orientation in space. We usually don’t think of our skin as a sense organ. But the skin is in fact the largest and heaviest sense organ. The skin of an average adult covers about
One important receptor involved with the sense of touch, called the Pacinian corpuscle, is located beneath the skin. When stimulated by pressure, the Pacinian corpuscle converts the stimulation into a neural message that is relayed to the brain. If a pressure is constant, sensory adaptation takes place. The Pacinian corpuscle either reduces the number of signals sent or quits responding altogether (which is fortunate, or you’d be unable to forget the fact that you’re wearing underwear). Sensory receptors are distributed unevenly among different areas of the body, which is why sensitivity to touch and temperature varies from one area of the body to another. Your hands, face, and lips, for example, are much more sensitive to touch than are your back, arms, and legs. That’s because your hands, face, and lips are much more densely packed with sensory receptors.
Pain
Pain is important to our survival. It provides us with important information about our body, telling us to pay attention, to stop what we are doing, or to pull away from some object or stimulus that is injuring us. A wide variety of stimuli can produce pain—the sensation of discomfort or suffering. Virtually any external stimulus that can produce tissue damage can cause pain, including certain chemicals, electric shock, and extreme heat, cold, pressure, or noise. Pain can also be caused by internal stimuli, such as disease, infection, or deterioration of bodily functions. Some areas of the body are more sensitive to pain than are other areas.
The most influential theory of pain is the gate-control theory, developed by psychologist Ronald Melzack and anatomist Patrick Wall (1965, 1996). The gate-control theory suggests that the sensation of pain is controlled by a series of “gates” that open and close in the spinal cord. If the spinal gates are open, pain is experienced. If the spinal gates are closed, no pain is experienced.
Taste
Taste, a chemical sense, is likewise a composite of five basic sensations—sweet, sour, salty, bitter, and umami—and of the aromas that interact with information from the taste buds. The influence of smell on our sense of taste is an example of sensory interaction.
Our sense of taste, or gustation, results from the stimulation of special receptors in the mouth. The stimuli that produce the sensation of taste are chemical substances in whatever you eat or drink. These substances are dissolved by saliva, allowing the chemicals to activate the taste buds. Each taste bud contains about 50 receptor cells that are specialized for taste.
The surface of the tongue is covered with thousands of little bumps with grooves in between. These grooves are lined with the taste buds. Taste buds are also located on the insides of your cheeks, on the roof of your mouth, and in your throat (Oakley, 1986). When activated, special receptor cells in the taste buds send neural messages along pathways to the thalamus in the brain. In turn, the thalamus directs the information to several regions in the cortex (O’Doherty & others, 2001b). There were long thought to be four basic taste categories: sweet, salty, sour, and bitter. Recently, the receptor cells for a fifth basic taste, umami, were identified (Chaudhari & others, 2000). Loosely translated, umami means “yummy” or “delicious” in Japanese. Umami is the distinctive taste of monosodium glutamate and is associated with protein-rich foods and the savory flavor of Parmesan and other aged cheeses, mushrooms, seaweed, and meat. Each taste bud shows maximum sensitivity to one particular taste, and lesser sensitivity to other tastes. Most tastes are complex and result from the activation of different combinations of basic taste receptors. Taste is just one aspect of flavor, which involves several sensations, including the aroma, temperature, texture, and appearance of food.
Smell
Like taste, smell is a chemical sense, but there are no basic sensations for smell, as there are for touch and taste. Unlike the retina’s receptor cells that sense color by breaking it into component parts, the 5 million olfactory receptor cells with their 1000 different receptor proteins recognize individual odor molecules. Some odors trigger a combination of receptors. Like other stimuli, odors can spontaneously evoke memories and feelings.
The sensory stimuli that produce our sensation of an odor are molecules in the air. These airborne molecules are emitted by the substance we are smelling. We inhale them through the nose and through the opening in the palate at the back of the throat. In the nose, the molecules encounter millions of olfactory receptor cells located high in the nasal cavity. Unlike the sensory receptors for hearing and vision, the olfactory receptors are constantly being replaced. Each cell lasts for only about 30 to 60 days. In 1991, neuroscientists Linda Buck and Richard Axel identified the odor receptors that are present on the hairlike fibers of the olfactory neurons. Like synaptic receptors, each odor receptor seems to be specialized to respond to molecules of a different chemical structure. When these olfactory receptor cells are stimulated by the airborne molecules, the stimulation is converted into neural messages that pass along their axons, bundles of which make up the olfactory nerves.
