1--One Brain...or Two?--2

(TheLeft Hemisphere)

How many brains do you have - 1 or 2? Actually, this is quiteeasy to answer...you have only 1 brain. However, the cerebral hemispheresare divided right down themiddle into a right hemisphere and a left hemisphere. Each hemisphereappears to be specialized for some behaviors. The hemispheres communicatewith each other through a thick band of 200-250 million nerve fiberscalled the corpuscallosum. (A smaller band of nervefibers called the anterior commissure also connects parts of the cerebralhemispheres.)

(The RightHemisphere)

Are you right-handed orleft-handed? As youprobably know, most people (about 90% of the population) are right-handed- they prefer to use their right hand to write, eat and throw a ball. Another way to refer to people who use their right hand is to say thatthey are "right hand dominant". It follows thatmost of the other 10% of the population is left-handed or "left handdominant". There are few people who use each hand equally; they are"ambidextrous". (Most people also have a dominant eye and dominantear...testyour "sidedness" here.) Exactly why people areright-handedor left-handed is somewhat of amystery. Dr. WilliamCalvin has developed a fascinating theory about the origin ofhandedness and has written an essay called The Throwing Madonna toexplain it.

Itis well-known thatthe right side of the brain controls muscles on the left side of the bodyand the left side of the brain controls muscles on the right side of thebody. Also, in general, sensory information from the left side of thebody crosses over to the right side of the brain and information from theright side of the body crosses over to the left side of the brain.Therefore, brain damage to one side of the brain will affect the oppositeside of the body.

In 95% of right-handers,the leftside of the brain is dominant forlanguage. Even in 60-70% of left-handers, the left side of brain is usedfor language. Back in the 1860's and 1870's, two neurologists (PaulBrocaand KarlWernicke) observed that when people had damage to a particulararea on the left side of the brain that they hadspeech and language problems. They noticed that people with damage tothese specific areas on the right side usually did not have any languageproblems. The two language areas of the brain that are important forlanguage now bear their names: Broca's area and Wernicke'sarea.

Left Hemisphere Language Math Logic

CerebralDominance Eachhemisphere of the brain is dominant for other behaviors. Forexample, it appears that the right brain is dominant for spatialabilities, face recognition, visual imagery and music. The left brain maybe more dominant for calculations, math and logical abilities. Of course,these are generalizations and in normal people, the two hemispheres worktogether, are connected, and share information through the corpuscallosum. Much of what we know about the right and left hemispheres comesfrom studies in people who have had the corpus callosum split - thissurgical operation isolates most of the right hemisphere from the lefthemisphere. This type of surgery is performed in patients suffering fromepilepsy.Thecorpus callosum is cut to prevent the spread of the "epileptic seizure" from one hemisphere to the other.

Right Hemisphere Spatial abilities Face recognition Visual imagery Music

Split-Brain Experiments Roger Sperry (who won the Nobel prize in 1981) and MichaelGazzaniga are two neuroscientists who studied patients who had surgeryto cut the corpus callosum. These studies are called "Split-Brain Experiments". After surgery, these peopleappeared quite "normal" - they could walk, read, talk, play sports and doall the everyday things they did before surgery. Only after carefulexperiments that isolated information from reaching one hemisphere, couldthe real effects of the surgery be determined. Dr. Sperry used a tachistoscope to present visualinformation to one hemisphere or the other. The tachistoscope requires people to focus on a point in the center of their visualfield. Because each half of the visual field projects to theopposite site of the brain (crossing in the optic chiasm), it is possible to project a picture to either the right hemisphere OR the lefthemisphere.

So, say a "typical" (language in the LEFThemisphere) split-brain patientis sitting down, looking straight ahead and is focusing on a dot in themiddle of a screen. Then a picture of a spoon is flashed to the right ofthe dot. The visual information about the spoon crosses in the opticchiasm and ends up in the LEFT HEMISPHERE. When the person is asked whatthe picture was, the person has no problem identifying the spoon and says"Spoon". However, if the spoon had been flashed to the left of the dot(see the picture), then the visual information would havetraveled to the RIGHT HEMISPHERE. Now if the person is asked what thepicture was, the person will say that nothing was seen!! But, when thissame person is asked to pick out an object using only the LEFT hand, thisperson will correctly pick out the spoon. This is because touchinformation from the left hand crosses over to the right hemisphere - theside that "saw" the spoon. However, if the person is again asked what theobject is, even when it is there in the person's hand, the person will NOTbe able to say what it is because the right hemisphere cannot "talk". So,the right hemisphere is not stupid, it just has little ability forlanguage - it is "non-verbal".

Anothertype of experiment performed with split brain patients uses chimeric figures, like this one to the right. In this figure,the face on the left is a woman and the face on the right is a man.Therefore,if the patient focuses on the dot in the middle of the forehead, thevisual information about the woman's face will go to the right cerebralhemisphere and information about the man's face will go to the lefthemisphere. When a split brain patient is asked to point to a whole,normal picture of the face that was just seen, the patient will usuallypick out the woman's picture (remember, the information about the woman'sface went to the RIGHT cerebral hemisphere). However, if the patient isrequired to say whether the picture was a man or a woman, the patient willSAY that the picture was of a man. Therefore, depending on what thepatient is required to do, either the right or left hemisphere willdominate. In this case, when speech is not required, the righthemisphere will dominate for recognition of faces.

Before different types of brain surgery, it is importantto identify whichcerebral hemisphere is dominant for language so that the neurosurgeon canavoid damaging speech areas. One way to test whichhemisphere is dominant for language is with a procedure called the WadaTest.During this test, a fast actinganesthetic called sodium amytal (amobarbital) is injected into the rightor left carotid artery. The right artery supplies the right cerebralhemisphere and the left artery supplies the left cerebral hemisphere.Therefore, either the right or left hemisphere can be "put to sleep" temporarily. If the left hemisphere is put to sleep in people who havelanguage ability in the left hemisphere, then when asked to speak, theycannot. However, if the right hemisphere is put to sleep, then thesepeople will be able to speak and answer questions. (Remember too thatsince the right hemisphere controls muscles on the left side, people willnot be able to move the left side of theirbodies.)

Another way to test for languagerepresentation in the brain is to electrically stimulate the cerebralcortex. A neurosurgeon can place an electrode on various areas of theexposed brain of an awake patient during surgery. The patient can saywhat he or she feels and thinks. Placement of the electrode on the braindoes NOT hurt because the brain itself does not have any receptors forpain. In people who have left side dominance for language, electricalstimulation of various locations on the left cerebral cortex willinterfere with speech.

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