In the last lecture, we examined which brain areas, in both humans and macaque monkeys, contribute to the learning of auditory sequences that form elementary grammars but present a partial analogy with the organization of spoken language. Chris Petkov's comparative work, based in particular on responses to violations of a learned auditory sequence, leads him to conclude that there is a mechanism of cerebral representation of sequences shared by both species and highly conserved in evolution. This is possible, but our own work, based on the local-global paradigm developed by Bekinschtein et al.(PNAS, 2009), suggests a major difference: in the human species, Broca's region (left inferior frontal gyrus, Brodmann's areas 44 and 45) displays unique responses for integrating several aspects of the sequence. We propose that this region is capable of representing sequences in the form of a compact formula that compresses all information into a single recursive symbolic tree.
In Abstract, language acquisition in humans relies on very specific circuits, notably the inferior prefrontal cortex and the superior temporal sulcus of the left hemisphere. Similar circuits exist in other primates, but they underwent a considerable expansion in surface area and connectivity during hominization. What's more, in our species, these circuits integrate more information than in the macaque monkey (work by Liping Wang). Although non-human primates are capable of learning arbitrary signs (signifier/signified associations), they do so without reversibility or mental construction of a vast system of symbols. They can learn arbitrary sequences, but are difficult or, at the very least, slow to learn systematic rules or complex grammars with recursive embedding. Although animals with vocal learning abilities, such as songbirds, exhibit circuits that are remarkably similar to those of the human species, including on the genetic level, their " language " does not feature embedded structures similar to those of our species.
Understanding the neurobiological origins of this singularity is one of the major questions facing researchers in the years to come.
