THE CONNECTIONS BETWEEN THOUGHT AND LANGUAGE

When someone is speaking to you, they do not separate each word from the next with a pause, like the spaces between written words on a page, yet your brain can still recognize each word individually. This ability is truly remarkable. (To realize just how hard it really is to isolate the components of spoken language, just try listening to someone speaking a language that you don’t know at all.)

In linguistics, the smallest unit of meaning, corresponding roughly to a word, is called a morpheme. To recognize words or morphemes from their sounds, the brain breaks them down into phonemes: the smallest units of sound that are used to construct the words of a given language.

There are two different disciplines that study the role of sounds in language. These disciplines are historically related to each other, but one is more relevant than the other for understanding the language functions of the brain.

The first of these disciplines is phonetics, which describes and classifies the sounds of all languages according to the way that these sounds are physically produced by our organs of speech. In phonetics, the sounds of words are represented by symbols placed between square brackets: [ ] (see the first sidebar on this page for an example). Examples of studies in phonetics would include a comparison of the use of different sounds in different languages, or a description of how the sounds in a given language have evolved.

The second discipline is phonology, which historically grew out of phonetics. Phonology is not interested in describing the sounds of language so precisely as phonetics, but is interested in examining the internal structure specific to a given language. In phonology, what is important is not the sound itself, but rather the way that it compares and contrasts with other sounds in the same phonological table typifying a given language. Phonological descriptions of words are represented by symbols between slashes: / / (see the first sidebar on this page for an example). Thus it is phonological analysis that lets us study the cerebral substrate of linguistic encoding and decoding with words, sentences, and meanings in a given language.

The ultimate function of language is to convey meaning. Once a word has been recognized from its phonemes, its meaning will depend on several factors: what it designates in the world, the context in which it is being spoken, and, most important, the way that it fits together with its neighbours in the sentence—that is, its syntax.

The order of the words in a sentence can be extremely important. The English sentences “The man eats the alligator.” and “The alligator eats the man.” contain the same words but mean two very different things. Only the order of the two nouns, and hence their relationship to the verb, has changed.

In every language, there are certain words that mean nothing in themselves but perform a syntactic function in the sequence of words that constitutes a sentence. The usefulness of these “relational” words, such as and, the, a, and with, becomes especially clear when they are left out, sometimes causing inadvertently comic ambiguities, in contexts such as classified advertisements or newspaper headlines where space is at a premium (“Bicyclist struck by car in fair condition.” “Nuns forgive break-in, assault suspect.”)

The American linguist Noam Chomsky demonstrated the importance of syntax in natural languages with his famous sentence “Colorless green ideas sleep furiously.” Obviously, this sentence has no real meaning, but its syntax is so correct that we try to find one anyway. Such observations led Chomsky to formulate his theory of universal grammar (follow the Tool module link to the left). According to this theory, syntax is independent of meaning, of context, of the information stored in the speaker’s memory, and of what the speaker wants to communicate. Chomsky’s approach is disputed, however, by linguists such as George Lakoff, who instead regard conceptual metaphors based on our bodily experiences as the central feature of language.

Be that as it may, the words that we know form a mental lexicon where each word can evoke several different meanings depending on the context in which is is spoken. When we speak, each word is thus related to several other words with which it shares connected meanings. This is what enables the brain to construct categories.

Many experiments have shown that language enables this transformation of information into abstract representations. For example, if a group of people listen to several sentences that form a paragraph, most of these people will then be able to state the general idea of the paragraph in their own words, but not to repeat the exact sentences they heard. It is as if two transformations have taken place. In the first, the people took the sentences that they heard and represented them mentally in more abstract, consolidated terms, which seem easier to memorize. In the second, the people retrieved this more abstract representation and converted it into their own words.

Scientists are still debating whether the meaning of a word and the characteristics of the real-world object that it designates are stored in the same location in the brain. Some researchers believe that there is a single storage site for every individual concept, idea, or object. For instance, all of the characteristics of lions would be stored together in one area of the brain, and the audible and written forms of the word “lion” would be stored in other areas that were connected to the one where this animal’s characteristics were stored.

But other scientists believe that information is processed in the brain in a much more distributed fashion: the lion’s smell, roar, and physical appearance would thus be stored in many areas of the brain that were closely interconnected. Thus, when you heard or read the word “lion”, all of these areas would be activated simultaneously.

In addition to its fundamental role in communication, language also gives us a powerful internal mechanism for retrieving, critiquing, and changing our thoughts. This internal mode of communication enables us to perform more complex mental operations in both the logical and the affective spheres. And the ability to predict consequences from these conceptual operations provides a definite adaptive advantage to a social species such as our own.

LEARNING TO SPEAK A FIRST AND SECOND LANGUAGE

Research by authors such as the American linguist Noam Chomsky has shown that for human language to be as sophisticated as it is, the brain must contain some mechanisms that are partly preprogrammed for this purpose (follow the Tool module link to the left). Babies are born with a language-acquisition faculty that lets them master thousands of words and complex rules of grammar in just a few years. This is not the case for our closest primate cousins, who have never succeeded in learning more than a few hundred symbols and a few simple sentences (follow the Experiment module link to the left).

