Sensation and Perception PDF

CHAPTER 3 SENSATION AND PERCEPTION Lesson 3.1: Sensation Introduction When talking about Sensation, we then think about our Sense organs- our eyes, ears, tongue, nose an...

CHAPTER 3 SENSATION AND PERCEPTION Lesson 3.1: Sensation Introduction When talking about Sensation, we then think about our Sense organs- our eyes, ears, tongue, nose and skin- allow us to appreciate our external world and make our living experience more wonderful. Our sensory organ’s primary function is to act as biological transducers. Transducers are devices that convert one kind of energy into another, for example a microphone. A microphone is a tool that changes sound into electricity. When you talk or make noise near the microphone, it picks up the sound. Inside the microphone, there’s a small part that moves with the sound waves. This movement turns the sound into an electric signal. The electric signal can then be sent to a speaker or saved in a recording. Essentially, a microphone helps turn what you say into something that can be heard through speakers or saved as a recording. In a similar context, the brain only works with one type of energy, which is electrical energy in the form of action potential. However, information that's out in the world comes to use as many different forms of energy and each one needs to be converted into electrical energy so that the brain can understand it. Considering our sense organs as biological transducers, it can be thought of as "translators" converting the different forms of energy in the world into electrical energy so that the brain can understand the information from the outside world. Sensation is the process of converting energy from the environment into a pattern of response by the nervous system. For example, our eyes convert light energy, the ears translate mechanical energy from sounds, and the nose and tongue translate chemical energy from odors and foods. Information arriving at the brain from the sense organs creates sensory impressions. When our sensory organs are damaged and unable to transduce energy, the brain cannot interpret the information from that sense, leading to difficulties with seeing, hearing, smell, and so on. But with modern technology, scientist are able to design artificial devices to help send electrical signal directly to the brain without passings to the damaged sense organs. Let us have a short recall of action potential and how we measure sensory impressions before we continue on Sensation. One of the main experts who tries to understand sensory impressions are called Psychophysicists. Psychophysics studies how our minds interpret physical properties of stimuli, like sound waves, light, or chemicals in food. It explores how the brain translates these physical properties into experiences, such as loudness, brightness, or sweetness. Moreover, psychophysicists have shown that a certain minimum amount of physical energy, known as the absolute threshold, is needed for a sensory impression to occur 50% of the time. Additionally, the difference threshold measures the minimum difference in energy needed between two stimuli for the difference to be noticed half the time. You might wonder how our brain is capable of selecting the sensations that are constantly stimulated in the different organs. Sensory transduction usually involves some selection. Psychophysical research has found that vision also narrows what we can possibly observe. Like the other senses, vision acts as a data selection system, meaning, it selects information in order to code and send to the brain only the most important sensory information for further processing. SENSORY SELECTION: THE FOUR WAYS TO REDUCE SENSORY OVERLOAD 1. Lack of Specific Transducers Considerable selection occurs because human sensory receptors do not transduce all energies that they encounter. For example, the eye transduces light waves, the ear transduces sound waves, and so on. But many other types of stimuli cannot be sensed directly because we lack sensory receptors to transduce their energy. For example, humans cannot sense the bioelectric fields of living creatures, but sharks have special organs that can. 2. Restricted Range of Transducers Further selection occurs because sense receptors transduce only part of their target range. For example, your eyes transduce only a tiny fraction of the entire range of electromagnetic energies- called the visible spectrum. For example, the eyes of the honeybees can transduce, and therefore see, parts of the electromagnetic spectrum that are invisible to humans. Another example are the bats, where they can hear their own reflected echoes, which is an ability called echolocation, this allows the bats to fly in total darkness, avoid collisions and catch insects. As you can see, our rich sensory experiences are only a small part of what could be sensed and what some animals can sense. 3. Sensory Adaptation Sensory Adaptation is a process were sensory receptors response less overtime to unchanging stimuli. Example is our sense of smell which can adapt quickly. When our olfactory receptors are exposed to a constant odor, they send fewer and fewer nerve impulses to the brain until the odor is no longer noticed. The reason for this concept is that there is usually little reason to keep reminding the brain that a sensory input is unchanged, sensory receptors generally respond best to changes in stimulation. 