Localisation of Language - Broca's area
The study of lateralisation of brain function first started to emerge in the 19th century when Jean-Baptiste Boullaud (1825) and Marx Dax (1836) observed that patients with damage to the left hemisphere invariably suffered from speech problems; yet the same was not true of individuals with damage to the right-hemisphere. Then in 1861, Simon Aubergine described the case of a failed suicide attempt where a man had blown away part of his left frontal cranium (using a shot gun), leaving the left part of his brain exposed (Wickens, 2005). Aubergine was intrigued to discover that if a spatula was pressed against the exposed brain region whilst the man was speaking his speech was immediately halted!
Thus, by the late 19th century it was becoming apparent that the left hemisphere played a major role in Language. Nonetheless, whilst such studies pointed towards regions of the left frontal lobe being important for language production, given the wavering support for phrenology, investigators were loath to ascribe specific brain regions with specific functions (because localisation of function could be taken as support for phrenology). Then, however, the French neurologist Paul Broca discovered Leborgne.
Leborgne was a patient of Broca's whose only utterance was the word 'Tan' (the name by which Leborgne became widely known). Despite this language deficit Leborgne was capable of comprehending spoken and written language, was able to communicate with gestures and by all accounts was an intelligent man (Wickens, 2005). Upon his death (1861), Broca performed an autopsy on Leborgne and discovered a large lesion towards the back of his left frontal lobe. By 1864 Broca had performed autopsies of eight more patients with similar language deficits and he was struck that all had damage to the same region of left frontal lobe (the left inferior prefrontal cortex). This region became known as Broca's area:
Individuals with damage to Broca's area typically suffer from a disturbance known as Broca's aphasia. This is language that is slow and laboured and lacking in grammatical structure; not unlike the language one might use in a telegram or text message. In addition, the speech of Broca's aphasics often lacks the intonation and inflection of normal language and, whilst the comprehension of spoken language appears normal in Broca's aphasics, they have problems understanding grammatically complex language, such as the sentence 'The man chased the horse' compared with 'The cat chased the mouse'. In the latter, real-world knowledge constrains the interpretation (i.e. it is less ambiguous). Thus Broca's area appears to be crucial for both the expression of language and the understanding of grammar.