One characteristic that clearly distinguishes us from non-human primates is our ability to understand and produce language. In particular, the human ability to apply complex linguistic rules has been held responsible for the fact that, in contrast to other species, we can produce and understand long sentences. When analysing language rules (syntax), one discovers two fundamentally different grammatical patterns. A simple rule governs the establishment of typical (probable) connections between words, like between article and noun ("a song") in contrast to article and verb ("a pleases"). The probability for a noun to follow an article is very high, while the probability of a following verb is very low. However, in order to understand longer sentences, a complex structural model is required - what is called a "hierarchy". Hierarchical dependencies serve to connect parts of a sentence - for example "around" an inserted subordinate clause: "The song [that the boy sang] pleased the teacher". The Max Planck study aimed to compare brain activities during the processing of both models - simple "local probability" and complex "hierarchy".
In a behavioural experiment, US scientists previously demonstrated that non-human primates (tamarin monkeys) are able to process local probability-based rules, but not hierarchical ones. This result led the researchers from Leipzig to hypothesise that complex grammatical rules are processed by brain areas that are "phylogenetically younger". The researchers investigated this assumption in an experiment using functional magnetic resonance imaging (fMRI) with humans.
To this end, the scientists created artificial grammars with meaningless but structured syllables (e.g., de bo gi to). The ordering of these syllables was based upon either the simple rule ("local probability") or the complex rule ("hierarchy"). 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). The simple rule involved alternating sequences from categories A and B (e.g., AB AB = de bo gi ku); the complex rules on the other hand required hierarchies to link both categories (e.g., AA BB = de gi ku bo). This principle was meant to reduce grammar into the simplest formal rules. The advantage of experimenting with artificial grammars - as opposed to naturally spoken grammars - lies in the fact that other elements of language (semantics, phonology, morphology) do not have additional influences on neurological processing.
The participants were trained with both types of grammars two days before the scanning session. One group learned "local probability", the other "hierarchy". During the fMRI session, new syllable sequences were presented on a monitor that were either syntactically "right" (correct sequences) or "wrong" (incorrect sequences). This measured the ability of the subjects to use the rules they had learned as they evaluated each sequence as grammatically "right" or "wrong".
In the processing of both rule types, the researchers from Leipzig were able to show activity in a phylogenetically older brain area, the frontal operculum. As they had suspected, a younger brain structure, Broca's Area, showed activity only when the participants processed hierarchical rules.
In the second step, diffusion tensor imaging (DTI) was used to investigate the structural connectivity of the two brain regions. The result was that both areas were again clearly differentiated from each other. The frontal operculum is connected to the anterior portion of the temporal lobe via special fibre connections (fasciculus uncinatus). By contrast, Broca's area is connected to posterior portions of the temporal lobe through the fasciculus longitudialis superior (see the figure).
Using two different procedures (fMRT and DTI), the Max Planck researchers were thus able to distinguish the two brain areas from each other in structure and function. When simple rules were processed - a task that apes can apparently also perform - the evolutionarily older area of the brain was activated. With more complex rules, on the other hand - which apes cannot apply - Broca's area became active.
This result is highly revealing with respect to the localisation of the functional regions governing language processing in the human brain. Furthermore, it exemplifies how complex questions - such as the origins of the human language faculty - can be investigated by the interdisciplinary combination of modern scientific methods. In future research, the scientific team in Leipzig will examine further consequences of these different connections to the temporal lobe for language processing.