So far, hundreds of different odor receptors have been identified (Mombaerts, 1999). We probably don’t have a separate receptor for each of the estimated 10,000 different odors that we can identify, however. Rather, each receptor is like a letter in an olfactory alphabet. Just as different combinations of letters in the alphabet are used to produce recognizable words, different combinations of olfactory receptors produce the sensation of distinct odors. Thus, the airborne molecules activate specific combinations of receptors, and the brain identifies an odor by interpreting the pattern of olfactory receptors that are stimulated (Buck, 2000).
As shown in Figure 3.10, the olfactory nerves directly connect to the olfactory bulb in the brain, which is actually the enlarged ending of the olfactory cortex at the front of the brain. Warren lost his sense of smell because the surgeon cut through the nerve fibers leading to his olfactory bulb. Axons from the olfactory bulb form the olfactory tract. These neural pathways project to different brain areas, including the temporal lobe and structures in the limbic system (Angier, 1995). The projections to the temporal lobe are thought to be part of the neural pathway involved in our conscious recognition of smells. The projections to the limbic system are thought to regulate our emotional response to odors. The direct connection of olfactory receptor cells to areas of the cortex and
limbic system is unique to our sense of smell. All other bodily sensations are first processed in the thalamus before being relayed to the higher brain centers in the cortex. Olfactory neurons are unique in another way, too. They are the only neurons that directly link the brain and the outside world (Axel, 1995). The axons of the sensory neurons that are located in your nose extend directly into your brain! As with the other senses, we experience sensory adaptation to odors when exposed to them for a period of time. In general, we reach maximum adaptation to an odor in less than a minute.We continue to smell the odor, but we have become about 70 percent less sensitive to it.
Body Position and Movement
Finally, our effective functioning requires a kinesthetic sense, which notifies the brain of the position and movement of body parts, and a sense of equilibrium, which monitors the position and movement of the whole body.
Pain begins when an intense stimulus activates small-diameter sensory fibers, called free nerve endings, in the skin, muscles, or internal organs. The free nerve endings carry their messages to the spinal cord, releasing a neurotransmitter called substance P. In the spinal cord, substance P causes other neurons to become activated, sending their messages through open spinal gates to the thalamus in the brain (Turk & Nash, 1993). Other areas of the brain involved in the experience of pain are the somatosensory cortex and areas in the frontal lobes and limbic system that are involved in emotion (Hunt & Mantyh, 2001; Rainville & others, 1997). When the sensory pain signals reach the brain, the sensory information is integrated with psychological information. Depending on how the brain interprets the pain experience, it regulates pain by sending signals down the spinal cord that either open or close the gates. If, because of psychological factors, the brain signals the gates to open, pain is experienced or intensified. If the brain signals the gates to close, pain is reduced.
Anxiety, fear, and a sense of helplessness are some of the psychological factors that can intensify the experience of pain. Positive emotions, laughter, distraction, and a sense of control can reduce the perception of pain. The experience of pain is also influenced by social and cultural learning experiences about the meaning of pain and how people should react to pain (Turk, 1994; Turk & Rudy, 1992). Psychological factors also influence the release of endorphins, the body’s natural painkillers that are produced in many parts of the brain and the body. Endorphins are released as part of the brain’s overall response to physical pain or stress. In the brain, endorphins can inhibit the transmission of pain signals. In the spinal cord, endorphins inhibit the release of substance P. Finally, a person’s mental or emotional state can influence other bodily processes that affect the experience of pain. Muscle tension, psychological arousal, and rapid heart rate can all produce or intensify pain (Turk & Nash, 1993). Today, a variety of techniques and procedures can effectively eliminate or reduce pain.
