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UT PSY 301 - Human Brain
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A LIFE-SIZEHUMAN BRAINEach of the five senses activates aseparate area of the cerebral cortex, thesheet of neurons that makes up the outerlayer of the brain’s hemispheres. Thisbrain, shown in actual size, is a computerreconstruction based on data from mag-netic resonance imaging (MRI).Approximate locations of the primarysensory areas are shown in color.Most of the activity takes place withinconvolutions that cannot be seenfrom the surface of the brain.VisionHearingTouchSmellTasteWe can recognize a friend instantly—full-face, in profile, or even by the back of hishead. We can distinguish hundreds ofcolors and possibly as many as 10,000 smells. Wecan feel a feather as it brushes our skin, hear thefaint rustle of a leaf. It all seems so effortless: weopen our eyes or ears and let the world stream in.Yet anything we see, hear, feel, smell, or tasterequires billions of nerve cells to flash urgent mes-sages along linked pathways and feedback loops inour brains, performing intricate calculations thatscientists have only begun to decipher.“You can think of sensory systems as little scien-tists that generate hypotheses about the world,”says Anthony Movshon, an HHMI investigator atNew York University. Where did that sound comefrom? What color is this, really? The brain makesan educated guess, based on the information athand and on some simple assumptions.SEEING, HEARING, ANDSMELLING THEWORLD• 7times “hear things” that are not reallythere. But suppose a leopard approached,half-hidden in the jungle—then our abilityto make patterns out of incomplete sights,sounds, or smells could save our lives.Everything we know about the worldcomes to us through our senses. Tradition-ally, we were thought to have just five ofthem—vision, hearing, touch, smell, andtaste. Scientists now recognize that we haveseveral additional kinds of sensations, suchas pain, pressure, temperature, joint posi-tion, muscle sense, and movement, butthese are generally included under “touch.”(The brain areas involved are called the“somatosensory” areas.)Although we pay little attention to them,each of these senses is precious and almostirreplaceable—as we discover, to our sor-row, if we lose one. People usually fearblindness above all other disabilities. Yetdeafness can be an even more severe handi-cap, especially in early life, when childrenlearn language. This is why Helen Keller’sachievements were so extraordinary. As aWhen you look at the illustration below,for instance, you see an X made of spheressurrounded by cavities. But if you turn thepage upside down, all the cavities becomespheres, and vice versa. In each case, theshapes seem real because “your brainassumes there is a single light source—andthat this light comes from above,” saysVilayanur Ramachandran, a professor ofneuroscience at the University of California,San Diego. As he points out, this is a goodrule of thumb in our sunlit world.To resolve ambiguities and make senseof the world, the brain also creates shapesfrom incomplete data, Ramachandran says.He likes to show an apparent triangle thatwas developed by the Italian psychologistGaetano Kanizsa. If you hide part of thispicture, depriving the brain of certain cluesit uses to form conclusions, the large whitetriangle disappears.We construct such images unconsciouslyand very rapidly. Our brains are just as fer-tile when we use our other senses. Inmoments of anxiety, for instance, we some-8 •SEEING, HEARING, ANDSMELLING THEWORLDILLUSIONS REVEAL SOME OF THE BRAIN’S ASSUMPTIONSThe shaded circles seem to form an X made ofspheres. But if you turn the page upsidedown, the same circles form an X made ofcavities, since the brain assumes that lightcomes from above.Are these triangles real? They appear to be,because the brain automatically fills in linesthat are missing. But if you block out parts ofthe picture, the triangles vanish.…the patientswere awake,since the braindoes not feelwhat is hap-pening to it.SEEING, HEARING, ANDSMELLING THEWORLD• 9result of an acute illness at the age of 19months, she lost both vision and hearingand sank into a totally dark, silent universe.She was rescued from this terrible isolationby her teacher, Anne Sullivan, who man-aged to explain, by tapping signs into thelittle girl’s palm, that things have names,that letters make up words, and that thesecan be used to express wants or ideas.Helen Keller later grew into a writer (herautobiography, The Story of My Life, waspublished while she was still an undergrad-uate at Radcliffe College) and a well-knownadvocate for the handicapped. Her remark-able development owed a great deal to herdetermination, her teacher, and her family.But it also showed that when a sense (ortwo, in Helen Keller’s case) is missing,another sense (in her case, touch) may betrained to make up for the loss, at least inpart. What we perceive through our senses isquite different from the physical character-istics of the stimuli around us. We cannotsee light in the ultraviolet range, thoughbees can, and we cannot detect light in theinfrared range, though rattlesnakes can.Our nervous system reacts only to a select-ed range of wavelengths, vibrations, orother properties. It is limited by our genes,as well as our previous experience and ourcurrent state of attention.What draws our attention, in manycases, is change. Our senses are finelyattuned to change. Stationary or unchang-ing objects become part of the scenery andare mostly unseen. Customary soundsbecome background noise, mostly unheard.The feel of a sweater against our skin issoon ignored. Our touch receptors, “so alertat first, so hungry for novelty, after a whilesay the electrical equivalent of ‘Oh, thatagain,’ and begin to doze, so we can get onwith life,” writes Diane Ackerman in A Nat-ural History of the Senses. If something in the environmentchanges, we need to take notice because itmight mean danger—or opportunity. Sup-pose an insect lands on your leg. Instantlythe touch receptors on the affected leg fire amessage that travels through your spinalcolumn and up to your brain. There it cross-es into the opposite hemisphere (the righthemisphere of the brain receives signalsfrom the left side of the body, and viceversa) to alert brain cells at a particularspot on a sensory map of the body.This map extends vertically along a stripof cerebral cortex near the center of theskull. The cortex—a deeply wrinkled sheetof neurons, or nerve cells, that covers thetwo


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