Language impairment is often caused by tragic conditions like aphasia, a type of brain damage. This condition can teach us a lot about how language works. To begin to understand such a complex, fascinating, and ever-changing field, we will first talk about the human brain and the field devoted to its study.

What Is neurolinguistics?

Neurolinguistics is a branch of neuroscience whose goal is to understand the neural aspects of language, such as how the brain processes language. To do research in neurolinguistics, neuroscientists depend largely on impaired language data, not normal language data. In other words, analyzing the patterns in the abnormal speech of someone who has suffered from an event like a stroke, or from someone who has a medical condition like dementia, provides information for scientists that normal speech cannot provide.

The basic components of our brains

The brain consists of more than 100 billion nerve cells—called “neurons”—which are connected with fibers. The brain’s surface is called the cortex, or “gray matter,” and is responsible for making decisions, storing memories, initiating action, and, of course, for our knowledge of language and grammar. (2) One of the coolest things about the brain is how it’s wrinkled and folded up, because if it weren’t, our heads would be enormous! The cortex is thin, but quite large in surface area, and we need for all of that surface area somehow to fit into the skull—it’s like taking a sheet of newspaper and crumpling it into a ball to fit inside a small bowl. (4) In proportion to our bodies, human brains are larger and more intricate than any other living creature. (6)

You may have learned that the brain has two halves, which are known as its hemispheres (“hemi” is a prefix that means “half”). They are connected by the “corpus callosum”—a network of 200 million fibers. Incredibly, some patients with severe epilepsy need to have this connection surgically removed, and they are still able to function normally! You may also have heard that brain function is “contralateral,” which means that the left side of the brain controls the right side of the body, and vice versa.

Lots of language processing happens in the left side of the brain

The answer to this question is complicated. You may have heard that the left hemisphere is “responsible for language,” and that is mostly true: It’s called “lateralization” when a brain function is localized to one hemisphere (“lateral” means “side”).

Scientists have figured this out using technology such as CT scans and PET scans, which show the areas of the brain that light up when subjects are given various types of language stimuli, and the areas that light up when they get non-linguistic stimuli, too.

Furthermore, those epileptic patients with the separated-hemisphere surgery provide a dramatic illustration of this language-on-the-left idea, because although those patients function just fine after the operation, controlled experiments show that separating the hemispheres results in some odd language changes.

For example, one experiment blindfolded the patients, and placed simple objects like keys and pens into one hand at a time. These subjects were able to easily name objects in their right hand, because the left hemisphere—where language is located—operates the right hand. However, in the left hand, they recognized the objects but were unable to name them, because the right hemisphere operates the left hand, but does not process language, and more importantly, they are blindfolded, and their right hemispheres can no longer communicate with their left hemispheres! (4)

Language isn’t limited to the left side of the brain though

So, it seems pretty clear that language lives on the left side of our brains, right? Well, it’s not that simple. Research, including that same brain-scan research, shows that small amounts of language processing are found in the right side of the brain, too, and that the amount varies from person to person. (4) Remarkably, left-handed people are less lateralized for language, in that they have significant language representation in both of their hemispheres. Left-handed people also recover from language loss after strokes better than right-handed people, because they have language spread out more evenly. In fact, this quicker recovery is also observed in right-handed people who have left-handed people in their family! (3)

Let’s back up: Despite the fact that the hemispheres of the brain do a lot of work in concert with each other, there absolutely is a general tendency for language to lateralize left, and the details of this tendency are very interesting. The history of neurolinguistics explains a lot about these basic facts, starting with Paul Broca, a French surgeon from the 1800s, who noticed language problems in patients who had experienced trauma to the front left portion of the head, near the temple. This area of the brain is now known as “Broca’s area.”

Around ten years later, a German neurologist named Carl Wernicke noticed that his language-impaired patients had lesions in the left temporal area, which is the same left hemisphere but farther back. (Back then, the discovery could only occur during autopsies, of course.) This part of the left hemisphere, like a neighbor to Broca’s area, is now known as “Wernicke’s area.” (2)

What is aphasia?

Aphasia is any disease- or trauma-induced brain damage that causes a language disorder. Though most commonly caused by stroke, it can also be caused by infection, tumors, hemorrhaging, and blows to the head. The worst type is called “global aphasia,” which causes the patient to be mute. (4)

Less severe types of aphasia, when some language is retained, are commonly split into two types called “fluent aphasia” and “non-fluent aphasia.”

Fluent aphasics speak fluidly, as the name implies, but they may not make sense when they speak.

