Why are we able to understand complex sentences, while our nearest cousins, apes, can only understand individual words? Neurologists from Max Plank Institute have discovered there are two different areas in the brain specialized in language processing. The area dealing with simple grammatical tasks - such as the relation between two words - is also the more primitive area. The area dealing with hierarchical linguistic structures is only found in humans.

By a simple linguistic task is meant that one can comprehend the probability that a certain word, e.g. an article, comes before another word, e.g. a substantive or a verb. For example it is much more likely to find an expression such as "a book" than one like "an eat". Such a rule can also be understood by primates.

On the other hand a complex linguistic structure, such as a sentence, involves a more complicated hierarchical composition of words. For example to understand a sentence such as "I take the bus from A to B" one is required to organize the words in more complex relationships. Researchers have now found that this task is done by a different area of the brain, the Broca's Area, which appeared later in the development of the brain.

The non-human primates' inability to deal with complex hierarchical linguistic structures had been previously demonstrated by behavioral tests. Angela D. Friederici and her colleagues from the Max Plank Institute for Human Cognitive and Brain Sciences wanted to get a grip on the neuronal basis of the difference.

They have used in the testing an artificial grammar because other elements of language (semantics, phonology, morphology) also have various influences on neurological processing. This would have prevented them to gather specific information.

The scientists created an artificial grammar with meaningless but structured syllables such as "de bo gi to". The syllables were divided into two categories. Syllables of category A ended with (in German) phonetically bright vowels (de, gi, le …), and category B with dark vowels (bo, fo, gu). Then, the rules involved the ordering of these syllables: the simple rule ("local probability") involved alternating sequences from categories A and B (e.g., AB AB = de bo gi ku); and the complex rule ("hierarchy") required hierarchies to link both categories (e.g., AA BB = de gi ku bo).

One group of participants was trained on "local probability", while another group was trained to learn "hierarchy". Then, during the brain scanning session, the participants were presented with either syntactically correct sentences or with incorrect sentences. The subjects had to use the rules they had learned to evaluate each sequence as grammatically "right" or "wrong".

The result was that people trained on "local probability" used a phylogenetically older brain area, called the frontal operculum, while people trained on "hierarchy" used a younger brain structure, called the Broca's Area.

This research not only demonstrates the brain localization of various types of language processing functions, but also gives a clue about the evolution of language.

Previous research, published last year in Nature, showed a very interesting aspect of the evolution of the Broca Area. Dr. Michael Petrides and colleagues at the Montreal Neurological Institute at McGill University have shown that the brain region that eventually developed into the Broca's Area had originally evolved to control the mouth and the jaw more accurately. In humans this area came to control the movements necessary for speech and eventually also the details of complex linguistic processes.

"Our study shows that nonlinguistic monkeys possess an area comparable to Broca's area - it is located in the same region and has the same anatomical characteristics as Broca's area in the human brain", explained Dr. Michael Petrides. "Broca's area may have evolved originally as an area exercising high-level control over oral and facial actions, including those related to communicative acts, and that, in the human brain, this area eventually came to control also certain aspects of the speech act."