The idea that specific functions of the brain, like language, emotion, or vision, are linked to specific brain areas, can be traced back to the 18th Century, when Franz Joseph Gall coined the then-popular (but now pseudoscientific) phrenology; linking bumps and abnormalities in the human skull to differences in personality. Our methods have got a little bit better since then, but before we could rely on MRI, PET, EEG, TMS, and all of the wonderful insights that brain-imaging technology brings, we unfortunately had to rely on observing the effects of brian damage.
If we assume that a specific brain area is responsible for certain abilities, we can deduce that damage to this area will hinder this ability. Paul Broca first documented that some of his patients had an almost completely impaired ability to produce language; one could only repetitively say the word “tan”. Post-mortem examination of these patients showed that their brains were lesioned in the same place; the left inferior frontal gyrus, or now “Broca’s area”.
Because of this, many assumed that Broca’s area was responsible for language, but this would soon be contested. In the folllowing century, Carl Wernicke observed patients who could not comprehend language, but could still produce language which was often jargon-like (but retained some grammatical ability), or “word salad”, e.g. “Well this is … mother is away here working her work o'here to get her better, but when she's looking, the two boys looking in the other part. One their small tile into her time here. She's working another time because she's getting, too.” Wernicke noted that these patients had lesions in the left posterior temporoparietal region, or now “Wernicke’s area”.
These two distinctions resulted in two different categorisations for aphasia (an impairment in language ability). Broca’s aphasia, from damage to Broca’s area, refers to an inability to produce language, though intelligence and the ability to understand language are intact; it has been described as the patients knowing what they want to say, but being unable to get it out. Wernicke’s aphasia is the opposite; these individuals cannot comprehend language, but the ability to produce language remains intact.
When the preserved brains of Broca’s patients were rexamined, it was found that other areas may have contributed to the reduced ability to produce language. When language ability is disrupted by temporarily interfering with the electrical activity in Broca’s area (a modern method known as transcranial magnetic stimulation), this supports the notion that Broca’s area plays a role in language production, because temporarliy stopping Broca’s area from working halts speech. However, due to the findings that some patients with damage to this area cope rather well, and that other areas seem to be involved, it seems that Broca’s area is one component of a larger complex network which contributes to language production, rather than the be-all-and-end-all. Similarly, it is now believed that the functional areas for language comprehension, though including Wernicke's area, are more broad. Nonetheless, the categorical diagnoses of Broca’s (or also “expressive”) aphasia and Wernicke’s (or “receptive”) aphasia remain.
So we don’t yet have a 100% clear picture of language in the brain. The technology we use on humans cannot see what is happening on a cell-by-cell level, leading us to emphasize the specific areas instead; furthermore, whereas we can do highly controlled experiments on animals, examining very specific brain areas and how they are connected to each other, the fact that animals don't have the same degree of language ability as we do would mean we cannot make strong inferences about human language processing.
Nevertheless, language research is striding on, using a variety of methods and opening up interesting new areas (I've written about some of these issues in my personal blog, so feel free to follow the links). Why does language occur in the left hemisphere? How do young children pick up language? Is there a “critical period” in which language must be learned? How deep is the link between language and memory? I hope that you have found this interesting, and that you are as excited about the insights the future might bring as I am!
- Rebecca (@reddyforthis)