Language, Neural Basis of

Investigations into the neural basis of language center around how the brain processes language. To do this, we must understand that language is a most complex function, one that encompasses numerous subprocesses, including the recognition and articulation of speech sounds, the comprehension and production of words and sentences, and the use of language in pragmatically appropriate ways. Underlying and interacting with these are also the functions of ATTENTION and MEMORY. All contribute in special ways to our ability to process language, and each may, in fact, be handled differently by the human brain. Classic neurolinguistic theories, developed over a hundred years ago, have suggested that certain brain areas play specific roles in the process of language. Since then, modern techniques are offering us new data on what these areas might actually do and how they might contribute to a network of brain areas that collectively participate in language processing.

Most of our information about how the brain processes language has been gleaned from studies of brain-injured patients who have suffered strokes due to blockage of blood flow to the brain or from gunshot wounds to the head during wartime. In these cases, the language deficits resulting from these injuries have been compared to the areas of the brain which became lesioned.

In the past, such investigations had to rely on autopsy data obtained long after the behavioral data had been collected. These days, structural neuroimaging using computed tomography (CT) and MAGNETIC RESONANCE IMAGING (MRI) can help us view the location and extent of the brain lesion while behavioral data can be collected concurrently. Modern electrophysiology studies as well as functional neuroimaging with POSITRON EMISSION TOMOGRAPHY (PET) and functional magnetic resonance imaging (fMRI) are now being conducted with normal non - brain-injured subjects and are also beginning to assist in our understanding of the brain mechanisms involved in language.

Classic descriptions of the brain areas involved in language have largely implicated those in the left cerebral hemisphere known as Broca's area, Wernicke's area, and the connecting bundle of fibers between them, the arcuate fasciculus. These descriptions began in 1861 when Pierre Paul BROCA described his examination of a chronically ill patient with an unusual speech deficit that restricted his ability to communicate (Broca 1861). Whenever the patient attempted to speak, his utterance was reduced to the single recurring syllable "tan," though he could intone it in different ways to change its meaning. When the patient died a few days after the examination, Broca discovered a lesion involving the posterior inferior frontal gyrus (i.e., the back part of the lowest section within the frontal lobe). Though Broca never cut the brain to examine the extent of this lesion, he suggested that this specific region was responsible for the articulation of speech. The area later became known as Broca's area and the behavioral deficit as Broca's APHASIA.

In 1874, Carl Wernicke reported on two patients with a language disturbance that was very different from the one Broca described (Wernicke 1874). These patients had difficulty with what Wernicke described as the "auditory memory for words." In short, they had trouble understanding spoken language, even though their own speech was fluent and unencumbered. Wernicke examined the brain of one of these patients at autopsy and thought that the most significant area of damage in this patient was in the superior temporal gyrus (i.e., the top part of the temporal lobe). He concluded that this region was crucial to the purpose of language comprehension, and subsequently the disorder in language comprehension was referred to as Wernicke's aphasia. Wernicke also developed an elaborate model of language processing that was revived by the neurologist Norman GESCHWIND in the 1960s, and which has formed the basis of our current investigations (Geschwind 1965, 1972).

The Wernicke-Geschwind model holds that the comprehension and formulation of language is dependent on Wernicke's area, after which the information is transmitted over the arcuate fasciculus to Broca's area where it can be prepared for articulation. In general, data from patients with lesions in these areas support this model. Those patients with injuries that involve temporal lobe structures have easily articulated speech but do not always understand spoken or written language. Those with frontal lobe lesions generally have speech or articulation difficulty but tend to understand simple sentences fairly well.

Like most theories, this one has its problems. First, the correlation between lesions to Broca's area and language deficits is far from perfect. Lesions to Broca's area alone never result in a persisting Broca's aphasia. The language deficits that are seen in these patients during the first few weeks after the injury always resolve into a mild or nonexistent problem (Mohr 1976). Several patients have also been reported that suffer from a persisting Broca's aphasia with no involvement of Broca's area whatsoever (Dronkers et al. 1992). Furthermore, Broca's original patient, on whom a significant part of the model is based, had multiple events that led to his aphasia. Since Broca never cut the brain, he could not have seen that other brain areas were also affected by these injuries.

