Overview Of Neural Structures Supporting Language

There is evidence that language processing involves the perisylvian association cortex—the pars triangularis and opercularis of the inferior frontal gyrus [Brod-man's areas (BA) 45 and 44 (Broca's area)], the angular gyrus (BA39), the supramarginal gyrus (BA40), and the superior temporal gyrus (BA22: Wernicke's area) —in the dominant hemisphere. Data regarding the functional neuroanatomy of language processing were originally derived from deficit-lesion correlations and, recently, have been obtained from functional neuroi-maging and electrophysiological studies in normal subjects. All these sources of data indicate that the perisylvian association cortex is involved in this function.

Patients with lesions in parts of this cortex have been described who have had long-lasting impairments of this function. Disorders affecting language processing after perisylvian lesions have been described in all languages that have been studied, in patients of all ages, with written and spoken input, and after a variety of lesion types, indicating that this cortical region is involved in syntactic processing, independent of these factors. Functional neuroimaging studies have documented increases in regional cerebral blood flow (rCBF) using positron emission tomography (PET) or blood oxygenation level-dependent (BOLD) signal using functional magnetic resonance imaging (fMRI) in tasks in associated with language processing. Event-related potentials (ERPs) whose sources are likely to be in this region have been described in relationship to a variety of language processing operations. Stimulation of this cortex by direct application electrical current during neurosurgical procedures interrupts language processing. From these data, it can be concluded that language processing is carried out in the dominant perisylvian cortex.

Regions outside the perisylvian association cortex might also support language processing. Working outwards from the perisylvian region, there is evidence that the modality of language use affects the location of the neural tissue that supports language, with written language involving cortex closer to the visual areas of the brain and sign language involving brain regions closer to those involved in movements of the hands than movements of the oral cavity and its contents. Some ERP components related to processing improbable or ill-formed language are maximal over high parietal and central scalp electrodes. This may suggest that these regions are involved in language processing, but two factors have to be considered before such a conclusion is drawn: The location in the brain of the tissue that generates an ERP wave is not easy to identify on the basis of the scalp location of that wave and may not be right below that wave, and some of these waves may reflect general processes related to detection of pragmatically implausible or unlikely events in general, not language processing per se.

Both lesion studies in stroke patients and functional neuroimaging studies suggest that the anterior temporal lobe, primarily in the dominant hemisphere, is involved in aspects of language processing. The leading candidate for such a function is the accessing of the sounds of words from their meanings in speech production. However, electrocortical stimulation studies and the effects of neurosurgical resections do not support this conclusion. On the other hand, both functional neuroimaging and electrocortical stimulation studies indicate that the inferior temporal lobe is involved in aspects of word processing, particularly the representation of meanings of nouns. Activation studies implicate the frontal lobe just in front of Broca's area in word meaning as well, although these activations may reflect switching sets rather than processing semantic representations. Injury to the supplementary motor cortex along the medial surface of the frontal lobe can lead to speech initiation disturbances; this region may be important in activating the language processing system, at least in production tasks. Activation studies have shown increased rCBF and BOLD signal in the cingulate gyrus in association with many language tasks. This activation, however, appears to be nonspecific because it occurs in many other, nonlinguistic, tasks as well. It has been suggested that it is due to increased arousal and deployment of attention associated with more complex tasks. The cerebellum also has increased rCBF in some activation studies involving both language and other cognitive functions. This may be a result of the role of this part of the brain in processes involved in timing and temporal ordering of events or because it is involved in many cognitive functions.

Subcortical structures may also be involved in language processing. Several studies report aphasic disturbances following strokes in the deep gray matter nuclei (the caudate, putamen, and parts of the thalamus), but studies of other diseases affecting the same nuclei fail to show significant language impairments. For instance, aphasias follow some caudate strokes, but language disorders are minimal in patients with Huntington's disease, even at the stage of the illness at which memory impairments are readily documented. It has been suggested that subcortical structures involved in laying down procedural memories for motor functions, particularly the basal ganglia, are involved in "rule-based" processing in language, such as regular aspects of word formation, as opposed to the long-term maintenance of information in memory, as occurs with simple words and irregularly formed words.

Some abnormal language behaviors seen after deep gray matter lesions probably reflect the effects of disturbances in other cognitive functions on language. An example of this is the fluctuation between neolo-gistic jargon and virtual mutism seen after some thalamic lesions. This corresponds to a more general fluctuation between states of delirium and near akinetic mutism, and it most likely reflects the effects of some thalamic lesions on arousal, alerting, and motivational functions, some of which are seen in the sphere of language. Intraoperative stimulation studies of the interference with language functions following dominant thalamic stimulation also suggest that the language impairments seen in at least some thalamic cases are due to disturbances of attentional mechanisms. Perhaps the most important consideration regarding language disorders following subcortical lesions is the question of whether they result from altered physiological activity in the overlying cortex and not from disorders of the subcortical structures. In general, subcortical lesions cause language impairments when the overlying cortex is abnormal (often, the abnormality can only be seen with metabolic scanning techniques), and the degree of language impairment is much better correlated with measures of cortical rather than subcortical hypometabolism. It may be that subcortical structures serve to activate the language processing system but do not process language.

The other major component of the subcortical region of the cerebral hemispheres is the white matter. White matter tracts transmit representations from one area to another. Lesions of white matter tracts disconnect regions of the brain from others and make the operations performed in one region unavailable to others. This can cause language disorders. The best known such disturbance is a pure alexia, in which a patient can write but not read-not even his or her own writing. This can result from a lesion that destroys the primary visual cortex in the dominant hemisphere and extends forward in the white matter to cut off visual information coming to the nondominant hemisphere from the dominant hemisphere language area. In addition to these "disconnection" syndromes, lan guage disturbances of all sorts occur with lesions affecting many white matter tracts, whereas sparing of language functions can follow lesions in identical subcortical areas. The fact that multiple language processing disturbances occur following subcortical strokes affecting white matter is consistent with the existence of many information transfers carried out by white matter fibers, suggesting that many of the areas of cortex and/or subcortical nuclei that carry out sequential language processing operations are not contiguous.

In summary, a large number of brain regions are involved in representing and processing language. Ultimately, they all interact with one another as well as with other brain areas involved in using the products of language processing to accomplish tasks. In this sense, all these regions are part of a "neural system,'' but this concept should not obscure the fact that many of these regions appear to compute specific linguistic representations in particular tasks. The most important of these regions is the dominant perisylvian cortex.

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