Movement, Position, and Balance
The phone rings. Without looking up from your textbook, you reach for the receiver, pick it up, and guide it to the side of your head. You have just demonstrated your kinesthetic sense—the sense that involves the location and position of body parts in relation to one another. (The word
kinesthetics literally means “feelings of motion.”) The kinesthetic sense involves specialized sensory neurons, called proprioceptors, which are located in the muscles and joints. The proprioceptors constantly communicate information to the brain about changes in body position and muscle tension. Closely related to the kinesthetic sense is the vestibular sense, which provides a sense of balance, or equilibrium, by responding to changes in gravity, motion, and body position. The two sources of vestibular sensory information, the semicircular canals and the vestibular sacs, are both located in the ear. These structures are filled with fluid and lined with hairlike receptor cells that shift in response to motion, changes in body position, or changes in gravity. When you experience environmental motion, like the rocking of a boat in choppy water, the fluids in the semicircular canals and the vestibular sacs are affected. Changes in your body’s position, such as falling backward in a heroic attempt to return a volleyball serve, also affect the fluids. Your vestibular sense supplies the critical information that allows you to compensate for such changes and quickly reestablish your sense of balance.
Maintaining equilibrium also involves information from other senses, particularly vision. Under normal circumstances, this works to our advantage. However, when information from the eyes conflicts with information from the vestibular system, the result can be dizziness, disorientation, and nausea. These are the symptoms commonly experienced in motion sickness, the bane of many travelers in cars, on planes, on boats, and even in space. One strategy that can be used to combat motion sickness is to minimize sensory conflicts by focusing on a distant point or an object that is fixed, such as the horizon.
In the first part of this chapter, we’ve described how the body’s senses respond to stimuli in the environment. Table 3.4 summarizes these different sensory systems. To make use of this raw sensory data, the brain must organize and interpret the data and relate it to existing knowledge. Next, we’ll look at the process of perception—how we make sense out of the information that we receive from our environment.
Selective Attention
At any moment we are conscious of a very limited amount of all that we are capable of experiencing. One example of this selective attention is the cocktail party effect—attending to only one voice among many. Another example is inattentional blindness, which refers to our blocking of a brief visual interruption when focusing on other sights.
Perceptual Illusions
Visual and auditory illusions were fascinating scientists even as psychology emerged. Explaining illusions required an understanding of how we transform sensations into meaningful perceptions, so the study of perception became one of psychology’s first concerns. Conflict between visual and other sensory information is usually resolved with the mind’s accepting the visual data, a tendency known as visual capture.
Perceptual Organization
From a top-down perspective, we see how we transform sensory information into meaningful perceptions when we are aided by knowledge and expectations.
The early Gestalt psychologists were impressed with the seemingly innate way we organize fragmentary sensory data into whole perceptions. Our minds structure the information that comes to us in several demonstrable ways:
Our senses are constantly registering a diverse range of stimuli from the environment and transmitting that information to the brain. But to make use of this raw sensory data, we must organize, interpret, and relate the data to existing knowledge.
Psychologists sometimes refer to this flow of sensory data from the sensory receptors to the brain as bottom-up processing. Also called data-driven processing, bottom-up processing is often at work when we’re confronted with an ambiguous stimulus. For example, imagine trying to assemble a jigsaw puzzle one piece at a time, without knowing what the final picture will be. To accomplish this task, you would work with the individual puzzle pieces to build the image from the “bottom up,” that is, from its constituent parts. But as we interact with our environment, many of our perceptions are shaped by top-down processing, which is also referred to as conceptually driven processing. Top-down processing occurs when we draw on our knowledge, experiences, expectations, and other cognitive processes to arrive at meaningful perceptions, such as people or objects in a particular context.
Both top-down and bottom-up processing are involved in our everyday perceptions. As a simple illustration, look at this photograph which sits on Don’s desk. Top-down processing was involved as you reached a number of perceptual conclusions about the image. You quickly perceived a little girl holding a black cat—our daughter Laura holding her cat, Nubbin. You also perceived a child as a whole object even though the cat is actually blocking a good portion of the view of Laura. But now look at the background in the photograph, which is more ambiguous. Deciphering these images involves both bottom-up and top-down processing. Bottom-up processes help you determine that behind the little girl looms a large, irregularly shaped, dark green object with brightly colored splotches on it. But what is it? To identify the mysterious object, you must interpret the meaning of the sensory data. Top-down processes help you identifythe large green blotch as a Christmas tree—a conclusion that you probably would not reach if you had no familiarity with the way many Americans celebrate the Christmas holiday. The Christmas tree branches, ornaments, and lights are really just fuzzy images at best, but other images work as clues—a happy child, a stuffed bear with a red-and-white stocking cap. Our learning experiences create a conceptual knowledge base from which we can identify and interpret many objects, including kids, cats, and Christmas trees.
