Fall 1995 Volume 4, Number 4

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BY DAVID CAPLAN, M.D., PH.D.

anguage is one of the pillars of the human intellect. It is the principal means whereby individuals formulate thoughts and convey them to others. It plays a role in analyzing the world, in reasoning, solving problems, and planning actions. It allows us to convey memories of the past and beliefs about the future, to engage others about events that have not taken place, and to express the relations between events.

Language is an indispensable part of human culture, without which jurisprudence, commerce, science and other human endeavors could not exist in the forms we know them. It is an object of beauty in its own right. A combination of semantic and artistic force can make writings such as Second Isaiah, the Gettysburg Address, or Shakespeare's sonnets, the definitive statements of spirituality, jurisprudence, or personal love for a culture or an individual.

Language is vital to individual success, and diseases affecting language can cripple a person in his or her family or social group. Ongoing research is making progress in understanding language, its neural basis, and how to successfully intervene in the course of language disorders.

The language code
Modern linguistics has taught us that, in its essence, language is a special kind of code. A "standard" code consists of a set of symbols that can be connected to the words and phrases in a language. When we crack a code, we understand an encoded message because we understand the language that we have translated the code into. Natural language is
a different sort of code, because its forms are related to meaning directly.

The forms of language are simple words, sentences, intonation, and other "representations." Words refer to objects, actions, properties and logical connections. Sentences relate words to each other to depict events and states of affairs in the conversation or whether a sentence is a statement or a question. Language is a complex code because all these types of representations interact to determine the meaning of each sentence in each context.

Processing language
Language processors activate these linguistic representations in speaking, understanding, reading and writing, in a remarkably fast and accurate way. For instance, when we speak, we select words in accordance with what we think our listener will understand. We activate the sounds for each word. We construct a syntactic structure to relate the words to each other, and an intonational contour to convey the syntax.

All this information is translated into movements of the mouth, jaw, tongue, palate, larynx and other articulators that are regulated on a millisecond-by-millisecond basis, so that we produce about three words per second or one sound every tenth of a second on average. Yet we only make about one sound error per million sounds and one word error per million words.

Watching the brain speak and listen
Scientists have tried for over a century to understand how the brain learns, stores, and processes language. The task is difficult because there are no animals who have symbol systems as rich as language. Therefore, for a long time, information about how the brain processed language could only come from the study of the effects on language of neurological disease in humans. In the past decade, exciting new techniques have allowed us to picture the normal brain at work processing language. What used to take decades to learn, as scientists waited for the opportunity to examine the brains of patients at post-mortem, can now be approached in months using positron emission tomography, special analyses of electroencephalograms, functional magnetic resonance imaging, magnetoencephalography, and other tools.

Left-brain/Right-brain
As is true for every other functional ability, parts of the brain specialize in language. The brain has two roughly identical halves -- the left and the right hemispheres. We now know that there are small differences in the sizes of some regions in the two hemispheres. These differences may form the basis for the first major brain specialization for language -- lateralization of language to the left hemisphere.

In about 98 percent of right-handers, the left hemisphere accomplishes most language processing functions. In non-right handers (which include left-handed and ambidextrous people), language functions are far more likely to involve the right hemisphere. There is some evidence that lateralization differs in males and females.

There is also evidence that the non-dominant hemisphere is primarily involved in functions that are just one step beyond the essential language functions of relating form to literal meaning. These include determining the emotional state of a speaker from his or her tone of voice, and appreciating humor and metaphor.

Can we be more specific about exactly where in this language region particular language operations are carried out? Where do we activate the sounds of specific words, or compute the meaning of a sentence? The jury is out on this question. Since the earliest investigations into the topic, some scientists have thought that the language region works more or less as a unit, while others have sworn by the idea that individual language operators are localized in specific parts of this region.

Our own data may indicate why this controversy has endured so long. We studied different types of language impairments. One was patients' abilities to construct syntactic structures -- the ability to structure the sentence The dog that chased the cat ate the cheese so that, despite the sequence of words the cat ate the cheese, the dog and not the cat is understood as the animal doing the eating. We found that damage to any part of the language zone can affect the ability to assign syntactic structure. There was even evidence for a mild impairment in syntactic processing after strokes in the right hemisphere, though the effect was much less than after left hemisphere damage

This suggests that there is some truth to the idea that the entire language zone is involved in syntactic processing. However, when we studied the areas of the brain that increased their blood flow while normal subjects read syntactically complex sentences, only a small part of this area increased its metabolic activity. This suggests that there is some specialization within the language area that is involved in syntactic processing.

The differences in the studies may be because larger brain regions are involved in more abstract operations -- like computing syntax -- while smaller regions that are nearer to sensory cortex are involved in operations that are closer to elementary sensory processing. Thus, putting together the pieces of the puzzle of the how the brain is organized to support language may be a very complex task. The application of new imaging techniques will move research in this area along at the most rapid pace ever known.

Starting the engine and driving the system
The language system is connected to other intellectual and motor systems. People use language to inform others, to ask for information, to get things done, etc. The mechanisms that trigger language use require motivation and arousal.

Functional neuroimaging studies have provided strong evidence that areas such as the frontal lobes and structures deep in the brain, such as the cingulate gyrus, become active during many language tasks.

Perhaps these structures are related to the level of arousal needed to activate language processors.

Once language use is initiated, it must be regulated in time and monitored. The relevant timing mechanisms may lie in the cerebellum and in subcortical gray matter, which recently have been found to be active during many language functions and to result in language impairments when lesioned.

Diseases affecting language
Although the deprivation of any function is onerous, diseases that affect cognition are devastating to humans in a particular way. Not being able to communicate thoughts efficiently can cut a person off from his or her livelihood and family and have immense effects on emotional state and social position. Language can be impaired by sudden events such as stroke or head injuries, insidiously progressive conditions such as Alzheimer's or Parkinson's Disease, or developmental disorders as happens in dyslexia.

We now are able to make highly specific diagnoses of what language processors are affected in a particular language disorder, and recent work has begun to demonstrate that targeting these specific impairments can improve language functioning. As we know more about the brain mechanisms involved, medical therapies such as those that improve attention will also become more tailored to remediation of particular language disorders. The future holds much promise for applying our rapidly-accruing knowledge regarding the neural basis of language to improving the quality of life of language-impaired individuals. *

Dr. Caplan is Associate Neurologist at Massachusetts General Hospital.