Non-fluent aphasics produce labored speech, and have difficulty structuring their sentences, but their messages are more understandable. (We’ll see some examples of these utterance types shortly.) In addition, the labored speech in the non-fluent type is usually “agrammatic,” which means it lacks functional elements of language such as prepositions, articles, and pronouns. Function words are also called closed-class words, because the closed classes don’t gain new members very quickly. So, people with non-fluent aphasia often form sentences without prepositions, articles, and pronouns, and struggle to get the utterances out. However, they have an easier time with content words like nouns, verbs and adjectives. Content words are also called open-class words, because they gain new members all the time! For a review of this fun language distinction, read  opens in a new windowPart II of the series “Why Do People Say ‘A-Whole-Nother’?

When people develop language disorders, it is devastating to them and to their loved ones. However, from a scientific perspective, the process of attempting to understand the disorders, and develop treatments, has provided us with vast amounts of information about how language works in a general sense. For example, we know that this linguistic distinction between function words and content words is not just something that linguists made up or imagined. Why? Because much of the non-fluent aphasia—the one that causes agrammatic speech—is classified as “Broca’s aphasia” because it is caused by damage to Broca’s area.

Broca’s aphasia

Here is an example of an utterance produced by a Broca’s sufferer, documented by linguist Victoria Fromkin. It was in response to the patient being asked if he had been going home on weekends:

Why, yes…. Thursday uh… uh… uh… no… Friday… Bar…ba…ra…wife… and oh car… drive… you know… rest… and TV.

Now here is an example of how Broca’s aphasia affects the sufferers’ ability to understand those grammatical components of language. Sentence (2) can be harder for Broca’s aphasics to understand than sentence (3):

The cat was chased by the dog.

The car was chased by the dog.

Sentence (2) requires knowledge of syntax in order to be interpreted, because the sentence is in the passive voice, whose word order is not the more common English order. (By the way, the “more common” English order, which is “subject + verb + object,” like “The cheetah ate the cake,” is called “canonical.”) So, imagine a sufferer who is lacking the ability to process those functional language pieces, such as the “-ed” past tense verb ending (that’s called a bound morpheme), and the preposition “by.” Even the verb “was” is a function word, because it is an auxiliary verb, not a main verb.

So, when aphasics can only process content words, like “cat,” “chase,” and “dog,” they are less likely to correctly process the doer and the receiver of the chasing action in sentence (2).  That’s because if you strip the sentence down to content words, it might be “cat chase dog,” so the person with Broca’s aphasia may be misled, and say that the cat must be doing the chasing, because canonical word order in English is the most common and straightforward.

However, sentence (3) has stronger semantic information (again, that means “meaning”): Cars are not animate, and cannot really chase things, so these aphasics may more aptly ascertain/guess that the car is the recipient of the chasing action, even with the tricky word order.

Whether something is animate or not is one of the many features that are encoded within every word that we learn in the languages we know. In other words, in sentence (2), semantics does not provide enough information, because both cats and dogs can be chasers, and this would be difficult for a speaker who is dependent on semantic knowledge, due to having suffered grammar loss.

A final example of this function/content distinction, which is both fascinating and sad, is the nature of the dyslexia that aphasics often acquire along with the spoken-language loss. In reading tests, because of their difficulty with function words, Broca’s patients are unable to read function words out loud from test cards, yet they can read homophones—those are words that sound the same but mean something else—if the homophone is a content word! For example, a patient may read the word “witch” without a problem, because “witch” is a content word. It is a noun that you can define, and picture in your mind. However, this same patient may struggle immensely to read the homophone “which” because “which” is a relative pronoun: a grammatical, closed-class word. (2)

Wernicke’s aphasia

Now, let’s talk about damage to Wernicke’s area of the brain. The resulting Wernicke’s aphasia is a type of the fluent aphasia, which means that the speech is not labored, and the sufferers are still able to use function words like articles and prepositions. Their words most often occur in a grammatically correct order (remember, this type is not “agrammatic,” like Broca’s aphasia). Instead, they have trouble with picking the right content words. So, the Wernicke’s sufferers may use one content word when they need another, but they speak at a normal pace. They tend to produce meaningless utterances and they also, unfortunately, tend not to be able to understand what is spoken to them, so meaning is impaired for them in a very general sense. (4)  opens in a new windowNoam Chomsky, the “grandfather” of linguistics, created the following sentence to show that in human language, meaning is separate from syntax/word order:

“Colorless green ideas sleep furiously.”

This sentence sounds a bit like something that someone with Wernicke’s aphasia could say. It is grammatically correct, in that the word order follows syntactic rules, and, for example, the subject-verb agreement is correct for most English dialects, but it makes no logical sense at all.