Neurobiology of human language and its evolution: Primate and Nonprimate Perspectives

Book excerpt: The evolution of human language has been discussed for centuries from different perspectives. Linguistic theory has proposed grammar as a core part of human language that has to be considered in this context. Recent advances in neurosciences have allowed us to take a new neurobiological look on the similarities and dissimilarities of cognitive capacities and their neural basis across both closely and distantly related species. A couple of decades ago the comparisons were mainly drawn between human and non-human primates, investigating the cytoarchitecture of particular brain areas and their structural connectivity. Moreover, comparative studies were conducted with respect to their ability to process grammars of different complexity. So far the available data suggest that non-human primates are able to learn simple probabilistic grammars, but not hierarchically structured complex grammars. The human brain, which easily learns both grammars, differs from the non-human brain (among others) in how two language-relevant brain regions (Broca’s area and superior temporal cortex) are connected structurally. Whether the more dominant dorsal pathway in humans compared to non-human primates is causally related to this behavioral difference is an issue of current debate. Ontogenetic findings suggest at least a correlation between the maturation of the dorsal pathway and the behavior to process syntactically complex structures, although a causal prove is still not available. Thus the neural basis of complex grammar processing in humans remains to be defined. More recently it has been reported that songbirds are also able to distinguish between sound sequences reflecting complex grammar. Interestingly, songbirds learn to sing by imitating adult song in a process not unlike language development in children. Moreover, the neural circuits supporting this behavior in songbirds bear anatomical and functional similarities to those in humans. In adult humans the fiber tract connecting the auditory cortex and motor cortex dorsally is known to be involved in the repetition of spoken language. This pathway is present already at birth and is taken to play a major role during language acquisition. In songbirds, detailed information exist concerning the interaction of auditory, motor and cortical-basal ganglia processing during song learning, and present a rich substrate for comparative studies. The scope of the Research Topic is to bring together contributions of researchers from different fields, who investigate grammar processing in humans, non-human primates and songbirds with the aim to find answers to the question of what constitutes the neurobiological basis of grammar learning. Open questions are: Which brain networks are relevant for grammar learning? Is there more than one dorsal pathway (one from temporal cortex to motor cortex and one to Broca’s area) and if so what are their functions? Has the ability to process sequences of a given hierarchical complexity evolved in different phylogenetic lines (birds, primates, other vocal production learners such as bats)? Is the presence of a sensory-to-motor circuit in humans a precondition for development of a dorsal pathway between the temporal cortex and Broca’s area? What role do subcortical structures (Basal Ganglia) play in vocal and grammar learning?