4. Feature Detection As the senses collect information, feature detectors in the brain also reduce the flow of sensory input by dividing the world into important perceptual features, which is the basic attributes or stimulus patters, such as lines, shape, edges, or colors. A feature detector is a cell, or collection of cells in the cerebral cortex that responds to a specific attribute of an object. Consequently, the brain need only further process the perceptual feature rather than the underlying sensory pattern. Visual pop-out is one example of feature detectors that our visual system have. Visual pop-out occurs because our visual system is highly sensitive to these perceptual features. Although our sensitivity to perceptual features is an innate characteristic of the nervous system, it also is influenced by the experiences early in life. In summary, our sensory organs have remarkable capacity to take in the rich and varied information that exists in the world around us. VISION When we open our eyes, we become aware of the visual richness of the world around us. However, our eyes transduce only the tiniest fraction of the entire range of electromagnetic energies- which is the visible spectrum. We cannot "see" the other electromagnetic spectrum such as microwaves, cosmic rays, X-rays, or radio waves. So how does sensory transduction in vision occur? Understanding how vision works is one of the complex sense organs to understand, but it is wonderfully made. To better understand how it works, we must first understand the characteristic of how the light is being processed by the eye, and then the process of how it is being transduced or translated from a light energy into electrical energy. Lastly, we will also consider the theories of color vision. Characteristic of Light The Visible spectrum- the sliver of electromagnetic energies to which the eyes respond- is made up of a narrow range of wavelengths of electromagnetic radiation. Visible lights starts at "short" wavelengths of 400 nanometers, which we sense as purple or violet. Successively longer light waves produce blue, green, yellow, orange, and red, which has a wavelength of 700 nanometers. There are three (3) characteristics of light that is important for us to remember: 1. Hue (Colors). This refers to the various colors of light: red, orange, yellow, green, blue, indigo, and violet. Note: Various hues, or color sensations, correspond to the wavelength of the light that reaches our eyes. Hue are the colors of light a determined by its corresponding wavelength. Additionally, white light, in contrast is a mixture of many wavelengths. 2. Saturation. Hues from a narrow band of wavelengths are very saturated, or pure. Example, an intense, fire-engine red is more saturated than a muddy, brick red that might include some degree of orange or brown. 3. Brightness. This corresponds roughly to the amplitude (or height) of light waves. Waves of greater amplitude are taller, carry more energy, and cause the colors that we see to appear brighter or more intense. For example, the same brick would look brighter in intense, high-energy illumination and duller in dim light. How does the eye actually work? Although the visual system is much more complex than any digital camera, both cameras and eyes have a lens to focus light rays on a light-sensitive surface at the back of an enclosed space, where the image is created. The structure of the eye serves two purposes, first, it begins by focusing the light waves coming in from the world, and second, it carries out the important work of transducing them so that the brain can make sense of the incoming information and create an image. Let us understand further the purpose of the eye one by one. 1. FOCUS! The Process of Accommodation in the Eye In cameras, focusing is done relatively simply- by changing the distance between the lens and the image sensor. In the eye, most focusing is done by the cornea, a curved, transparent, protective layer at the front of the eye that bends light inward. The lens, which is the clear structure behind the pupil that bends light toward the retina, makes additional, smaller adjustments. Our eye's focal point changes when the ciliary muscles attached to the lens alter its shape. This process is called accommodation, and it's what allows you to focus on objects regardless of whether they are several feet away or right in front of your nose. There are two factors that can compromise our ability to focus, first is the shape of our eye, and second is the flexibility of the lens. ⮚ The shape of our eye If the eye is too short, nearby objects will be blurred, but distant objects will be sharp. This is also called as Hyperopia (farsightedness), which is having difficulty focusing on nearby objects. If the eyeball is too long, images fall short of the retina, this is called Myopia (nearsightedness) and the person won't be able to focus on distant objects.  