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Source Knol: The Five Senses
by Kevin Spaulding, Sunnyvale, CA
Turn off all the lights, electronics, or other contraptions in your room, close your eyes, sit very still and, if you can, try to imagine that you cannot see, hear, taste, smell, or feel. You would be nothing but a brain hovering in the middle of an endless abyss of space. The world or your existence would have absolutely no meaning. Lacking feedback from your surroundings you would most likely die of starvation unless someone was there to take care of you. If you were born like this then you would hardly think and would never learn anything at all. This is why the senses are of utmost importance, for they connect our brains to the outside world.
Humans have five senses that are based on signals we receive through the skin, eyes, nose, tongue, and ears.  The brain takes these signals and makes sense of them, creating our interpretation of the world. Thinking about what this means can be fun because it raises philosophical questions. For instance, how can we really be sure that this supposed world we perceive actually exists outside of our own brain's imagination? After all, everything we've ever come to know since conception has been based on sensory experiences. Let's explore what the senses are and where they come from.
Sensory experiences start with special cells called sensory receptors, also called afferent nerves because they take information to the brain.  A sensory receptor will detect stimuli such as heat, light, sweetness, etc. and then convert it into an electrical signal that courses its way into the brain, where the signal is translated into perception like hot, bright, sweet, etc. We are constantly being bombarded by sensory information. The brain decides what information to keep and promote to consciousness and what to discard or keep in unconsciousness. By ignoring information from the senses we are able to focus on doing specific tasks so that we are not always overwhelmed by the world around us.
Lysergic acid diethylamide, better known as LSD, is a chemical compound that unlocks the barrier between the unconscious and conscious sensory information. This leads to psychedelic experiences that interfere with a person's ability to function.  It makes sense that the brain has evolved to block off some of the sensory receptor's messages since we would otherwise be in drowned in sensory overload.
Sensory information is processed by the side of the brain opposite to the sensory receptors it is coming from. For instance, things that we see with our right eye is processed by the left side of the brain, and when we touch something with our left hand the information is processed by the right side of the brain. 
The study of the relationship between physical stimulus and a person's conscious experience of that stimulus is called psychophysics. Studies in this field are done with the intent of understanding how much stimulus is needed to produce a psychological reaction to that stimulus. This helps us understand how and why we interpret the world around us like we do. 
The eyes send sensory information to the brain, which is then translated into vision. The eyes contain around 70% of all the body's sensory receptors, making sight the most information heavy of all the senses.  When light first gets to the eye it pass through the cornea, a covering over the iris. The iris will constrict/dilate to make the pupil smaller or larger so it can focus on objects.  Behind the pupil is a four millimeter thick crystalline lens which works with the pupil to form two-dimensional imagery of the world.  A constricting iris will sharpen the image hitting the retina and also decrease the amount of light entering the eye.
To understand how the image gets from the eye to the brain, we must understand the retina. The retina is the section of the eye that is covered by photoreceptors called rods and cones. These rods and cones will code light (electromagnetic radiation) into electrical signals through a process called transduction. There are around 120 to 125 million rod cells and 6 to 7 million cone cells in each eye.  Rod cells are shaped like rods and perform best in dim lit conditions. Cone cells are shaped like cones and perform best in bright conditions.
There is a specific cone cell for the colors blue, green and red. Each of these kinds of cone cells will interpret all the possible kinds of colors in the world by measuring the level of blue, green, or red in any object. This means that there are three kinds of cone cells but just one kind of rod cell. Inside of cone cells there are color detectors called photopsins.  These photopsins have a light sensitive chemical called retinol, which is made from vitamin A.  Retinol is found inside of shells that are made of opsin protein.  The type of opsin will determine whether the cell will detect the color blue, green, or red. Cyanolabe is the opsin for blue, chlorolabe the opsin for green, and erythrolabe the opsin for red. The retinol will change molecular shape when light hits it, which then causes the opsin to change shape. These reactions cause a series of nerves to be excited, helping electrical information get to the optic nerve.
The optic nerve is made of many neurons and runs out from the back of the retina and into the brain.  The optic nerve goes through the thalamus and ends at the back of the cortex in the occipital lobe. The visual cortex has cortical representations of the retina called retinotopic maps.  There are separate retinotopic maps for motion, depth, color, and form.  When our eyes send information to the visual cortex it is in the form of a two-dimensional pattern. The retinotopic maps and temporal lobe will then work together to build the three-dimensional representation we actually 'see'. We don't consciously recognize this because it happens at such a fast speed.