Clearly, bottom-up and top-down processing are both necessary to explain how we arrive at perceptual conclusions. But whether we are using bottom-up or top-down processing, a useful way to think about perception is to consider the basic perceptual questions we must answer in order to survive. We exist in an ever-changing environment that is filled with objects that may be standing still or moving, just like ourselves. Whether it’s a bulldozer or a bowling ball, we need to be able to identify objects, locate objects in space, and, if they are moving, track their motion.
Thus, our perceptual processes must help us organize our sensations to answer three basic, important questions: (1) What is it? (2) How far away is it? And (3) Where is it going? In the next few sections, we will look at what psychologists have learned about the principles we use to answer these perceptual questions. Much of our discussion reflects the work of an early school of psychology called Gestalt psychology, which was founded by German psychologist Max Wertheimer in the early 1900s. The Gestalt psychologists emphasized that we perceive whole objects or figures (gestalts) rather than isolated bits and pieces of sensory information. Roughly translated, the German word Gestalt means a unified whole, form, or shape. Although the Gestalt school of psychology no longer formally exists, the pioneering work of the Gestalt psychologists established many basic perceptual principles (Palmer, 2002).
Form Perception
To recognize an object, we must first perceive it (see it as a figure) as distinct from its surroundings (the ground). We must also organize the figure into a meaningful form. Several Gestalt principles—proximity, similarity, continuity, connectedness, and closure—describe this process.
When you look around your world, you don’t see random edges, curves, colors, or splotches of light and dark. Rather, you see countless distinct objects against a variety of backgrounds. Although to some degree we rely on size, color, and texture to determine what an object might be, we rely primarily on an object’s shape to identify it.
Figure–Ground Relationship
How do we organize our perceptions so that we see an object as separate from other objects? The early Gestalt psychologists identified an important perceptual principle called the figure–ground relationship, which describes how this works. When we view a scene, we automatically separate the elements of that scene into the figure, which is the main element of the scene, and the ground, which is its background. You can experience the figure– ground relationship by looking at a coffee cup on a table. The coffee cup is the figure, and the table is the ground. Notice that usually the figure has a definite shape, tends to stand out clearly, and is perceptually meaningful in some way. In contrast, the ground tends to be less clearly defined, even fuzzy, and usually appears to be behind and farther away than the figure. The early Gestalt psychologists noted that figure and ground have vastly different perceptual qualities (N. Rubin, 2001). As Gestalt psychologist Edgar Rubin (1921) observed, “In a certain sense, the ground has no shape.” We notice the shape of the figure but not the shape of the background, even when that ground is used as a well-defined frame. It turns out that brain neurons also respond differently to a stimulus that is perceived as a figure versus a stimulus that is part of the ground (Baylis & Driver, 2001).
Particular neurons in the cortex that responded to a specific shape when it was the shape of the figure did not respond when the same shape was presented as part of the background. The separation of a scene into figure and ground is not a property of the actual elements of the scene you’re looking at. Rather, your ability to separate a scene into figure and ground is a psychological accomplishment.
Depth Perception
Research on the visual cliff revealed that many species perceive the world in three dimensions at, or very soon after, birth. We transform two-dimensional retinal images into three-dimensional perceptions by using binocular cues, such as retinal disparity, and monocular cues, such as the relative sizes of objects.
Many of the forms we perceive are composed of a number of different elements that seem to go together (Prinzmetal, 1995). It would be more accurate to say that we actively organize the elements to try to produce the stable perception of well-defined, whole objects. This is what perceptual psychologists refer to as “the urge to organize.” What sort of principles do we follow when we try to organize visual elements? The Gestalt psychologists studied how the perception of visual elements becomes organized into patterns, shapes, and forms. They identified several laws, or principles, that we tend to follow in grouping elements together to arrive at the perception of forms, shapes, and figures. These principles include similarity, closure, good continuation, and proximity.