Until babies are about one year old, they cannot utter anything but babble. This limitation is due to the immaturity of their temporal lobe, which includes Wernicke’s area. This area, by associating words with their meanings, is directly involved in the memorization of the signs used in language. The acquisition of vocabulary during the first years of life seems to closely track the maturation of Wernicke’s area, which eventually enables adults to maintain a vocabulary of some 50 000 to 250 000 words.

Our ability to retain such an impressive number of words involves two different types of memory, depending on whether the language in question is our mother tongue or a second language that we learned later in life (follow the Tool module link to the left).

To learn our mother tongue, we rely on procedural memory (also known as implicit memory), the same kind that is involved when we learn skills that become automatic, like riding a bike or tying our shoelaces. Because we are so immersed in our mother tongue, we end up using it just as automatically, without even being aware of the rules that govern it.

Learning and using a language may thus involve applying either implicit linguistic skills or explicit metalinguistic knowledge. Because each of these skill sets is supported by different structures in the brain (see sidebar), language disorders can affect people’s first languages and second languages selectively. Following brain injuries, bilingual people may selectively lose the use of one of their two languages. But the language that they retain is not necessarily their mother tongue, nor is it necessarily the language that they spoke most fluently before their accident.

Some types of brain damage can make people amnesic without affecting their ability to speak their mother tongue (which depends on procedural memory). But other types can cause serious problems in someone’s automatic use of speech without affecting their ability to remember a language that they learned consciously (using declarative memory). Other observations have been made that reflect this same distinction. For example, some people who have aphasia seem to recover their second language more successfully than their first, whereas some people with amnesia lose access to their second language completely. Alzheimer’s patients retain those language functions based on procedural memory but lose those, such as vocabulary, that are based on declarative memory.

But even in one’s mother tongue, not all aspects of language rely on procedural memory. It is believed, for example, that the lexicon for a person’s first language, which consists of the association of groups of phonemes with meanings, may have close connections with declarative memory. Vocabulary thus seems to constitute a special aspect of language: the great apes are capable of learning a large number of symbols related to words (follow the Experiment module link to the left); “wild children” who are deprived of language at the start of their lives can also learn many words, but comparatively little syntax; and people who have anterograde amnesia, though they can acquire new motor or cognitive skills, cannot learn new words.

While the lexicon for a person’s first language depends on declarative memory, which involves the parietal and temporal lobes, the grammar of this language depends on procedural memory, which involves the frontal lobes and the basal ganglia. Procedural memory is used for unconscious learning of motor and cognitive skills that involve chronological sequences of operations. This description clearly applies to grammatical operations, which consist in sequencing the lexical elements of a language in real time.

Simultaneous interpretation may well be the most complex verbal task imaginable. To take a speech that is being delivered in one language and simultaneously translate it into another language orally, the interpreter must understand the words that the speaker is saying, hold them in working memory while encoding them into the other language, then speak them in this other language. At the same time, the interpreter must continue listening to the speaker so as to be able to keep repeating this process as the speech goes on.

LANGUAGE DISORDERS

Many language disorders still remain mysterious or surprising. Such is the case with dyslexia, which is one of the developmental language deficits (dysphasias), as well as with certain types of aphasia resulting from highly localized brain lesions.

Dyslexia consists of various degrees of difficulty in learning to read and write. This is a developmental disorder that is discovered when children are learning to read (around age 6 or 7) and that is more common among boys and among children who are left-handed. (Some reading problems can be acquired in adult life, as the result of brain injuries, in which case they are referred to as alexia.) People with dyslexia may confuse certain sounds (for instance, p with b, or f with v) or letters that are visually similar (such as m and n). To dyslexics, some letters may seem reversed (perhaps a d that looks like a b) or even words may appear so (“bag” looks like “gab”). In cases of what is known as profound dyslexia, when people read out loud, they will actually substitute one word for another with a related meaning (for example, read “cow” in place of “horse”). Dyslexics represent 5 to 10% of the total population, and their other cognitive abilities are completely normal. The severity of this disorder varies widely; some dyslexics have only slight trouble in reading, while others are completely illiterate. Dyslexics may be able to express themselves orally in a completely normal way, but the problem begins when they are confronted with written words. However, the nature of dyslexia is probably more complex than a simple difficulty in reading. Some authorities even regard it as more of a problem in sensory processing, while others consider it a memory disorder. The purpose of most research on dyslexia is to establish the entire chain of causal links between certain genes, certain parts of the brain, certain cognitive functions, and the ability to read and write. Thus researchers have begun to identify various signs of pathology in the brains of dyslexics. For example, some obvious abnormalities have been reported in the arrangement of cortical cells, especially in certain areas of the left frontal and temporal cortexes. The scientists who uncovered these unusual cellular configurations in language-related areas of the brain believe that they probably start developing in the middle period of fetal gestation, when active cell migration is observed in the cerebral cortex. In most people, the left temporal planum is considerably larger than the right. But some authors say that in dyslexics, these two structures are similar in size. The presence or absence of temporal planum asymmetry in dyslexics remains a controversial topic, however. When differences in age, sex, and overall brain size are taken into account, the anatomical differences between the temporal planum in dyslexics and in control groups are far smaller. Other studies suggest that some changes in dyslexics’ sensory pathways may be responsible for their problems with reading. Autopsies have shown that in dyslexics, the neurons in the magnocellular layers of the lateral geniculate nucleus were smaller than in control groups and were arranged in a disorganized fashion. These abnormalities might interfere with the rapid processing required for changing visual signals such as those involved in reading. Brain-imaging studies of dyslexics have shown reduced activity in visual area V5/MT, which is responsible for detecting movement, or in the lower part of the left temporal lobe.