Now that we understand the electrochemical and biological processes that give us our vision, we should acknowledge the psychological processes that go along with sight. We form memories about how things look and how the world works, aiding in our visual perception. For instance, we know that when we place something down and walk away it will look like it is getting smaller but that it is not actually shrinking in size. We can also recognize a dog no matter what angle we see one at. These psychological truths are explained as size constancy and shape constancy, which are the abilities of the perceptual system to know that an object remains constant in size and shape regardless of distance or angle orientation.  Both of these things require that a person form memories of the object. For instance, someone who spots an elephant in the far distance, and has never seen an elephant before, may think that the elephant is small. If they know elephants are large, then the size they appear at from a distance won't trick them. It can also help to have other objects act as references. Someone who knows their basketball hoop is ten feet tall will automatically be able to determine the height of someone standing next to that basketball hoop.
There are other facets of perception that form through experience and the formation of memories, such as depth perception.  When we look at photographs we are able to determine what is up close and what is far away even though the picture only portrays two-dimensions. This is because our brains do this constantly, all of our lives, even while we are looking at three-dimensional environments. Cues that give us perception of depth and only require one eye are called monocular depth cues.  Monocular depth cues include motion parallax, kinetic depth effect, linear perspective, interposition, texture, clearness, and shadowing. Depth cues that require both eyes are called binocular depth cues, and include retinal disparity and convergence. 
Eyes are usually always in motion, including when we sleep. Rapid movements, called saccades, are the most common type of eye movement.  Saccades occur when people read, drive, or just look around. The eye can make four or five saccades a second. Each movement takes just 20 to 50 milliseconds but it takes 200 to 250 milliseconds before the eye will be able to make its next movement. These rapid movements create snapshots in our memory that fuse together and create a stable view of the world. 
When a person's eyes are not perfectly shaped their vision will be affected. Eyes shaped in such a way that they allow a person to see things that are close to them but not far away are called myopic, or nearsighted. The opposite of myopic eyes are hypermetropic, or farsighted eyes. Hypermetropic eyes can see things at a distance but have trouble seeing things up close. Both of these problems result in eyes sending images to the brain that are not well focused. 
Sometimes the lense of an eye will be irregularly shaped, causing visual distortions. This is called astigmatism.  It can sometimes accompany nearsightedness and farsightedness. Many times a person's astigmatism will not be pronounced enough for corrective actions, but when it is they usually have to get eye surgery.
As people age they will usually get fuzzy vision as their lenses thicken and become less pliable.  This usually results in people having to get glasses or contact lenses of some kind.
Sometimes a person will be born without the ability to see, or will experience heavy damage to the eyes through infection or disease that leaves them visually impaired.  When someone has no visual capability they are called totally blind. If a person is able to see but it is less than 20/200 after the best correction, then they are declared legally blind and treated as someone who is totally blind.
Hearing allows us to listen to music, the voices of loved ones, and many other things. In fact, humans probably wouldn't have evolved spoken language if it were not for the ability to hear. Our ears are the tools which collect sound. Because we have two of them, and because they are so sensitive, we are able to make out volume, pitch, direction, and distance of noise.  By measuring these factors we can even determine whether or not something is moving toward or away from us and at what rate.
The visible skin of the ear directs air vibrations into the ear canal. These vibrations are then picked up by the tympanic membrane, or eardrum. The eardrums will then send the vibrations to three different bones, all of which are very tiny. These bones are the malleus (hammer), incus (anvil), and stapes (stirrup).  The stirrup connects to the cochlea, which is spiral shaped and contains three fluid-filled cavities. The stirrup acts like a piston on the cochlea, causing a soft part called the oval window to move fluid around. The movement created by the fluid in the cochlea is detected by microscopic hair cells. Each hair cell has 20 to 30 hairs known as stereocilia that are arranged in a semi-circle from small to large.  The stereocilia are flexible and vibrations cause protein channels to open up between them and the hair cell, which results in the formation of chemical signals that will eventually be sent to the brain.
Hair cells in the cochlea are arranged in such a way that certain frequencies will affect specific hairs only. This is called a tonotopic map.  Each hair cell can send information to ten nerve fibers which carry the signal to the brain stem, where it is briefly analyzed. The brain stem then passes the information to the primary auditory cortex (temporal lobes) where it is analyzed in full.  The front of the temporal lobes will work with low frequency signals while high frequency signals are sent to the back. This will tell us the volume, pitch, and direction of the sound.
We can determine the direction a noise is coming from by way of sound localization. Because we have two ears, we can usually hear something in one ear before we hear it in the other, which helps us determine where the sound is coming from. This is called interaural time difference.  Sound can also enter one ear at a higher intensity than it enters the other. By registering that one ear found the frequency more intense, we can decide which direction the sound must be coming from. This is called interaural intensity difference.  When we are uncertain of a sound's direction we will turn our head and body to use interaural time and intensity difference.
Hearing impairment is another way of describing damaged or incorrectly functioning auditory systems. There are different kinds of impairment ranging from minor hearing loss to total deafness.  The two most common forms of hearing impairment are conduction deafness nerve deafness.