When the cornea or lens is misshapen, part of the person’s vision will be focused, and part will be fuzzy. In this case, the eye has more than one focal point, and this is called Astigmatism, whish is a defect in the cornea, lens, or eye that cause some areas of vision to be out of focus. Just remember that all three visual defects can be corrected by placing glasses (or contact lenses) in front of the eye to change the path of light. ⮚ Flexibility of the Lens As people age, the lens becomes less flexible and accommodating becomes more difficult for the eye, this is called Presbyopia which comes from the Latin for “Old vision “ or farsightedness caused by aging. As people age, people need bifocal lenses, which correct near vision and distance vision. 2. The Process of Transduction in the Eye Back to the concept similar to a camera, it is the job of the cornea and lens to focus light rays on a light-sensitive surface at the back of an enclosed space. In a camera, the light-sensitive surface is a layer of pixels in the digital image sensor. In the eye, it is a layer of light-sensitive cells called photoreceptors that are located in the retina. The retina is the surface at the back f the eye onto which the lens focuses light rays. It has an area about the size and thickness of a postage stamp. The eye has two types of photoreceptors, namely the rods and cones, which are responsible for transduction. Again, transduction is the process by which energy out there in the world- in this case, light energy- is converted into electrical energy (action potentials) that can be understood by the brain. PHOTORECEPTORS: THE NERVE CELLS IN THE EYE ⮚ Cones There are 5 million cones in each eye, which work best in bright light. They also produce color sensations and fine details of what we can see. Cones, by definition, are the photoreceptors that are sensitive to color, and it mainly lies at the center of the eye. In fact, the fovea, a tiny spot in the center of the retina, contains only cones- about 50,000 of them. Like high-resolution digital sensors made of many small pixels, the tightly packed cones in the fovea produce the highest level of acuity (that is the sharpest images). Normal acuity (sharpness of vision) is designated as 20/20 vision, meaning at a distance of 20 feet, you can distinguish what the average person can see at 20 feet. Therefore, the cones are primarily responsible for your color vision, though in some cases individuals are not able to see colors in the normal way. In such cases, we say that they are experiencing color blindness or color weakness. What is it like to be color blind? What causes color blindness? A person with color blindness cannot perceive colors. It is as if the world were a black-and-white movie. The color-blind person either lacks cones or has cones that do not function normally. Such total color blindness is rare. In color weakness, or partial color blindness, a person can't see certain colors. How can color-blind individuals drive? Don't they have trouble with traffic lights? Red-Green color blind individuals have normal vision for yellow and blue, so the main problem is telling red lights from green. In practice, that’s' not difficult. The red light is always on top, and the green light is brighter than the red. Also, red traffic signals have yellow light mixed in with the red, and a green light is really blue-green.  ⮚ Rods Areas outside the fovea also get light, creating a large region of peripheral (side) vision. Peripheral vision are visions we see at the edge of the visual field. This is the area where the rods take over from the cones. The rods are most numerous about 20 degrees from the center of the retina, so much of our peripheral vision is rod vision. Inside the rods and cones is where transduction occurs in the eye, but they also have a role to play in visual acuity, or the sharpness of visual perception. Once those photoreceptors have converted light energy into electrical energy, those action potentials travel along a series of interneurons to the optic nerve, and then on to the brain. The rods are also quite sensitive to movement in peripheral vision. People who suffer from tunnel vision (a loss of peripheral vision) feel as if they are wearing blinders. There are about 120 million rods in the eye, however, rods can’t detect colors. Pure rod vision is black and white, with this, rods are much more sensitive to light than cones. This allows the rods to help us see in very dim light. Dark adaptation is the dramatic increase in the eye's sensitivity to light that occurs after a person enters the dark or under low-light conditions. Consider yourself walking into a movie theater. If you enter from a brightly lit lobby, you practically need to be led to your seat. Almost immediately, the pupil (the black opening inside the iris that allows light to enter the eye) the opening surrounded by the colored iris ( the colored structure on the surface of the eye surrounding the pupil) , begins to open to allow lighter to enter the eye. After a short time, you can see the entire room in detail. The retina, however, also becomes more sensitive, taking about 30 to 35 minutes of complete darkness to reach maximum visual sensitivity. At that point, your eye will be 100,000 times more sensitive to light. What causes dark adaptation? Like cones, which contain a pigment called