Similar discrepancies can be seen with regard to Wernicke's area. Lesions to this area alone do not result in a persisting language comprehension problem, nor do all chronic Wernicke's aphasia patients have lesions in Wernicke's area (Dronkers, Redfern, and Ludy 1995). In fact, the data that originally predicted the participation of Wernicke's area in language comprehension were based on Wernicke's two problematic cases. One of these was noted to have resolved the comprehension problem after seven weeks and was never even brought to autopsy, while the second was a demented patient with numerous other pathological changes besides just those in the superior temporal gyrus. Current findings are showing that lesions must encompass far more than just the inferior frontal gyrus or superior temporal gyrus to produce the respective chronic Broca's or Wernicke's aphasia.

Still, those who study language and the brain have clung to traditional theory for several good reasons. First, there have been no good substitute theories that can explain as much of the data as the classic theories have been able to do. Most aphasic patients do show the pattern of behaviors described by Broca, Wernicke, and Geschwind. Furthermore, physicians and speech pathologists have found it easy to diagnose and treat their aphasic patients within this framework. While the original cases may have been faulty, the theories that resulted have served to answer most of the questions that surround these patients.

Modern techniques and technologies may gradually be changing the classic model. Most of these contributions concern the roles of traditional language areas, the possibility that other brain areas might also be important, and the likelihood that language processing involves a network of brain areas that contribute in individual but interactive ways. Being so new, these conclusions have not yet made it into neuroscience or linguistics textbooks. Still, it is clear that the classic model, despite its important contributions to understanding language mechanisms in the brain, will see some revision in the next decade.

Take the elusive role of Broca's area as an example. Though Broca thought it was concerned only with the articulation of speech, it has since been associated with many functions. Work in the 1970s included the manipulation of grammatical rules as a function of Broca's area, since those patients with a "Broca's aphasia" and lesions encompassing Broca's area had difficulty in using and comprehending grammatical information (Zurif, Caramazza, and Meyerson 1972). Recent functional imaging work with PET has suggested that it may play a role in short-term memory for linguistic information (Stromswold et al. 1996). Still other PET studies conclude that it is part of an articulatory loop (Paulesu, Frith, and Frackowiak 1993), while those that involve electrically stimulating the exposed brain during neurosurgery specify it as an end stage for motor speech (Ojemann 1994).

Lesion studies, coupled with high-resolution structural neuroimaging, continue to give us new information regarding other brain regions that might participate in speech and language. One new area that may participate in the articulation of speech is deep in the insula, the island of cortex that lies beneath the frontal, temporal, and parietal lobes. A recent study used computerized lesion reconstructions to find that a very specific area of the insula (high on its precentral gyrus) was lesioned in twenty-five stroke patients who all had a disorder in coordinating the movements for speech articulation (Dronkers 1996). Nineteen stroke patients without this particular disorder had lesions that spared the same area.

The insula is also lesioned in the majority of cases of Broca's aphasia (Vanier and Caplan 1990). This is not surprising, since Broca's aphasia patients have trouble in coordinating articulatory movements in addition to their other language deficits. Even Broca's original case had a large lesion that included the insula, as confirmed by a CT scan of the preserved brain done a hundred years later (Signoret et al. 1984). The fact that Broca's aphasia requires a large lesion that involves multiple brain areas supports the idea that many different regions must participate in the normal processing of language.

The functional imaging literature has given us new insight into areas of the brain that are actively engaged during a language task. Some of these areas would not have been detected from traditional lesions studies because the vascular supply to the brain is more susceptible to stroke in certain areas than others. For example, the supplementary motor area is consistently activated in functional imaging studies involving speech, but strokes to this area are relatively uncommon or do not come to the attention of those who study or treat language disorders. The same holds for posterior temporal areas that appear to be active in word form recognition. Also, functional imaging studies can signal the involvement of the right hemisphere in a given speech or language task, in addition to activation of traditional areas within the left hemisphere.

