Localisation: We know that different areas of the brain are responsible for different processes. In linguistics, localisation is the idea that specific regions of the brain are responsible for different parts of language. Localisation implies the existence of different levels of language, such as the lexicon (words and vocabulary), syntax (grammar), phonology (sound), and morphology (word structure). Language is certainly localised to some extent. Following a stroke, many individuals develop aphasia, a language disorder that occurs when a blood vessel in the brain becomes blocked or ruptures. Broca's aphasia (caused by damage near Broca's area in the frontal cortex) is characterised by more jolted, effortful speech. Individuals with Broca's aphasia may also present with agrammatism, where sentences often miss core functional words. So, instead of saying "the dog barked at the child", "dog…bark…child" may be produced instead.

On the other hand, individuals with Wernicke's aphasia can produce perfectly grammatical and fluent sentences, yet they are often meaningless. From this, we can infer that Broca's area must play a crucial role in sentence structure, whilst Wernicke's area must be more involved in meaning and comprehension. These specific areas of language seem to be localised to specific brain regions.

But is language entirely localised? Not fully. A 2007 study explored the relationship between the proximity of a brain lesion to Wernicke's area and the severity of phonemic identification disorders. For example, patients with such disorders may struggle to distinguish between words such as "pat" and "bat". Since Wernicke's area is associated with speech comprehension, we would assume that the nearer the lesion is to Wernicke's area, the more pronounced these disorders are. This was the case 94.7% of the time, which means there is still a significant minority of patients who do not fit this pattern. Some report the same symptoms despite having a lesion much further away. However, we should not immediately discredit localisation theories: no two lesions are identical, and neither is the quality and immediacy of the treatment received, meaning that not all variables are controlled.

Lateralisation: This is the idea that specific functions and processes are taken care of by one hemisphere of the brain. It is generally accepted that language is lateralised to the left hemisphere. There have been several studies on split-brain patients who have undergone commissurotomy (a procedure that essentially separates the two hemispheres). When sentences such as "the girl is drinking" and "the girl will drink" are presented to the right hemisphere in such studies, participants struggle to tell the difference between the two. This suggests that the recognition of syntactic and phonological forms is largely left-lateralised.

Fernández, E. & Smith Cairns, H. (2010). Fundamentals of psycholinguistics. Malden, MA: Wiley-Blackwell. Ch. 3.

Ingram, John C. L. (2007). Neurolinguistics: An introduction to spoken language processing and its disorders. Cambridge: Cambridge University Press. Part I. 

Gazzaniga, M. S., & Hillyard, S. A. (1971). Language and speech capacity of the right hemisphere. Neuropsychologia, 9(3), 273–280.

Lateralization of brain function

Localisation of Function in the Brain and Hemispheric Lateralisation

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Tuesday 19th May

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Friday 22nd May

Tuesday 26th May

26th — 28th May: Various Subject Meetings with Directors of Studies at Christ’s College, Cambridge