The Gestalt psychologists also formulated a general principle called the law of Prägnanz, or the law of simplicity. This law states that when several perceptual organizations of an assortment of visual elements are possible, the perceptual interpretation that occurs will be the one that produces the “best, simplest, and most stable shape” (Koffka, 1935). To illustrate, look at Figure 3.16. Do you perceive the image as two six-sided objects and one four-sided object? If you are following the law of Prägnanz, you don’t. Instead, you perceptually organize the elements in the most cognitively efficient and simple way, perceiving them as three overlapping squares. According to the Gestalt psychologists, the law of Prägnanz encompasses all the other Gestalt principles, including the figure–ground relationship. The implication of the law of Prägnanz is that our perceptual system works in an economical way to promote the interpretation of stable and consistent forms (van der Helm, 2000). The ability to efficiently organize elements into stable objects helps us perceive the world accurately. In effect, we actively and automatically construct a perception that reveals “the essence of something,” which is roughly what the German word Prägnanz means. Being able to perceive the distance of an object has obvious survival value, especially regarding potential threats, such as snarling dogs or oncoming trains. But simply walking through your house or apartment also requires that you accurately judge the distance of furniture, walls, other people, and so forth. Otherwise, you’d be constantly bumping into doors, walls, and tables. The ability to perceive the distance of an object as well as the three-dimensional characteristics of an object is called depth perception.
Monocular Cues
We use a variety of cues to judge the distance of objects. The following cues require the use of only one eye. Hence, they are called monocular cues (mono means “one”). After familiarizing yourself with these cues, look at the photographs on the next page. Try to identify the monocular cues you used to determine the distance of the objects in each photograph.
1. Relative size. If two or more objects are assumed to be similar in size, the object that appears larger is perceived as being closer.
2. Overlap. When one object partially blocks or obscures the view of another object, the partially blocked object is perceived as being farther away.
3. Aerial perspective. Faraway objects often appear hazy or slightly blurred by the atmosphere.
4. Texture gradient. As a surface with a distinct texture extends into the distance, the details of the surface texture gradually become less clearly defined. The texture of the surface seems to undergo a gradient, or continuous pattern of change, from crisp and distinct when close to fuzzy and blended when farther away.
5. Linear perspective. Parallel lines seem to meet in the distance. For example, if you stand in the middle of a railroad track and look down the rails, you’ll notice that the parallel rails seem to meet in the distance. The closer together the lines appear to be, the greater the perception
of distance.
6. Motion parallax. When you are moving, you use the speed of passing objects to estimate the distance of the objects. Nearby objects seem to zip by faster than do distant objects. When riding on a commuter train, for example, houses and parked cars along the tracks seem to whiz by, while the distant downtown skyline seems to move very slowly.
When monocular cues are used by artists to create the perception of distance or depth in paintings, they are referred to as pictorial cues. If you look at the cover of this book, you can see how artist Phoebe Beasley used pictorial cues, including overlap and relative size, to create the perception of depth in her artwork. Another monocular cue is accommodation. Unlike pictorial cues, accommodation utilizes information about changes in the shape of the lens of the eye to help us estimate distance.When you focus on a distant object, the lens is flat, but focusing on a nearby object causes the lens to thicken. Thus, to some degree, we use information provided by the muscles controlling the shape of the lens to judge depth. In general, however, we rely more on pictorial cues than on accommodation for depth perception.
Binocular Cues
Binocular cues for distance or depth perception require information from both eyes. One binocular cue is convergence—the degree to which muscles rotate your eyes to focus on an object. The more the eyes converge, or rotate inward, to focus on an object, the greater the strength of the muscle signals and the closer the object is perceived to be. For example, if you hold a dime about six inches in front of your nose, you’ll notice the slight strain on your eye muscles as your eyes converge to focus on the coin. If you hold the dime at arm’s length, less convergence is needed. Perceptually, the information provided by these signals from your eye muscles is used to judge the distance of an object.