One area of the frontal lobe that has received a fair amount of attention in the functional imaging literature is the left inferior prefrontal cortex, the area of the brain in front of and below Broca's area. Peterson and colleagues found it to be activated in semantic retrieval tasks where subjects generated verbs associated with nouns presented to them (Peterson et al. 1988). Others have also found it involved in tasks requiring word retrieval or semantic encoding (e.g., Demb et al. 1995; Warburton et al. 1996). Still, it is not clear whether the role played by this area is one that is truly related to language or whether it is related to attention or executive functioning and merely plays an assistive role in language processing. Patients with lesions to this area do show deficits on constrained verbal fluency tasks in which they must generate words that begin only with certain letters or belong only to certain semantic categories, yet these patients are not obviously aphasic. Thus, the true contribution of this area to language must still be determined.

The basal temporal area is another region that was not implicated in classic models. This region lies at the base of the temporal lobe and is not usually affected by stroke, though it can be the source of epileptic seizure activity. Some epileptic patients have had electrodes temporarily placed under the skull directly on the cortex to monitor seizure activity. These electrodes can also deliver small electrical charges to the cortex that interfere with normal functioning. When placed over the basal temporal area, stimulation disrupts patients' ability to name objects, implying that this area is somehow involved in word retrieval (Luders et al. 1986). In a different kind of study, an epileptic patient with a seizure focus in the basal temporal area had an aphasia associated with the duration of the seizures that resolved once the seizures were stopped (Kirshner et al. 1995). All these data are derived from a different source than are most of the data from stroke patients, but still provide strong evidence that this area may also be important for normal language processing.

There are several other areas that may also contribute to language in their own way. The cingulate gyrus has been implicated in word retrieval, possibly because of its role in maintaining attention to the task. The anterior superior temporal gyrus, just in front of primary auditory cortex, may also play a role in sentence comprehension because of its rich connections to hippocampal structures important in memory. The fact that there are so many new brain regions emerging in modern lesion and functional imaging studies of language suggests that the classic Wernicke-Geschwind model, though useful for so many years, is now seen as oversimplified. Areas all over the brain are recruited for language processing; some are involved in lexical retrieval, some in grammatical processing, some in the production of speech, some in attention and memory. These new findings are still too fresh for any overarching theories to have developed that might explain how these areas interact. Future imaging and electrophysiological studies will undoubtedly show us not only the areas involved in language but also the recruitment of these areas at any stage of the process, the manner in which they interact, the time course of these activities, and the change in activation and allocation of resources relative to task complexity. The study of the neural mechanisms of language is evolving rapidly in conjunction with advances in the technologies that allow us to study it.

Other avenues of interest that are being pursued include the intriguing possibility that the brain may choose where to store lexical information depending on the semantic category to which the word belongs (Damasio et al. 1996; Martin et al. 1996). Another is that the brain might store and process language in different ways depending on the modality of acquisition (auditory vs. visual) (Neville, Mills, and Lawson 1992). Others question whether the brain mechanisms involved in language may differ for men and women, left-handers and right-handers, or monolinguals and bilinguals. These are all challenges for continued exploration whose findings are shaping contemporary models of language processing.

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Language and the Brain

-- Nina F. Dronkers

References

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Dronkers, N. F. (1996). A new brain region for coordinating speech articulation. Nature 384:159-161.

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Dronkers, N. F., J. K. Shapiro, B. Redfern, and R. T. Knight. (1992). The role of Broca's area in Broca's aphasia. Journal of Clinical and Experimental Neuropsychology 14:52-53.

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Signoret, J., P. Castaigne, F. Lehrmitte, R. Abelanet, and P. Lavorel. (1984). Rediscovery of Leborgne's brain: Anatomical description with CT scan. Brain and Language 22:303-319.

Stromswold, K., D. Caplan, N. Alpert, and S. Rauch. (1996). Localization of syntactic comprehension by positron emission tomography. Brain and Language 52:452-473.

Vanier, M., and D. Caplan. (1990). CT-scan correlates of agrammatism. In L. Menn and L. Obler, Eds., Agrammatic Aphasia: A Cross-Linguistic Narrative Sourcebook. Amsterdam: John Benjamins, pp. 37-114.

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Zurif, E. B., A. Caramazza, and R. Meyerson. (1972). Grammatical judgements of agrammatic aphasics. Neuropsychologia 10:405-417.

Language, Neural Basis of

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