7.5 Neurolinguistics: Syntactic Category Differences in the Brain

In linguistics, we group words into categories according to how they behave in their morphology and in sentences. This unit shows that different categories of words also lead to different responses in the brain.

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We’ve seen that we can group words into categories according to how they behave. We know that words within a particular syntactic category behave similarly to the other words in that category. They’re similar in their inflectional morphology, and in their syntactic distribution, that is, what positions they can occupy in a sentence. That’s some linguistic evidence that syntactic categories are real. There’s also neurolinguistic evidence that our brains respond differently to words from different categories.

Lorraine Tyler and her colleagues used functional magnetic resonance imaging (fMRI) to measure blood flow to different regions of the brain. The idea behind fMRI is that brain activity consumes oxygen, and when a particular area of the brain is active, then more blood flows there to bring it more oxygen. The researchers asked people to do an easy task. They showed them lists of three words and asked them to decide if the third word, the one in all caps, was related to the first two. So in this example, sparrows, thrushes, and wrens are all kinds of birds, so the participants would respond Yes. In this next example, hammer, wrench, banana, the first two are tools but the third one is a fruit, so it’s not related to the first two, so the participants would answer No.

Some of the words in this task were nouns, like the lists we just saw, and some were verbs, like these ones: eating, grazing, dining. All of those words are related to eating, so the participant would decide Yes. This is a pretty simple task, but what the researchers found in the fMRI was that there were several areas of the brain that showed greater blood flow for verbs than for nouns! Apparently, the brain was reacting differently to words from these two different syntactic categories, even though the task was the same for both categories.

We also see differences between nouns and verbs in the brains of people with aphasia. Aphasia is the name for any kind of language disorder that results from an injury to the brain, such as a stroke or a tumour. There are different kinds of aphasia that have different kinds of symptoms.  Louise Zingeser and Rita Sloan Berndt found a dissociation between nouns and verbs in the speech of two different groups of people with aphasia.

The researchers asked their participants to do a few simple tasks. One was a picture naming tasks, where the researcher would show a line drawing and ask the participant to say what it was, like a fish or a car. In another task, they asked participants to describe how they would go about a particular action, such as making a birthday cake or attending a concert. And in the last task, they gave participants a picture book that depicted a well-known fairy tale but didn’t have any words in it and asked them to tell the story. So from all these tasks, they had a good collection of each person’s speech. For each person, they calculated the ratio of nouns to verbs.

It turns out that in people without any brain injury, the ratio of nouns to verbs is pretty close to one. That means there are about the same number of nouns as there are verbs in the average person’s speech for these tasks. But for people with agrammatic aphasia, verbs are very difficult to produce. These people had more than twice as many nouns as verbs. And for people with anomic aphasia, nouns are quite difficult. This group of people had fewer nouns than verbs.

Aphasia researchers call this kind of pattern a dissociation. We say that nouns are verbs are dissociated from each other because it’s possible to have verb production impaired while noun production is ok, or vice versa. This dissociation is consistent with the idea that verbs and nouns are processed differently in the brain.

So what does all this mean? We’ve seen that, in typical brains, a simple task with verbs involves greater brain activity than the same task with nouns. And in people with brain injuries, some people have an impairment of verbs but not nouns, while others have an impairment of nouns but not verbs.

All this suggests that our brains treat words differently depending on what category they’re in. Or in other words, different syntactic categories exist not just in language, but also in the brain!


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