Conduction deafness is defined as interference in the delivery of sound to the neural mechanism of the inner ear.  This interference can be caused by hardening of the tympanic membrane, destruction of the tiny bones in the ears, diseases that create pressure in the middle ear, head colds, or buildup of wax in the outer ear canal.
Nerve deafness is damage to the ear that is usually results from very high intensity sound emitted by things like rock bands and jet planes.  Constant presence of loud noise can increase a person's sound threshold. This means that a higher-ampiltude sound will be needed to create the same effect that lower-amplitude sound has on someone with normal hearing. Of course, this also means that the person with conduction deafness will have trouble making out sounds that are of a normal decibel. It is important to keep headphones at a reasonable volume and to protect the ears when around loud machinery. 
Older people will often have hearing impairment, especially in the high-frequency ranges. Sometimes this can cause difficulties that can generally be helped with the use of hearing aids. Serious deafness such as the kind caused by genetics are much harder to help. Though hearing aids can help slightly, these people generally find it impossible to communicate with hearing people. Since speech and language is directly tied to the auditory systems of our brain, those with severe hearing impairment are unable to use their voice successfully for communication. Though they can read lips and hearing aids can sometimes help, deaf people often feel alienated unless they can talk with someone who knows sign language. 
Smell and Taste
Smell and taste are perhaps the most important senses in terms of evolutionary significance. They cause us to want to eat, and help us know what we should be eating. Smell actually works with taste to help us decide whether we're eating something pleasing or disgusting, which can save our lives!  When we eat, the tongue is stimulated, of course, and molecules from the food will travel up the nose. These molecules excite millions of nasal sensors (neurons) that are on a sheet of tissue called the olfactory epithelium. The nasal sensors will live only 30 to 60 days before they are replaced by a sheet of stem cells that are waiting in line for their turn as nasal neurons. At the end of each of these cells there are five to twenty little hairs, called cilia. These hairs extend into the nasal cavity where they're protected and aided by mucus. 
The human nose can detect millions of different kinds of scents. We have at least one thousand olfactory neuron types, each one capable of detecting a different kind of odorant molecule.  When one of these neurons binds with an odorant molecule it will send an electrical signal to its axon. These axons go up into the skull through something called the cribriform plate and connect with the brain. The signals are passed along through the thalamus and into the temporal lobe, or olfactory cortex.  Since the pathways of the brain that analyze smell are closely connected with parts of the brain that are responsible for emotions (amygdala) and memories (hippocampus), smell has a way of bringing up emotions and memories from the past. 
The tongue has sensory cells, called taste buds, that determine whether something is bitter, sweet, salty, sour, or savory.  Each taste bud has about one hundred taste cells with little projections called microvilli.  Chemical signals created by these sensory cells are converted to electrical signals and sent to the cortex. Smell and taste information is analyzed by the cortex and transformed into a single perception we call taste. Of course, we can smell things without having to taste them, but taste would be very different without the ability to smell.
The sensation of feeling things touch the skin of our bodies is called touch. Skin is made of three different layers: the epidermis, dermis, and hypodermis. The sense of touch occurs because of sensory receptors built into our skin, called mechanoreceptors.  These mechanoreceptors are neurons with stretch-sensitive gateways that open up when pressure is applied.  When the skin is touched it will deform, opening up the gates that cause receptors to fire electrical signals through the nervous system and into the brain.
We also have thermoreceptors, which have a protein that varies the cell's activity depending on the amount of heat it is exposed to.  Cold receptors will fire signals at around 77 degrees fahrenheit while warm receptors will fire signals at around 113 degrees.  When skin is exposed to dangerous temperatures nociceptors will activate.
Nociceptors are sensory cells responsible for creating pain. When activated they will release a range of chemical messengers which bind to and activate nearby nerves whose sole purpose is to transmit pain information to the brain. These pain signals are routed through the thalamus and into the cerebral cortex. In fact, all the information collected by the skin is sent to the cerebral cortex by way of the spinal cord and thalamus. This information is then processed in a sequential map by nerve cells that specialize in texture, shape, orientation, temperature, and more. 
These are the main five senses that humans recognize. Some of them obviously overlap, like smell and taste, while others work together in more subtle ways. There is still a lot to learn about how humans perceive the world around them, and there may be senses that the scientific community has not yet proven exist. For example, there is the idea that some people posess extrasensory perception (ESP), or heightened perceptual abilities that the normal person doesn't have. ESP includes telepathy, which is the transfer of thoughts from one person to another without the use of anything external, and clairvoyance, which is the ability to recognize objects or events without the use of normal sensory receptors. Then there is precognition, which is the ability to see into the future, and psychokinesis, which is the ability to move object's using only the mind. None of these abilities are tangible or proven, but there is some evidence which hints that there are senses we don't fully understand yet. On any note, the five senses are both incredible and undeniably important. It will be interesting to see what future research reveals about our senses and ability to perceive the world.
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Source Knol: The Five Senses