LANGUAGE AND THE BRAIN

A.Language and The Brain

                Language is brain stuff--not tongue, lip, ear, or hand stuff. The language organ is the mind. More specifically, the language faculty seems to be located in certain areas of the left hemispheric cortex in most healthy adults.  A special branch of linguistics, called neurolinguistics, studies the physical structure of the brain as it relates to language production and comprehension.  

B. Structure of the human brain

. The human brain displays a number of physiological and structural characteristics that must be understood before beginning a discussion of the brain as language organ. 

First, the cerebrum, consisting of a cortex (the outer layer) and a subcortex, is also divided into two hemispheres joined by a membrane called the corpus callosum.  There are a few points which must be made about the functioning of these two cerebral hemispheres.         

    1) In all humans, the right hemisphere controls the left side of the body; the left hemisphere controls the right side of the body. This arrangement--called contralateral neural control is not limited to humans but is also present in all vertibrates--fish, frogs, lizards, birds and mammals. On the other hand, in invertibrates such as worms, the right hemisphere controls the right side, the left hemisphere controls the left side. The contralateral arrangement of neural control thus might be due to an ancient evolutionary change which occurred in the earliest vertibrates over half a billion years ago. The earliest vertibrate must have undergone a 180° turn of the brain stem on the spinal chord so that the pathways from brain to body side became crossed. The probability that such a primordial twist did occur is also born out by the fact that invertibrates have their main nerve pathways on their bellies and their circulatory organs on their backs, while all vertibrates have their heart in front and their spinal chord in back--just as one would expect if the 180° twist of the brain stem vis-a-vis the body did take place.

    2.) Another crucial feature of brain physiology is that each hemisphere has somewhat unique functions (unlike other paired organs such as the lungs, kidneys, breasts or testicles which have identical functions). In other words, hemisphere function is asymmetrical. This is most strikingly the case in humans, where the right hemisphere--in addition to controlling the left side of the body--also controls spatial acuity, while the left hemisphere--in addition to controlling the right side of the body-- controls abstract reasoning and physical tasks which require a step-by-step progression. It is important to note that in adults, the left hemisphere also controls language; even in most left-handed patients, lateralization of language skills in the left hemisphere is completed by the age of puberty.

How do we know that the left hemisphere controls language in most adults. There is a great deal of physical evidence for the left hemisphere as the language center in the majority of healthy adults.

    1) Tests have demonstrated increased neural activity in parts of the left hemisphere when subjects are using language.  (PET scans--Positron Emission Tomography, where patient injects mildly radioactive substance, which is absorbed more quickly by the more active areas of the brain). The same type of tests have demonstrated that artistic endeavor draws normally more heavily on the neurons of the right hemispheric cortex.

    2) In instances when the corpus callosum is severed by deliberate surgery to ease epileptic seizures, the subject cannot verbalize about object visible only in the left field of vision or held in the left hand.) Remember that in some individuals there seems to be language only in the right brain;  in a few individuals, there seems to be a separate language center in each hemisphere.)

    3.) Another clue has to do with the evidence from studies of brain damage. A person with a stroke in the right hemisphere loses control over parts of the left side of the body, sometimes also suffers a dimunition of artistic abilities. But language skills are not impaired even if the left side of the mouth is crippled, the brain can handle language as before. A person with a stroke in the left hemisphere loses control of the right side of the body; also, 70% of adult patients with damage to the left hemisphere will experience at least some language loss which is not due only to the lack of control of the muscles on the right side of the mouth--communication of any sort is disrupted in a variety of ways that are not connected with the voluntary muscles of the vocal apparatus. The cognitive loss of language is called aphasia, and we will discuss various types of aphasia in great detail tomorrow; only 1% of adults with damage to the right hemisphere experience any permanent language loss.

    Aphasics can blow out candles and suck on straws, even sing and whistle, but they cannot produce normal, creative speech in either written, spoken, or gestural form.  Sign language users also store their linguistic ability in the left hemisphere. If this hemisphere is damaged, they cannot sign properly, even though they may continue to be able to use their hands for such things as playing the drums, giving someone a massage, or other non-linguistic hand movements.

    4.) In 1949 it was discovered that if sodium amytal is injected into the left carotid artery, which services blood to the left hemisphere, language skills are temporarily disrupted.  If the entire left hemisphere is put to sleep, a person can think but cannot talk.

    5.) If an electrical charge is sent to certain areas of the left hemisphere (exactly which areas we will discuss tomorrow), the patient has difficulty talking or involuntarily utters a vowel-like cry  (although the production of specific speech sounds has never been induced by electrical charge). An electrical charges to the right hemisphere produces no such effect.

    6.) Musical notes and tones are best perceived through the left ear (which is connected to the spacial-acuity-controlling right hemisphere. In contrast, the right ear better perceives and processes the sounds of language, even linguistic tones (any form with meaning); the right ear takes sound directly to the left hemisphere language center.

    7.) When repeating after someone, most individuals have a harder time tapping with the fingers of the right hand than with the left hand. /Perform this experiment in class./

    8.) The language centers in the left hemisphere of humans actually make the left hemisphere bulge out slightly in comparison to the same areas of the right hemisphere. This is easily seen without the aid of the microscope. For this reason, some neurolinguists have called humans the lopsided ape.  Some paleontologists claim to have found evidence for this left-hemispheric bulging in Homo neanderthalus and Homo erectus skulls.