3.2. Linguistic function in the brain
Before the 19th century, the common theory of brain function assumed that the brain was not a structure made up of discrete independent centers; each specialized for different functions, but must be looked upon as a single working unit. This theory was termed ‘holism’. However, at the beginning of the 19th century, scientists began to assign function to certain neuronal structures and started to favor the theory of localization of function within the brain, which in particular was grounded on the discovery of Broca’s and Wernicke’s areas. Wernicke, who was neither a pure localizationist nor a pure holist, developed an elaborate model of language processing, where he proposed that only basic perceptual and motor activities are localized to single cortical areas, and more complex intellectual functions result from interconnections between these functional sites. He stated that different components of a single behavior are processed in a number of different regions of the brain and thus advanced the first evidence for the idea of distributed processing and connectionism. In the first part of the 20th century, the idea of functional segregation fell into disrepute, and was replaced by the proposal that higher cognitive abilities depended on the function of the brain as a whole. In contrast, the situation concerning memory and language was still an active battleground for the opponents and defenders of localization.
Until the 1960s, most of the information about the localization of linguistic function was based primarily on patients with brain lesions. In this case, the language deficits resulting from brain injuries have been compared to the areas of the brain which became lesioned. However, even if the lesion can be accurately located, the function that is examined after injury does not reflect the simple equation of the normal function missing, but it represents a new state of reorganization of the brain. In the 1960s, the American neurologist Norman Geschwind (1926–1984) refined Wernicke’s model of language processing, and this so-called Wernicke–Geschwind model still forms the basis of current investigations on normal and disturbed language function. Based on data from a large number of patients, Geschwind argued that regions of the parietal, temporal, and frontal brain lobes (тім'яної, скроневої та лобової часток мозку) were critically involved in human linguistic capacities. This model holds that the comprehension and formulation of language are dependent on Wernicke’s area, after which the information is transmitted to Broca’s area where it can be prepared for articulation.
The further expansion and improvement of Wernicke–Geschwind model of language processing within the brain has been made possible by the application of advanced neurophysiological methods (PET and fMRI in particular) which contributed interesting localization information, and provided evidence that a large region of the left hemisphere is clearly involved in language production and comprehension and that language localization varies from patient to patient. Contemporary studies are offering new data on what brain areas might actually do and how they might contribute to a network of neuronal structures that collectively participate in language processing. The results obtained by neurophysiologic methods support the notion that besides the classical Broca’s and Wernicke’s areas, several additional distributed cortical and subcortical (коркових і підкоркових) neuronal structures of both hemispheres clearly make a significant contribution to language function (Fig. 3.4).
Fig. 3.4.
Most of the areas consistently activated by linguistic processing lie within the left temporal, parietal, and frontal brain lobes (лівої скроневої, тім’яної та лобової часток мозку) and partly in right hemispheric areas. Extended areas in the left temporal cortex (лівої скроневої кори) are engaged in word retrieval, naming, morphosyntactic processing, parsing, syntactic comprehension and semantic analysis, and also in the articulation of speech. Moreover, several areas in the left frontal cortex (лівої лобної кори) including Broca’s area are found to be concerned with word retrieval, verbal working memory, syntactic processing, semantic encoding and retrieval, and also articulation, comprehension, and global attentional or executive functions. Right hemispheric cortical areas (зони кори правої півкулі) that correspond to Broca’s and Wernicke’s areas in the left hemisphere are associated with prosodic elements that impart additional meaning to verbal communication and additional linguistic functions that have not been clearly defined up to now. Moreover, the cerebellum (мозочок), and subcortical regions (підкоркові зони) like the basal ganglia (базальні ганглії) and especially the thalamus (таламус), which is presumably a lexical-semantic interface, may not only play supportive roles during linguistic processing.
The fact that there are so many new brain regions emerging in modern lesion and functional imaging studies of language suggests that the classical Wernicke-Geschwind model, although useful for so many years, has now been seen to be 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, and some in attention and memory. These new findings are still too fresh for any overarching theories that might explain how these areas interact. However, there is a definite need for hybrid models of language processing. Language is both localized and distributed, which means that specific language processing operations are carried out in particular brain locations organized in a distributed fashion.