Language and the brain

The human brain is roughly organized like a peach, in that there is a large outer layer (the cerebrum) surrounding an inner kernel (the brainstem), which keeps people alive. The outer layer is extensively folded, and is the source of all intentional thought and movement. It is grey in colour, so giving rise to the phrase: 'Use your grey matter' for Think!', and is divided into two halves, the cerebral hemispheres. The left hemisphere controls the right side of the body, and the right hemisphere the left: so if you meet someone paralysed down the right side of their body after a stroke, you know that the stroke affected the left side of their brain.

The hemispheres look roughly similar, but this is an illusion. One of them, usually the left, is the more powerful dominant

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hemisphere. This is not only because it controls the right side of the body - and the majority of humans are right-handed - but also because it normally controls language. Approximately 90 percent of the human race are born with their brain 'wired' for language in the left hemisphere. Humans who do not have language in their left hemisphere, are often, though not inevitably, left-handers. This much is fairly uncontroversial.

But disputes begin when attempts are made to locate language precisely within the left hemisphere. Once again, the outline is clearer than the details. Those who have problems with speech production, such as agrammatic aphasics, mostly have injuries towards the front of the brain, while those who have problems with comprehension, such as fluent aphasics, have injuries towards the back.

These disputes began in the nineteenth century, when Paul Broca, a French surgeon, pinpointed an area in front of, and slightly above, the left ear. According to him, postmortems showed that this area had been destroyed in the brain of two patients who could produce hardly any speech. Even today, dam­age to the general region known as Broca's area is statistically likely to cause severe speech problems - so much so that agram­matic speech is still sometimes known as Broca's aphasia. (Figure 51).

Some years after Broca's claims, Karl Wernicke, a German neu­rologist, noted that several patients who talked fluent nonsense had severe damage towards the back of the brain, in an area under and surrounding the left ear. This became known as Wernicke's area, and fluent aphasics are sometimes said to be suffering from Wernicke's aphasia (Figure 51).

But brain areas cannot be as accurately located as the heart or liver. So over the years, patients have been found who can speak perfectly well, even though Broca's region has been damaged, as well as people who cannot speak when Broca's area is apparently intact. Some neurolinguists argue that speech is therefore located slightly differently in each person, others that connections within the brain matter more than specific areas. And the matter is turn­ing out to be more complex than was once thought. Recent tech­niques are able to study electrical activity in the brain. The huge amount of activity taking place when people are speaking shows that it might be impossible to pinpoint minute areas. This will clearly continue to be a topic for future research.

This chapter has looked at how humans acquire, comprehend and produce speech. It has also considered briefly how the study of language and mind overlaps with that of language and the brain. Recent advances in both areas suggest that in the future, the link­up may become closer.

QUESTIONS

1. What is meant by maturationally controlled behaviour?

2. What is the content-process controversy?

3 Child language is rule-governed. Explain this statement.

4 What are perceptual strategies?

5 How might a psycholinguist find out about sentence production?

6 Outline the main symptoms of two types of aphasia

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