iodopsin, rods contain a light-sensitive visual pigment, rhodopsin, which allows them to see in black and white. Moreover, he rods are insensitive to extremely red light. That's why submarines, airplane cockpits, and ready rooms for fighter pilots are illuminated with red light. In each case, people can move quickly from a light place into a dark one without having to adapt. Because the red light doesn't stimulate the rods, it is as if they had already spent time in the dark. Fun Fact: The Retina Has A Hole In It! Each retina has a blind spot because there are no photoreceptors at the location where the optic nerve exits the eye to convey visual information to the brain, and blood vessels enter. Blind spot is the area in the retina where the optic nerves exits that contains no photoreceptor cells. Moreover, Optic nerve is a structure in the eye that conveys visual information away from the retina to the brain. The blind spot shows that vision depends greatly on the brain. If you close an eye, some of the incoming light will fall on the blind spot of your open eye. If there is a hole in the Retina, then why isn't there a gap in our vision? The answer is that the visual cortex of the brain actively fills in the gap with patterns from surrounding areas. The brain can also "erase" distracting information. If you roll your eyes all the way to the right and then close your right eye. You should clearly see your nose in your left eye's field of vision. Now open your right eye again, and your nose nearly disappears because your brain disregards its presence. Theories of Color Vision 1. Trichromatic Theory of Color Vision The trichromatic theory holds that there are three types of cones, each most sensitive to either red, green, or blue. Other colors result from combinations of these three. However, there are 2 basic problems with the trichromatic theory that have been identified, first is that four colors of light- red, green, blue, and yellow- seem to be primary. Second, this theory doesn’t account for the fact that it's impossible to have a reddish green color, or a yellowish blue. These problems led to the development of a second view of color vision. 2. Opponent-Process Theory of Color Vision Opponent-process theory of color vision, states that vision analyzes colors into "either-or" messages. That is the virtual system can produce messages for either red or green, yellow or blue, or black or white. Coding one color in a pair (red, for instance) seems to block the opposite message (green) from coming through. As a result, a reddish green is impossible, but a yellowish red (orange) can occur. According to opponent-process theory, fatigue caused by repeatedly having the cones respond to one color produces an afterimage of the opposite color as the system recovers. Afterimages are visual sensations that persist after a stimulus is removed-like seeing a spot after a flashbulb goes off. Which color theory is correct? Both theories are correct! The three-color theory applies to the retina, in which three different types of cones have been found. Each contains a different type of iodopsin, a light-sensitive pigment that breaks down when struck by light. This triggers action potentials and send neural messages to the brain. Each type of iodopsin is most sensitive to light in roughly the red, green, or blue region. Other colors result from combination of these three. Thus, the three types of cones fire nerve impulses at different rates to produce various color sensation. In contrast, the opponent-process theory better explains what happens beyond the retina- in the optic pathways and the brain-after information leaves the cones. HEARING Characteristics of Sound: What the Ear Hears The sound travels as a series of invisible waves of compression (peaks) and rarefaction (valleys) in the air. Any vibrating object - example a tuning fork, the string of a musical instrument, or the vocal cords-will produce sound waves (rhythmic movement of air molecules). The sound wave has two most important characteristics : its frequency and its amplitude. The frequency of sound waves (the number of waves per second) corresponds to the perceived pitch (higher or lower tone) of a sound. The amplitude (or physical "height" of a sound wave) tells how much energy it contains. Psychologically, amplitude corresponds to sensed loudness (the volume of a sound, related to the amplitude of a sound wave) or sound intensity. The Process of Transduction in the Ear Transduction, again, is the process by which a sensory organ- the ear in this case- converts some form of energy into electrical energy (in the form of action potentials) so that the brain can interpret it. The ear is responsible for transducing mechanical energy in the form of sound waves. Hearing involves a chain of events that begins with the pinna, the visible, external part of the ear. The pinna also acts like a funnel to focus sounds. After they are guided into the ear canal, sound waves collide with the eardrum (tympanic membrane), setting it vibrating in response, thus transmitting them inward. By definition, the Eardrum, is a membrane that vibrates in response to sound waves and transmits them inward. This in turn, causes three small middle ear ossicles (or bones): the malleus (or hammer), incus (or anvil), and stapes (stirrup) to vibrate. The ossicles link the eardrum with the cochlea (a snail-shaped organ in the inner ear that contains sensory receptors for hearing.). The stapes is attached to a membrane on the cochlea called the oval window. As the oval window moves back and forth, it makes waves in a fluid inside the cochlea. Inside the cochlea, the fluid waves trigger vibrations in the basilar membrane (the structure in the cochlea containing hair cells that convert sound waves into action potentials.), which is the "floor" of the organ of Corti. In turn, tiny hair cells (are receptor cells within the cochlea that transduce vibrations into nerve impulses) embedded in the basilar membrane are pushed up against the tectorial membrane. As a consequence a set of stereocilia (or bristles) atop each hair cell brush against the tectorial membrane whenever waves ripple through the fluid surrounding the organ of Corti. As the stereo-cilia are bent, action potentials are triggered, which then flow to the brain. Hearing Loss There are 2 common types of hearing loss: 1. Conductive hearing loss occurs when the transfer of vibrations from the outer ear to the inner ear weakens or poor transfer of sounds from the eardrum to the inner ear. Example, the eardrums or ossicles may be damaged or immobilized by disease or injury. In many cases, conductive hearing loss can be overcome with a hearing aid, which amplifies sounds, making them louder and clearer. 2. Sensorineural hearing loss is quite different, and results from damage to the inner ear hair cells or the auditory nerve. Many jobs, hobbies, and pastimes can cause noise-induced hearing loss (damage caused by exposing the hair cells to excessively loud sounds) which is a common subtype of sensorineural hearing loss that occurs when very loud sounds damage fragile hair cells. Note: If you work in a noisy environment or enjoy loud music, motorcycling, snowmobiling, hunting, or similar pursuits, you may be risking noise-induced hearing loss. Be forewarned: Dead hair cells are never replaced. When you abuse them, you lose them. By the time you are 65, more than 40 percent of them will be gone, mainly those that transduce high pitches. How loud must a sound be to be hazardous?Daily exposure to 85 decibels or more may cause permanent hearing loss. Decibels are a measure of sound intensity. Every 20 decibels increases the sound pressure by a factor of 10. In other words, a rock concert at 120 decibels is 1000 times stronger than a voice at 60 decibels. Short periods at 120 decibels can cause temporary hearing loss, and even one brief exposure to 150 decibels (a jet airplane nearby) may cause permanent hearing loss. Hearing aids are no help in cases of sensorineural hearing loss because auditory messages are being blocked from reaching the brain. In many cases, however, the hair cells are damaged but the auditory nerve is intact. This finding has spurred the development of cochlear implants that bypass hair cells and stimulate the auditory nerves directly. Wires from a microphone carry electrical signals to an external coil. A matching coil under the skin picks up the signals and carries them to one or more areas of the cochlea. Theories of Hearing Pitch Remembering the two important characteristics of a sound are its loudness (which comes from the amplitude of the sound wave) and its pitch (which is determined by the frequency). Two theories have been put forward to explain how we distinguish different pitches. The frequency theory of hearing (proposition that pitch is decoded from the rate at which hair cells of the basilar membrane are firing) states that as pitch rises, nerve impulses of a corresponding frequency are fed into the auditory nerve- that is, a 1,200-hertz tone produces 1,200 nerve impulses (action potentials) per second. The term hertz refers to the number of vibrations per second. This explains how sounds up to about 4,000 hertz reach the brain, but the theory cannot account for how we hear pitches (that is, tones) that are higher than that. Place theory of hearing To explain how the brain can make out tones higher than 4,000 hertz, we turn to the place theory of hearing, which states that higher and lower tones excite specific places in the cochlea. The place theory of hearing (proposes that higher and lower tones excite specific areas of the cochlea). High tones register most strongly at the base of the cochlea (near the oval window). Lower tones on the other hand, mostly move hair cells near the narrow outer tip of the cochlea. Pitch is then signaled by the area of the cochlea that is most strongly activated. Place theory is most useful in explaining why hunters sometimes lose their hearing in a narrow pitch range. "Hunter's notch" as this condition is called, occurs when hair cells are damaged in the specific area of the cochlea affected by the pitch of gunfire. CHEMICAL SENSES: OLFACTION & GUSTATION OLFACTION (SMELL) Our sense of smell is called Olfaction. Smell receptors respond to chemical molecules that are airborne. Anything that you can smell- from flowers to fudge to formaldehyde- produces chemical molecules that are picked up by your nose. The Process of