Another binocular distance cue is binocular disparity. Because our eyes are set a couple of inches
apart, a slightly different image of an object is cast on the retina of each eye. When the two retinal images are very different, we interpret the object as being close by. When the two retinal images are more nearly identical, the object is perceived as being farther away. Here’s a simple example that illustrates how you use binocular disparity to perceive distance. Hold a pencil just in front of your nose. Close your left eye, then your right. These images are quite different— that is, there is a great deal of binocular disparity between them. Thus you perceive the pencil as being very close. Now focus on another object across the room and look at it first with one eye closed, then the other. These images are much more similar. Because there is less binocular disparity between the two images, the object is perceived as being farther away. Finally, notice that with both eyes open, the two images are fused into one. A stereogram is a picture that uses the principle of binocular disparity to create the perception of a three-dimensional image (Kunoh & Takaoki, 1994). Look at the stereogram shown here. When you first look at it, you perceive a twodimensional picture of leaves. Although the pictorial cues of overlap and texture gradient provide some sense of depth to the image, the elements in the picture appear to be roughly the same distance from you. However, a stereogram is actually composed of repeating columns of carefully arranged visual information. If you focus as if you are looking at some object that is farther away than the stereogram, the repeating columns of information will present a slightly different image to each eye. This disparate visual information then fuses into a single image, enabling you to perceive a three-dimensional image—three rabbits! To see the rabbits, follow the directions in the caption.
Binocular Disparity and the Perception of Depth in Stereograms This stereogram, Rustling Hares, was created by artist Hiroshi Kunoh (Kunoh & Takaoki, 1994). To see the three-dimensional images, first hold the picture close to your face. Focus your eyesas though you are looking at an object that is beyond the book and farther away. Without changing your focus, slowly extend your arms and move the picture away from you. The image of the leaves will initially beblurry, then details will come into focus and you should see three rabbits. The threedimensional images that can be perceived in stereograms occur because of binocular disparity—each eye is presented with slightly different visual information.
Motion Perception
Our brain computes motion as objects move across or toward the retina. Large objects appear to move more slowly than smaller objects. A quick succession of images, as in a motion picture or on a lighted sign, can also create an illusion of movement.
Perceptual Constancy
Having perceived an object as a coherent figure and having located it in space, how then do we recognize it—despite the varying images that it may cast on our retinas? Size, shape, and lightness constancies describe how objects appear to have unchanging characteristics regardless of their distance, shape, or motion. These constancies explain several of the well-known visual illusions. For example, familiarity with the size-distance relationships in a carpentered world of rectangular shapes makes people more susceptible to the Mьller-Lyer illusion.
Perceptual Interpretation
The most direct tests of the nature-nurture issue come from experiments that modify human perceptions.
Sensory Deprivation and Restored Vision
For many species, infancy is a critical period during which experience must activate the brain’s innate visual mechanisms. If cataract removal restores eyesight to adults who were blind from birth, they remain unable to perceive the world normally. Generally, they can distinguish figure from ground and can perceive colors, but they are unable to recognize shapes and forms. In controlled experiments, animals have been reared with severely restricted visual input. When their visual exposure is returned to normal, they, too, suffer enduring visual handicaps.
Perceptual Adaptation
Human vision is remarkably adaptable. Given glasses that shift the world slightly to the left or right, or even turn it upside down, people manage to adapt their movements and, with practice, to move about with ease.
Perceptual Set
Clear evidence that perception is influenced by our experience—our learned assumptions and beliefs—as well as by sensory input comes from the many demonstrations of perceptual set and conBy one month before birth, the fetus demonstratestext effects. The schemas we have learned help us to interpret otherwise ambiguous stimuli, a fact that helps explain why some of us “see” monsters, faces, and UFOs that others do not.
Perception and the Human Factor
Perceptions vary, and may not be what a designer assumes. Human factors psychologists therefore study how people perceive and use machines, and how machines and physical environments can be better suited to that use. Such studies have improved aircraft safety and spawned user-friendly technology.
Is There Extrasensory Perception?
Many believe in or claim to experience extrasensory perception (ESP). To believe in ESP is to believe that the brain can perceive without sensory input. Most US scientists are skeptical, yet five British universities have parapsychology departments.
Examples of ESP include astrological predictions and communication with the dead. Three forms of ESP, telepathy, clairvoyance, and precognition, are deemed the most testable. However, parapsychologists have tried to documents several forms of ESP but for several reasons, especially the lack of a reproducible ESP effect, most research psychologists remain skeptical.
