Neuropsychology of Language

The neuropsychological approaches are gradually leading to important discoveries about many aspects of brain function, and language is no exception. Progress has certainly been made in identifying the structure and form of language(s), its universal features, its acquisition and so on, but, until recently, this work has tended to ignore pathologies of language. More recently, neuropsychologists have begun to draw parallels between aphasic disorders and disruption to specific linguistic processes.

This work provides evidence of a double dissociation between semantic and syntactic processes, and illustrates clearly that no single brain ‘language centre’ exists. The development of research tools such as the Wada test, and, more recently, structural and functional imaging procedures, has enabled researchers to examine language function in the brains of normal individuals.

This work considers the various ways that scientists have examined lateralisation, and the conclusions that they have drawn from their research. The work supports the view that language is mediated by a series of interconnected cortical regions in the left hemisphere, much as the 19th century neurologists proposed. In addition, this work considers recent explorations of language functions in the brain using neurophysiological techniques.

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At first glance, the two cortical hemispheres look rather like mirror images of each other. The brain, like other components of the nervous system, is superficially symmetrical along the midline, but closer inspection reveals many differences in structure, and behavioural studies suggest differences in function too. The reason for these so-called asymmetries is unclear, although they are widely assumed to depend on the action of genes. Some writers have suggested that they are particularly linked to the development in humans of a sophisticated language system (Crow, 1998). Others have argued that the asymmetries predated the appearance of language and are related to tool use and hand preference.

Scientific interest in language dates back to the earliest attempts by researchers to study the brain in a systematic way, with the work of Dax, Broca and Wernicke in the 19th century. Since then, interest in all aspects of language has intensified to the point where its psychological study (psycholinguistics) is now recognised as a discipline in its own right. In 1874 Karl Wernicke described two patients who had a quite different type of language disorder. Their speech was fluent but incomprehensible and they also had profound difficulties understanding spoken language.

Wernicke later examined the brain of one of these patients and found damage in the posterior part of the superior temporal gyrus on the left. At the same time as characterising this second form of language disorder, which we now call Wernicke's aphasia, Wernicke developed a theory of how the various brain regions with responsibility for receptive and expressive language function interact. His ideas were taken up and developed by Lichtheim and later, by Geschwind.

In Broca's aphasia, as with most neurological conditions, impairment is a matter of degree, but the core feature is a marked difficulty in producing coherent speech (hence the alternative names of 'expressive' or 'non-fluent' aphasia). Broca's aphasics can use well-practised expressions without obvious difficulty, and they may also be able to sing a well-known song faultlessly.

These abilities demonstrate that the problem is not related to 'the mechanics' of moving the muscles that are concerned with speech. Wernicke's first patient had difficulty in understanding speech yet could speak fluently, although what he said usually did not make much sense. This form of aphasia clearly differed in several respects from that described by Broca. The problems for Wernicke's patient were related to comprehension and meaningful output rather than the agrammatical and telegraphic output seen in Broca's patients.

Broca's and Wernicke's work generated considerable interest among fellow researchers. In 1885, Lichtheim proposed what has come to be known as the 'connectionist model of language' to explain the various forms of aphasia (seven in all) that had, by then, been characterised. Incidentally, the term 'connectionist' implies that different brain centres are interconnected, and that impaired language function may result either from damage to one of the centres or to the path-In Lichtheim's model, Broca's and Wernicke's areas formed two points of a triangle (Franklin 2003).

The third point represented a 'concept' centre where word meanings were stored and where auditory comprehension thus occurred. Each point was interconnected, so that damage, either to one of the centres (points), or to any of the pathways connecting them would induce some form of aphasia. Lichtheim's model explained many of the peculiarities of different forms of aphasia, and became, for a time, the dominant model of how the brain manages language comprehension and production.

Three new lines of inquiry - the cognitive neuropsychology approach, the functional neuro-imaging research of Petersen, Raichle and colleagues, and the neuroanatomical work of Dronkers and colleagues - have prompted new ideas about the networks of brain regions that mediate language. Researchers in the newly emerging field of developmental cognitive neuroscience seek to understand how postnatal brain development relates to changes in perceptual, cognitive, and social abilities in infants and children (Johnson 2005).

The cognitive neuropsychological approach has underlined the subtle differences in cognitive processes that may give rise to specific language disorders. The functional imaging research has identified a wider set of left brain (and some right brain) regions that are clearly active as subjects undertake language tasks. The emerging view from these diverse research approaches is that language is a far more complex and sophisticated skill than was once thought.

A universal design feature of languages is that their meaning-bearing forms are divided into two different subsystems, the open-class, or lexical, and the closed-class, or grammatical (Johnson 1997). Open classes have many members and can readily add many more. They commonly include (the roots of) nouns, verbs, and adjectives.

Closed classes have relatively few members and are difficult to augment. They include such bound forms as inflections (say, those appearing on a verb) and such free forms as prepositions, conjunctions, and determiners. In addition to such overt closed classes, there are implicit closed classes such as the set of grammatical categories that appear in a language (say, nounhood, verbhood, etc., per se), and the set of grammatical relations that appear in a language (say, subject status, direct object status, etc.).

The work supports a model of hemispheric specialisation in humans. While it would be an oversimplification to call the left hemisphere the language hemisphere and the right hemisphere the spatial (or non-language) hemisphere, it is easy to see why earlier researchers jumped to this conclusion. Whether this is because the left hemisphere is preordained for language, or because it is innately better at analytic and sequential processing, is currently a matter of debate.

The classic neurological approach to understanding the role of the brain in language relied on case studies of people with localised damage, usually to the left hemisphere. Broca and Wernicke described differing forms of aphasia, the prominent features of the former being non-fluent agrammatical speech, and those of the latter being fluent but usually unintelligible speech. Their work led to the development of Lichtheim's 'connectionist' model of language, which emphasised both localisation of function and the connections between functional areas.

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