Transduction in the Nose Again, transduction refers to the process by which one form of energy that's picked up by the senses (in this case, chemical energy) is converted into electrical energy (in the form of action potentials) that can be interpreted in the brain. As air enters the nose, it flows over roughly 5 million nerve fibers embedded in the lining of the upper nasal pages. Receptor proteins on the surface of the fibers are sensitive to various airborne chemical molecules. When a fiber is stimulated, it creates an action potential that then travels to the brain. Theory of Odor Detection The specific way that different odors are detected is still an unfolding mystery. One hint about the process comes from a type of anosmia, a sort of "smell blindness" to a single odor. Risks for anosmia include infections, allergies and blows to the head (which may tear the olfactory nerves). Moreover, repeated exposure to chemicals such as ammonia, paints, solvents, and hairdressing "potions” also can cause anosmia. Discovering that there exists a loss of sensitivity to specific types of odors in anosmia suggests the presence of receptors in the nose for specific odors.  The lock-and-key theory of olfaction suggests that these receptors may bind with airborne chemical molecules that have a matching "Shape" to create odors. By definition, it is a theory holding that odors are related to the shapes of chemical molecules. Like a piece fit into a puzzle, airborne chemical molecules produce odors when part of the molecule matches a hole on the receptor of the same shape (hence the name, "lock" and key" theory). Furthermore, chemical molecules trigger activity in different combinations of odor receptors. Thus, humans can detect at least 10,000 different odors. Just as you can make hundreds of thousands of words in English from the 26 letters of the Roman alphabet, there are many combinations of the 400 types of receptors that can be activated, resulting in many different odors. Additionally, researchers have noted that scents are also identified, in part, by the location of the receptors in the nose that a particular odor activates. And finally, the number of activated receptors tells the brain the strength of an odor. The brain uses these distinctive patterns of messages it gets from the olfactory receptors to recognize particular scents. GUSTATION (TASTE) Taste is also a chemical sense, in which chemical molecules are found in our food. Our sense of taste is also called gustation. There are five basic taste sensations: sweet, salty sour, bitter, and umami. We are generally most sensitive to bitter and sour going back to many generations, this may have helped prevent poisonings when most humans foraged for food because bitter and sour foods are more likely to be inedible. Umami which is a Japanese word describes a pleasant savory or "brothy" taste associated with certain amino acids in chicken soup, some meat extracts, kelp, tuna, human milk, cheese, and soybeans. The receptors for umami are sensitive to glutamate, a substance found in monosodium glutamate (MSG). The Process of Transduction on the Tongue Taste buds, which are the clusters of taste-receptor cells, are located mainly on the top side of the tongue, especially around the edges. As food is chewed, it dissolves and the chemical molecules enter the taste buds, where they set off action potentials that travel to the brain. Theory of Taste Detection Like smell, sweet, bitter, and umami tastes appear to be based on a lock-and-key match between the molecules and intricately shaped receptors. Saltiness and sourness, however, are triggered by a direct flow of charged atoms into the tips of taste cells. If there are only five tastes, how can there be so many different flavors? Flavors seem more varied because we include sensations of texture, temperature, pain (think "hot" chili peppers), and smell when we taste things. Smell, in particular, is important in determining flavor. If you plug your nose and eat small bits of apple, potato, and onion, they will "taste" almost exactly alike. So do gourmet jellybeans! That's also why food loses its "taste" when you have a cold. It is probably fair to say that subjective flavor is at least half based on smell. SOMESTHETIC SENSES: SKIN SENSES, KINESTHETIC, & VESTIBULAR SKIN SENSES (TOUCH) Touch (the skin senses) is the first of the somesthetic, or body-related senses. Skin senses includes the senses of touch, pressure, pain, heat, and cold. It is difficult to imagine what life would be like without the sense of touch, a condition called anaphia. The Process of Transduction on the Skin Skin receptors produce at least five different sensations: light, touch, pressure, pain, cold, and warmth. Receptors with particular shapes appear to specialize somewhat in various sensations. However, free nerve endings alone can produce all five sensations. Altogether, the skin has about 200,000 nerve endings for temperature, 500,000 for touch and pressure, and 3 million for pain. Does the number of receptors in an area of skin relate to its sensitivity? Yes. Your skin can be "mapped" by applying heat, cold, touch, pressure, or pain to points all over your body. Such testing would show that the number of skin