Through careful behavior analysis, psychologists identify specific brain areas responsible for particular language skills.

By Beth Azar
Monitor staff

It?s time to throw out those old psychology textbooks that highlight just two main language centers in the brain?Wernicke?s area in the temporal lobe and Broca?s area in the frontal lobe.

Although these areas are important to human language ability, 'it?s much more interesting than that,' said Nina Dronkers, PhD, at an APA 1998 Annual Convention symposium on language. 'There are many, many other areas involved in language processing.'

Psychologists are discovering that by homing in on highly specific language skills, they can pinpoint small areas of the brain responsible for those skills. And many fall outside the two traditional language locales. Dronkers, of the Northern California Health Care System, and Michael Posner, PhD, of the University of Oregon, detailed some of these discoveries as part of the 'Mind, brain and behavior' symposium series hosted by APA divisions 3 (Experimental) and 6 (Behavioral Neuroscience and Comparative Psychology).

Redefining language areas

In one set of studies, Dronkers and her colleagues isolated a small patch of cerebral cortex that appears to be responsible for word articulation. They used MRI and CT imaging to examine the brain damage of people with speech and language disorders, such as the articulation problem that is part of Broca?s aphasia.

The disorder is named after Paul Broca, who in the 1860s studied a man who seemed to understand language but who couldn?t produce words. At autopsy, Broca discovered the man had damage to the left frontal area of the brain?the area now named Broca?s area and long thought to control articulation. Clinicians have since defined Broca?s aphasia more broadly to include symptoms such as agrammatic speech, impaired comprehension for complex grammatical structures and difficulty repeating sentences.

But this simplistic association between the many symptoms of Broca?s aphasia and the rather poorly defined large patch of brain known as Broca?s area doesn?t hold up, said Dronkers. She and her colleagues began trying to better understand the role of Broca?s area in language by using computer software they helped develop to compare the lesions of different patients with varying symptoms.

One of their first questions was whether all patients with a diagnosis of chronic Broca?s aphasia had a lesion in Broca?s area. They didn?t. Ten out of 50 aphasic patients studied had both Broca?s aphasia and a lesion that included Broca?s area. Another 10 didn?t have Broca?s aphasia but had lesions in Broca?s area. And two patients had Broca?s aphasia but had no damage in Broca?s area.

'Broca?s area may indeed be involved with aspects of articulation, but it is probably not as critical to the syndrome of Broca?s aphasia as we?ve been led to believe,' said Dronkers, who also holds an appointment at the University of California?Davis.

When she and her colleagues looked more closely at patients? scans they found one area that was damaged in all of them. It?s a tiny piece within an area known as the insula?a patch of cortex buried deep inside the cerebral hemisphere. In addition, all the patients with lesions in this part of the insula had a particular kind of motor-speech problem known as apraxia of speech in which the patient can identify a word, but has trouble speaking it.

All of the 25 patients diagnosed with apraxia of speech had lesions in the prefrontal gyrus of the insula, and none of 19 patients without apraxia had damage to that area, said Dronkers. 'Apraxia of speech is a hallmark of Broca?s aphasia,' she said.

'There are many different specific problems involved in these [language] syndromes,' said Dronkers. 'But, really, each of these individual problems, if we narrow them down enough, can in fact be localized to different areas of the brain.'

A language pathway

Posner and his colleagues have also found that by narrowing down a problem they can identify specific areas of the brain involved in certain language processes. In particular, they think they?ve found an area that handles lexical information and another that relates words to each other.

They began by examining how the brain handles a word task that requires someone to think of and then say a word?s use?saying 'pound' after hearing 'hammer,' for example. They used functional brain imaging to locate specific areas of the brain that are active during the one-second task. And then they used event-related potential (ERP) data to map the electrical activity temporally during individual phases of the task?attending to the word 'hammer' and locating and articulating 'pound,' for example.

They found that early on, there is a burst of electrical activity that correlates spatially with a patch of activity seen in the brain?s frontal midline during brain imaging. Shortly thereafter there is a burst of activity that correlates with a patch of left frontal activity. And much later there is more activity that correlates with activity seen in the left p osterior part of the brain.

'I think this data tells you something really rather surprising,' said Posner. 'That the frontal areas get into the picture very early and then recruit areas toward the back of the brain.'

This discovery led him to hypothesize that the frontal activity might be related to processing the individual lexical item?deciphering a word?s meaning?while the posterior activity might be more involved in relating that information to other aspects of the situation?the task of stating the word?s use, for example.

Posner and Antonella Pavese tested this hypothesis in another study where they had participants engage in two similar language tasks: one where they attended to the lexical meaning of a word and another where they had to make a decision about how the word fit into a sentence.

When participants attended to the lexical meaning of the word, there was early activity over an area of the left frontal brain that researchers believe is involved in lexical meaning. And when they attended to the relation of the word to a sentence, activity was increased over the posterior area, supporting the hypothesis, said Posner.

His findings and those of Dronkers imply that if researchers carefully analyze the individual mental operations responsible for a task?'as psychologists have been trying to do for the past 25 years or so,' said Posner?then they can link those mental operations to specific, rather than general, brain areas.