Depending on the extent of the brain damage that causes them, the various forms of aphasia range from subtle speech impairments to a complete inability to speak. Global aphasia is equivalent to having both expressive and receptive aphasia. In global aphasia, extensive damage to the frontal, temporal, and parietal cortexes, including in particular Broca’s area, Wernicke’s area, and the supramarginal gyrus, leads to the total loss of the ability to understand, speak, read, or write language. People with global aphasia can manage to pronounce just a very few words, unconnected by any syntax. At best, global aphasics have an automatic form of expressive language, composed especially of emotional exclamations. They may also still have control over facial expressions, hand gestures, and vocal intonations. Their prognosis for recovering use of language is nevertheless extremely poor. In conduction aphasia, language comprehension and spontaneous oral expression are normal, but individuals have a great deal of difficulty when asked to repeat words or phrases.When they try to do so, they mix up the sounds in words and make numerous transformations and omissions of words. The location of the brain lesions that cause conduction aphasia is still controversial. Wernicke, and later Geschwind, believed that conduction aphasia was caused by the destruction of the arcuate fasciculus, the fibre bundle, in the suprasylvian parietal cortex, that connects Wernicke’s area to Broca’s area. But other authors have proposed that the symptoms of conduction aphasia might be produced by dysfunctions in areas such as the auditory cortex, the insula, and the supramarginal gyrus. In anomic aphasia (also known as nominal aphasia), oral expression and syntactic structure remain intact, and the main difficulty is in finding certain words. People with anomic aphasia compensate for their trouble in finding the right words by using other, vaguer words such as “thing” or “whatsit”, or they may use circumlocutions such as “the instrument that you wear on the wrist and that tells you the time”. If you show someone with anomic aphasia a photo of John F. Kennedy, for example, she may say that he was president of the United States, and that he was assassinated, but will not find his name until you help her by hinting “John F… ”. It is still possible to communicate with anomic aphasics, however, if you know the context or subject of the conversation. Anomic aphasia is often caused by parietal lobe damage that is limited to the angular gyrus or the area just above it. This disorder has also been associated with damage to the pulvinar in the thalamus. Because the language processing system in the human brain forms a densely interconnected network, damage just about anywhere in the left hemisphere can cause some form of anomic aphasia. Depending on the location of the lesion, an individual may, for example, be unable to name an item when it is shown to him (because of a disconnection between the visual cortex and the inferior parietal cortex) but still be able to name it if he is allowed to touch it or if it is described to him out loud (if the auditory and tactile pathways to the parietal cortex remain intact). Many other, less common forms of aphasia have also been described. In alexia, caused by damage to the inferior part of the left occipital and temporal lobes, the individual cannot read but can still write. In agraphia, the individual can reason normally, but cannot write. In anarthria, a malfunction in the system that controls the motor aspects of speech prevents individuals from articulating the words that would convey their thoughts. Progressive aphasia develops insidiously, resulting in a gradually worsening loss of speech. Subcortical aphasia is caused by small lesions in the subcortical areas of the left hemisphere and presents a variety of the symptoms seen in other kinds of aphasia. Transcortical motor aphasia (TMA) is characterized by abnormalities in spontaneous expression but distinguished from Broca’s aphasia in that people with TMA can repeat long sentences, whereas Broca’s aphasics can repeat only simple words. The fact that each of these different types of aphasia typically includes several subtypes clearly shows just how complex language pathologies can be.

In deaf people who use sign language, left hemisphere damage can cause language deficits comparable to those observed in verbal aphasics. For example, in some cases very similar to Broca’s aphasia, it becomes very difficult for people to sign, even though neither their comprehension nor their non-sign-language gestures are impaired. Likewise, there is a manifestation of Wernicke’s aphasia that occurs in deaf people. In these cases, people can still sign fluently, but make frequent mistakes, and they have difficulty in understanding other people’s signs. There was also one very rare case involving a man who could speak but also knew sign language because his parents were deaf. After suffering a left-hemisphere stroke, he displayed global aphasia, from which he recovered gradually. Interestingly, he recovered his ability to express himself in both spoken language and sign language at the same time. Other studies have shown that the two areas of the left hemisphere that are involved in these two types of language overlap, though not completely.