receptors varies, and that sensitivity generally matches the number of receptors in a given area. Important areas such as the lips, tongue, face, hands, and genitals have a higher density of receptors. Of course, what you ultimately feel will depend on brain activity. Yes. About 230 pain points per square centimeter (about a half-inch) are found behind the knee, 180 per centimeter on the buttocks, 60 on the pad of the thumb, and 40 on the tip of the nose. Regarding pain, we have a body’s warning system, which is pain based on large nerve fibers, this warns that bodily damage may be occurring. Pain carried by large nerve fibers is sharp, bright, and fast and seems to come from specific body areas. Give yourself a small nab with a pin and you will feel this type of pain. As you do this, notice that warning pain quickly disappears. Much as we dislike warning pain, it is usually a signal that the body has been, or is about to be, damaged. Without warning pain, we would be unable to detect or prevent injury. A second type of somatic (bodily) pain is carried by small nerve fibers. This type of pain is slower, nagging, aching, widespread, and very unpleasant. It gets worse if the pain stimulus is repeated. This is the body's reminding system, which reminds the brain that the body has been injured. For instance, lower-back pain often has this quality. Sadly, the reminding system can cause agony long after an injury has healed, or in terminal illnesses, when the reminder is useless. The Pain Gate Gate Control Theory (by Ronald Melzack's, 1999), suggests that pain messages from the two different types of nerve fibers pass through the same neural "gate" in the spinal cord. If the gate is "closed" by one pain message, other messages may not be able to pass through. How is the gate closed? It may depend on what types of nerve fibers are carrying information about the pain. Messages carried by large, fast nerve fibers seem to close the spinal pain gate directly. Doing so can prevent slower, "reminding system" pain from reaching the brain. But messages from small, slow fibers seem to take a different route. After going through the pain gate, they continue to a "central biasing system" in the brain which sends a message back down the spinal cord, closing the pain gates. Melzack believes that gate control theory may explain the painkilling effects of acupuncture, which is the Chinese medical art of relieving pain and illness by inserting thin needles into the body. As the acupuncturist’s needles are twirled, heated, or electrified, they activate small pain fibers. These relay through the biasing system to close the gates to intense or chronic pain. The Kinesthetic and Vestibular Systems Along with touch, the kinesthetic and vestibular senses are somesthetic senses and they are primarily concerned with the position of your body in relation to the rest of the world. KINESTHETIC SENSES (BODY POSITION & MOVEMENT) Kinesthetic senses, is defined as, the senses of body movement and positioning. Kinesthetic transducers are special receptors that are located in the muscles and joints. They send information to the brain about where your various body parts are located in space, relative to one another. The kinesthetic senses also allow you to touch your nose (or any other part of your body) even when your eyes are closed, and you cannot see your hand where it is moving. VESTIBULAR SENSES (GRAVITY, BALANCE, AND ACCELERATION) Vestibular system is somewhat different. Its primary function is related to the position of the body as a whole in space, and it has an important role to play in the ability to keep our balance. Vestibular senses is then defined as the perception of balance, gravity, and acceleration. The Process of Transduction in the Vestibular System Transducers for the vestibular system are located in the inner ear. There fluid-filled sacs called otolith organs are sensitive to movement, acceleration, and gravity. The otolith organs contain tiny crystals in a soft, gelatinlike mass. The tug of gravity or rapid head movements can cause the mass to shift. This in turn, stimulates hairlike receptor cells, allowing us to sense gravity, acceleration, and movement through space. Three fluid-filled tubes- called the semicircular canals- are the sensory organs for balance. When head movements occur, the fluid inside the tubes will also swirl in accordance with the head movement. As the fluid moves, it bends a small "flap", or "float," called the crista, that detects movement in the semicircular canals. The bending of each crista again stimulates hair cells and signals head rotation. Sensation in Everyday Life: Motion Sickness Information coming from a variety of senses (or "modalities") isn't necessarily processed by the brain separately. Our tendency to integrate, or combine, the sensory impression from several modalities is referred to as multimodal integration. Multimodal Integration is the process by which the brain combines information coming from multiple senses. For example, information from the vestibular system, vision, and kinesthetic is often integrated to give you a more complete sense of your body's orientation in space. On solid ground, the information coming from those senses will align.