Touch Taste Hearing Smell Vision A LIFE SIZE HUMAN BRAIN Each of the five senses activates a separate area of the cerebral cortex the sheet of neurons that makes up the outer layer of the brain s hemispheres This brain shown in actual size is a computer reconstruction based on data from magnetic resonance imaging MRI Approximate locations of the primary sensory areas are shown in color Most of the activity takes place within convolutions that cannot be seen from the surface of the brain W e can recognize a friend instantly fullface in profile or even by the back of his head We can distinguish hundreds of colors and possibly as many as 10 000 smells We can feel a feather as it brushes our skin hear the faint rustle of a leaf It all seems so effortless we open our eyes or ears and let the world stream in Yet anything we see hear feel smell or taste requires billions of nerve cells to flash urgent messages along linked pathways and feedback loops in our brains performing intricate calculations that scientists have only begun to decipher You can think of sensory systems as little scientists that generate hypotheses about the world says Anthony Movshon an HHMI investigator at New York University Where did that sound come from What color is this really The brain makes an educated guess based on the information at hand and on some simple assumptions S EEING H EARING AND S MELLING THE W ORLD 7 When you look at the illustration below for instance you see an X made of spheres surrounded by cavities But if you turn the page upside down all the cavities become spheres and vice versa In each case the shapes seem real because your brain assumes there is a single light source and that this light comes from above says Vilayanur Ramachandran a professor of neuroscience at the University of California San Diego As he points out this is a good rule of thumb in our sunlit world To resolve ambiguities and make sense of the world the brain also creates shapes from incomplete data Ramachandran says He likes to show an apparent triangle that was developed by the Italian psychologist times hear things that are not really there But suppose a leopard approached half hidden in the jungle then our ability to make patterns out of incomplete sights sounds or smells could save our lives Everything we know about the world comes to us through our senses Traditionally we were thought to have just five of them vision hearing touch smell and taste Scientists now recognize that we have several additional kinds of sensations such as pain pressure temperature joint position muscle sense and movement but these are generally included under touch The brain areas involved are called the somatosensory areas Although we pay little attention to them ILLUSIONS REVEAL SOME OF THE BRAIN S ASSUMPTIONS The shaded circles seem to form an X made of spheres But if you turn the page upside down the same circles form an X made of cavities since the brain assumes that light comes from above Gaetano Kanizsa If you hide part of this picture depriving the brain of certain clues it uses to form conclusions the large white triangle disappears We construct such images unconsciously and very rapidly Our brains are just as fertile when we use our other senses In moments of anxiety for instance we some8 S EEING H EARING AND S MELLING THE W ORLD Are these triangles real They appear to be because the brain automatically fills in lines that are missing But if you block out parts of the picture the triangles vanish each of these senses is precious and almost irreplaceable as we discover to our sorrow if we lose one People usually fear blindness above all other disabilities Yet deafness can be an even more severe handicap especially in early life when children learn language This is why Helen Keller s achievements were so extraordinary As a result of an acute illness at the age of 19 months she lost both vision and hearing and sank into a totally dark silent universe She was rescued from this terrible isolation by her teacher Anne Sullivan who managed to explain by tapping signs into the little girl s palm that things have names that letters make up words and that these can be used to express wants or ideas Helen Keller later grew into a writer her autobiography The Story of My Life was published while she was still an undergraduate at Radcliffe College and a well known advocate for the handicapped Her remarkable development owed a great deal to her determination her teacher and her family But it also showed that when a sense or The black line in the back seems much longer than the one in the front because your brain assumes it is seeing the effects of perspective Take a ruler to find out for yourself two in Helen Keller s case is missing another sense in her case touch may be trained to make up for the loss at least in part What we perceive through our senses is quite different from the physical characteristics of the stimuli around us We cannot see light in the ultraviolet range though bees can and we cannot detect light in the infrared range though rattlesnakes can Our nervous system reacts only to a selected range of wavelengths vibrations or other properties It is limited by our genes as well as our previous experience and our current state of attention What draws our attention in many cases is change Our senses are finely attuned to change Stationary or unchanging objects become part of the scenery and are mostly unseen Customary sounds become background noise mostly unheard The feel of a sweater against our skin is soon ignored Our touch receptors so alert at first so hungry for novelty after a while say the electrical equivalent of Oh that again and begin to doze so we can get on with life writes Diane Ackerman in A Natural History of the Senses If something in the environment changes we need to take notice because it might mean danger or opportunity Suppose an insect lands on your leg Instantly the touch receptors on the affected leg fire a message that travels through your spinal column and up to your brain There it crosses into the opposite hemisphere the right hemisphere of the brain receives signals from the left side of the body and vice versa to alert brain cells at a particular spot on a sensory map of the body This map extends vertically along a strip of cerebral cortex near the center of the skull The cortex a deeply wrinkled sheet of neurons or nerve cells that covers the two hemispheres of the brain governs all our sensations
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